HT Relay
Ian Bell
discharge the HT when off). Most relays I can find have contacts rated
at 250VAC which translates into a peak of about 350V. However, data is
scarce on what dc voltage these relays can switch. So far I have found
only one that gives a dc current versus voltage curve and that stops at
210V dc (and 200mA) and I really want to be able to switch up to 350V at
up to 200mA. The rest just give a dc voltage at max current value.
So, I am guessing that this problem has been faced before and there are
relays that are known to work fine in this application. Any recommendations?
By the way I prefer a 5V coil.
Cheers
Ian
David Looser
news:hgvuml$ovn$2...@localhost.localdomain...
The ratings of relays (indeed switches generally) is lower with DC than AC.
The usual general purpose relays available these days are rated to 250Vac,
but often only about 30Vdc at rated current. You could use a separate heater
transformer and use the relay to switch the primary of the HT transformer.
David.
Ian Bell
> "Ian Bell" <ruffr...@yahoo.com> wrote in message
> news:hgvuml$ovn$2...@localhost.localdomain...
>> I am trying to select a relay for a delayed HT switch (which will also
>> discharge the HT when off). Most relays I can find have contacts rated at
>> 250VAC which translates into a peak of about 350V. However, data is scarce
>> on what dc voltage these relays can switch. So far I have found only one
>> that gives a dc current versus voltage curve and that stops at 210V dc (and
>> 200mA) and I really want to be able to switch up to 350V at up to 200mA.
>> The rest just give a dc voltage at max current value.
>>
>> So, I am guessing that this problem has been faced before and there are
>> relays that are known to work fine in this application. Any
>> recommendations?
>>
>> By the way I prefer a 5V coil.
>>
>
> The ratings of relays (indeed switches generally) is lower with DC than AC.
> The usual general purpose relays available these days are rated to 250Vac,
> but often only about 30Vdc at rated current.
Yes, I realise that, but what happens if a use a relay rated at a much
higher current than I am actually using - say a 20 amp relay for a
0.2amp supply.
You could use a separate heater
> transformer and use the relay to switch the primary of the HT transformer.
>
The current (sic) design does have a separate heater transformer so that
is a viable alternative except I want to ensure the HT is discharged
when the mains is turned off.
Cheers
ian
> David.
>
>
Brian Gaff
involved would spark at that current I'd say.
Brian
--
Brian Gaff - bri...@blueyonder.co.uk
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Ian Bell" <ruffr...@yahoo.com> wrote in message
news:hgvuml$ovn$2...@localhost.localdomain...
Brian Gaff
Another thing to ask is is the load inductive? When I could see Ive welded
many a rely like this!
I guess for the crobar effect one could use a semiconductor, and maybe one
the other way around so to speak to do the soft rise in voltage. Its a
highish current, but not impossible.
Brian
--
Brian Gaff - bri...@blueyonder.co.uk
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"David Looser" <david....@btinternet.com> wrote in message
news:7phgkk...@mid.individual.net...
David Looser
> David Looser wrote:
>
> You could use a separate heater
>> transformer and use the relay to switch the primary of the HT
>> transformer.
>>
>
> The current (sic) design does have a separate heater transformer so that
> is a viable alternative
Then that is what I would do. It is a technique that was often used in
valve-based industrial equipment, such as broadcast transmitters.
> except I want to ensure the HT is discharged when the mains is turned
> off.
>
Bleeder resistor?
David.
David Looser
news:SxNYm.18638$Ym4....@text.news.virginmedia.com...
>I got the impression he wanted to short the ht out to make it safe though.
Shorting out the HT is a pretty drastic thing to do! Guaranteed to wreck the
relay that does it and the smoothing capacitors.
I suspect he intends to use the relay connect a resistor across the HT to
discharge the capacitors. But such a resistor can be left permanently
connected at the cost of a few wasted milliamps of HT current.
David.
Serge Auckland
Use a Triac. The gate can then be triggered by a low voltage relay like a
reed. I built such a delay into a valve amp I designed many years ago, which
used a SS bridge rectifier, and I wanted to delay the HT until the heaters
had warmed up. Used a Unijunction transistor timer to create a 30 second
delay, then that closed the reed relay which fired the Triac. 1000 volt 1
amp or more Triacs are cheap.
S.
Ian Bell
Yes, but unless it consumes a significant fraction of the load then its
decay time will be rather long.
Cheers
Ian
>
> David.
>
>
Ian Bell
Except its discharge time would them be rather too long.
Cheers
ian
> David.
>
>
Ian Bell
That sounds like a good idea.
Thanks
ian
> S.
David Looser
>
> Yes, but unless it consumes a significant fraction of the load then its
> decay time will be rather long.
>
True, but it's normal. How many bits of valve kit have relay connected
discharge resistors? - almost none. Is there any particular reason why you
need a very short discharge time?
David.
David Looser
>
> Except its discharge time would them be rather too long.
>
David.
David Looser
>
> That sounds like a good idea.
>
essential unless you add a second triac.
(Actually, since this is DC, wouldn't a thyristor be more appropriate?)
David.
Ian Bell
Safety. One of the functions of the relay is to disconnect the HT if the
PSU connector is unplugged. If I just switch the ac and bleed the HT
there will be a significant HT voltage on the PSU connector for long
enough for someone to get a shock from it. That's one reason why I
prefer to switch the dc HT.
Cheers
Ian
> David.
>
>
Ian Bell
The PSU is remote and the relay has an interlock to turn off the HT if
the PSU HT output lead is disconnected. I need to either disconnect the
HT or bleed it very quickly to avoid a possible shock hazard.
Cheers
ian
> David.
>
>
Phil Allison
"Ian Bell"
>
> Safety. One of the functions of the relay is to disconnect the HT if the
> PSU connector is unplugged. If I just switch the ac and bleed the HT there
> will be a significant HT voltage on the PSU connector for long enough for
> someone to get a shock from it. That's one reason why I prefer to switch
> the dc HT.
** Bell must be the biggest idiot on the face of the planet !!!
What sort of UTTER MORON uses a connector with exposed and ** touchable
pins** for the output of a high voltage PSU ??
Has the pommy ASS never noticed how AC supply leads ALWAYS use
connectors with female pins on the end that goes to the equipment ????????
Unbelievable.
.... Phil
John
vacuum relay
See
http://www.jenningstech.com/ps/jen/prodsearch.cgi?action=relays
I've been using one for a number of years for switching 1200V dc @ 0.5A and
have had no problems.
Coil voltages are normally 12 or 24VDC but I don't think they are available
in 5V
"Ian Bell" <ruffr...@yahoo.com> wrote in message
news:hgvuml$ovn$2...@localhost.localdomain...
Serge Auckland
I can't now remember why I used a triac rather than a thyristor. A thyristor
will certainly do the job. It's possible that I just happened to have a
triac lying around! Long time ago.
S.
Arny Krueger
news:SxNYm.18638$Ym4....@text.news.virginmedia.com
> make it safe though. Another thing to ask is is the load
> inductive? When I could see Ive welded many a rely like
> this!
> I guess for the crobar effect one could use a
> semiconductor, and maybe one the other way around so to
> speak to do the soft rise in voltage. Its a highish
> current, but not impossible.
Legacy crowbar - fire a thyratron.
Jim Lesurf
For the HT source side I assume you use female connectors, so physically
preventing accidental contact with fingers, etc.
For the HT sink side you can use a silicon diode following a shunt resistor
to ground. Once the plug is pulled this isolates the HT from the prongs of
the male plug.
Note this means the cable is *not* reversable as it has a male at one end
and a female at the other to ensure that whichever end is opened you can
apply the above correctly. The shunt resistor before the diode will also
help remove the charge from any cable capacitance if the disconnection is
at the source end.
Afraid I don't recall you saying where the smoothing/reservoir caps are
located. So I'm assuming both/either ends of the link.
Also use a bleed resistor on the caps.
You could use something like a crowbar which trips if a loop via two
conductors of the cable is broken. However though neat in some ways it can
fall into the trap of being over-complex so designing in extra failure
modes. Hence if you are very safety conscious I'd still do the above.
My experience is that almost any type of conventional relay will end up
causing problems if the unit is used for many years. Avoid if at all
possible. Solid state relays would be much better if you can find one that
suits your purpose.
Slainte,
Jim
--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scots_Guide/intro/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html
Ian Bell
Of course I use female pins. Unfortunately, female pins are not by
themselves sufficient to stop a stupid enough person from connecting
themselves to them. Disconnection is the only really safe option.
Cheers
Ian
Ian Bell
> In article <hh0rma$t9r$3...@localhost.localdomain>, Ian Bell
> <ruffr...@yahoo.com> wrote:
>> David Looser wrote:
>>> "Ian Bell" <ruffr...@yahoo.com> wrote
>>>> Yes, but unless it consumes a significant fraction of the load then
>>>> its decay time will be rather long.
>>>>
>>> True, but it's normal. How many bits of valve kit have relay connected
>>> discharge resistors? - almost none. Is there any particular reason
>>> why you need a very short discharge time?
>>>
>
>> The PSU is remote and the relay has an interlock to turn off the HT if
>> the PSU HT output lead is disconnected. I need to either disconnect the
>> HT or bleed it very quickly to avoid a possible shock hazard.
>
> For the HT source side I assume you use female connectors, so physically
> preventing accidental contact with fingers, etc.
Of courcse.
>
> For the HT sink side you can use a silicon diode following a shunt resistor
> to ground. Once the plug is pulled this isolates the HT from the prongs of
> the male plug.
>
I don't quite follow that. Could you explain a little further.
> Note this means the cable is *not* reversable as it has a male at one end
> and a female at the other to ensure that whichever end is opened you can
> apply the above correctly. The shunt resistor before the diode will also
> help remove the charge from any cable capacitance if the disconnection is
> at the source end.
>
Yes, that is the plan - female connector on PSU, male connector to
female connector in the cable and male at the equipment end. Still not
clear how the diode and shunt R work.
> Afraid I don't recall you saying where the smoothing/reservoir caps are
> located. So I'm assuming both/either ends of the link.
>
Mostly at the PSU end but there will be some in the equipment too (
order of magnitude less capacitance).
> Also use a bleed resistor on the caps.
>
> You could use something like a crowbar which trips if a loop via two
> conductors of the cable is broken. However though neat in some ways it can
> fall into the trap of being over-complex so designing in extra failure
> modes. Hence if you are very safety conscious I'd still do the above.
>
That's effectively what the relay is supposed to do:
1. A NO contact disconnects the HT when the relay drops out
2. A NC contact rapidly discharges the resevoir caps..
> My experience is that almost any type of conventional relay will end up
> causing problems if the unit is used for many years. Avoid if at all
> possible. Solid state relays would be much better if you can find one that
> suits your purpose.
I think I just grokked the series diode shunt R at the equipment end.
Thanks
Ian
>
> Slainte,
>
> Jim
>
David Looser
>
> Of course I use female pins. Unfortunately, female pins are not by
> themselves sufficient to stop a stupid enough person from connecting
> themselves to them. Disconnection is the only really safe option.
Wouldn't a better option being to simply house the thing somewhere where a
stupid enough person can't get anywhere near it?
David.
Ian Bell
Unfortunately, it is surprising how enterprising a health and safety,
sorry I meant stupid, person can be.
Cheers
ian
David Looser
>
> My experience is that almost any type of conventional relay will end up
> causing problems if the unit is used for many years. Avoid if at all
> possible. Solid state relays would be much better if you can find one that
> suits your purpose.
>
I don't share your jaundiced view of relays. Whilst they are not suitable
for every switching requirement, there are many applications for which they
are the most logical choice. I suspect your negative view is based on the
effects of cost-cutting by equipment manufacturers. SSRs are a lot more
expensive than ordinary relays and they have their own limitations. They
only make economic sense when very many operate-release cycles are expected
over the equipment life.
I might cite the two relays in the control box of my 30-year old central
heating boiler. The boiler is well past it's sell-by date and due for
replacement in 2010 (esp if I can get Gordon Brown to chip in �400 :-), but
the relays have never been any trouble.
David.
mick
<snip>
>
> My experience is that almost any type of conventional relay will end up
> causing problems if the unit is used for many years. Avoid if at all
> possible. Solid state relays would be much better if you can find one
> that suits your purpose.
>
For goodness sake don't tell the thousands of small generating stations
that have been using relays for generator control continuously for the
last 40 years or so... They might get suspicious if they find out that
their technology is so unreliable. ;-)
Providing relays are specified correctly (and not run at too high a
temperature) they can be very reliable indeed - often better than solid
state switching as they aren't affected by moderate spikes on either the
coil or contact circuits. When that is coupled with the very simple plug-
n-play maintenance (which is often all the systems get) it makes them
ideal for use in remote generation plant.
The main problem is wiring. They are very labour intensive when a lot are
needed, not to mention the space needed for relay racks. I've yet to find
a 4-pole SSR with changeover contacts... :-)
--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
Filtering everything posted from googlegroups to kill spam.
Phil Allison
"Ian Bell"
** Works for several * BILLION * mains power leads in use round the world.
YOU FUCKING AUTISTIC MORON !!!
> Disconnection is the only really safe option.
** Execution is the only safe option for congenital anencephalics like YOU.
.... Phil
Jim Lesurf
<ruffr...@yahoo.com> wrote:
> Jim Lesurf wrote:
> > In article <hh0rma$t9r$3...@localhost.localdomain>, Ian Bell
> > <ruffr...@yahoo.com> wrote:
> >
> > For the HT sink side you can use a silicon diode following a shunt
> > resistor to ground. Once the plug is pulled this isolates the HT from
> > the prongs of the male plug.
> >
> I don't quite follow that. Could you explain a little further.
Think of the connector on the unit at the receiving end of the cable.
First put a shunt resistor between the pins. The put a series diode in the
internal lead from the connector into the unit so that it is in between the
connector and any internal capacitor. Put this to way round so it will
conduct in normal use.
Now pull out the cable from the connector.
The power supply has been removed but there is still a charge in the
capacitance of the unit. However to reach the connector it would have to
pass back though the diode. But for this current diection the diode is
reverse biassed, and its other end is connected to zero by the shunt
resistor. So the diode isolates the internal capacitance from the
connector's 'live' pin as the resistor connects it to zero.
If the cable is still connected to the unit, but disconnected at the other
(PSU) end, then the shunt resistor also bleeds away the charge in the cable
capacitance.
This clearly means using a diode with a high enough reverse bias limit, and
accepting a diode drop in the supplied voltage in normal operation. But it
is a cheap and effective way to have a 'one way valve' in the system.
> I think I just grokked the series diode shunt R at the equipment end.
OK, the above should confirm that. :-)
A loop sensing relay should work. And physical relays do have a low
insertion resistance, and good isolation, when working well. But I do have
a worry about reliability over time and with any physical abuse. Solid
state methods like diodes, crowbars, etc are better in my view.
Based partly on signal relays. But also with working in the past with 2k5V
PSUs for klystrons and higher voltages for BWOs, etc. ...and with having
shocks when other farted up the system without telling me. That was how I
found I *could* do a back flip. 8-]
No harm in having a loop relay as *well* of course. :-) But I'd then
wonder about things like leakage in the cable if using the system for
safety critial purposes in the long term where abuse and degradation might
occur.
BTW One 8kV 100mA PSU I used to use had a habit of tripping at a given time
of day. Turned out to be sunlight getting into a crack in the case and
shining on a photodiode in an isolator that sensed a connection problem.
:-)
Jim Lesurf
<david....@btinternet.com> wrote:
> "Jim Lesurf" <no...@audiomisc.co.uk> wrote
> >
> > My experience is that almost any type of conventional relay will end
> > up causing problems if the unit is used for many years. Avoid if at
> > all possible. Solid state relays would be much better if you can find
> > one that suits your purpose.
> >
> I don't share your jaundiced view of relays. Whilst they are not
> suitable for every switching requirement, there are many applications
> for which they are the most logical choice. I suspect your negative
> view is based on the effects of cost-cutting by equipment
> manufacturers.
Based also on kit I designed myself and where I chose relays. The problems
may take decades, but in my experience eventually turn up. If you think
about it, physical contacts all eventually wear or corrode, springs weaken,
etc.
> SSRs are a lot more expensive than ordinary relays and
> they have their own limitations. They only make economic sense when
> very many operate-release cycles are expected over the equipment life.
> I might cite the two relays in the control box of my 30-year old central
> heating boiler. The boiler is well past it's sell-by date and due for
> replacement in 2010 (esp if I can get Gordon Brown to chip in �400 :-),
> but the relays have never been any trouble.
However the problem Ian is converned with is a safety issue, and for all I
know might affect users in some decades time from now.
And FWIW I had to change the controls for our central heating some time
ago.
David Looser
news:50cfa62...@audiomisc.co.uk...
> In article <7pkr07...@mid.individual.net>, David Looser
> <david....@btinternet.com> wrote:
>> "Jim Lesurf" <no...@audiomisc.co.uk> wrote
>> >
>> > My experience is that almost any type of conventional relay will end
>> > up causing problems if the unit is used for many years. Avoid if at
>> > all possible. Solid state relays would be much better if you can find
>> > one that suits your purpose.
>> >
>
>> I don't share your jaundiced view of relays. Whilst they are not
>> suitable for every switching requirement, there are many applications
>> for which they are the most logical choice. I suspect your negative
>> view is based on the effects of cost-cutting by equipment
>> manufacturers.
>
> Based also on kit I designed myself and where I chose relays. The problems
> may take decades, but in my experience eventually turn up. If you think
> about it, physical contacts all eventually wear or corrode, springs
> weaken,
> etc.
>
Yes physical contacts will wear, but nothing lasts forever. Electronic
components have their own failure mechanisms. I remember reading only a few
months ago about concern from the avionics industry about the short MTBFs of
modern high-density ICs.
>
> However the problem Ian is converned with is a safety issue, and for all I
> know might affect users in some decades time from now.
Indeed. But that statement carries an unspoken assumption that using a relay
will be more of a safety issue in years to come than using electronic
components instead. No components are 100% reliable, and correctly specified
relays have as good a record as any.
> And FWIW I had to change the controls for our central heating some time
> ago.
>
Was that change required because of relay failure? Anyway I wasn't talking
about "controls", I was talking about the boiler control unit, which
sequences the operation of the blower motor, the ignition transformer and
the oil valve at start-up. It also shuts down the boiler in the event of no
flame being detected, which *is* a safety issue. Over the years the boiler
has had to have new baffle plates, a new blower motor and a new drive
coupling to the oil pump, but the 30 year-old relays in the control box are
working as well as ever.
David.
mick
> I am trying to select a relay for a delayed HT switch (which will also
> discharge the HT when off). Most relays I can find have contacts rated
> at 250VAC which translates into a peak of about 350V. However, data is
> scarce on what dc voltage these relays can switch. So far I have found
> only one that gives a dc current versus voltage curve and that stops at
> 210V dc (and 200mA) and I really want to be able to switch up to 350V at
> up to 200mA. The rest just give a dc voltage at max current value.
>
> So, I am guessing that this problem has been faced before and there are
> relays that are known to work fine in this application. Any
> recommendations?
>
Did you get this sorted out Ian? The thread in rat seems to have
degenerated into the usual slanging match with Phil. :-(
By the way, you may not find it too easy getting hold of a suitable relay
with a 5v coil. Manufacturers often list them down to 6v but nobody
stocks them! The nearest that you will get easily will be 12vdc and the
next one up is 24vdc. 5v coils are usually only on small pcb relays,
which don't have enough voltage rating on the contacts, even in series
(if you can ever find a complete data sheet!).
Ian Bell
I'm getting there. Despite Phil's rantings there's quite a bit of useful
info in there if you take the time to sift it out. A bright lad but
sadly disabled.
Anyway, I have found a number of 5V relays that should do the trick,
along with some careful bits at both ends of the cable to ensure the
voltages broken are manageable. I can use 6V relays if necessary - the
current plan is to make a local dc supply from the heater 6.3VAC via a
5V low drop out regulator for the little PIC that decides when the relay
should operate. The relay could either work from the regulated 5V or the
raw smoothed dc. Next step is to make a simple test rig and try a life test.
Cheers
Ian
David Looser
>
> PIC that decides when the relay should operate.
A PIC? good lord you are making a meal out of this one aren't you? Anyway I
thought you had decided to go for Don's Triac option?
Had you thought about simply hard-wiring the two units together? then the
problem vanishes. Or if you *must* use a plug & socket make it impossible to
remove it without unplugging the mains first. I really do think that you are
likely to end up with an electrical interlock system that is so complex, and
based on potentially unreliable components, as to fail to provide the safety
that is supposedly your aim.
David.
David Looser
>
> By the way, you may not find it too easy getting hold of a suitable relay
> with a 5v coil. Manufacturers often list them down to 6v but nobody
> stocks them! The nearest that you will get easily will be 12vdc and the
> next one up is 24vdc. 5v coils are usually only on small pcb relays,
> which don't have enough voltage rating on the contacts, even in series
> (if you can ever find a complete data sheet!).
>
ISTM that 5V coils are now rather more common than 6V ones, and 5V coils are
just as likely to be available on power relays as any other coil voltage.
BTW why do you think PCB relays won't have enough voltage rating?, I've just
found a relay with a 10kV rating for the contact, and that is a PCB mounting
type.
David.
Ian Bell
> "Ian Bell" <ruffr...@yahoo.com> wrote
>
>> PIC that decides when the relay should operate.
>
> A PIC? good lord you are making a meal out of this one aren't you?
No, the initial reason for the relay was to delay the switch on of the
HT by 30 seconds or so to allow the heaters to warm up. That was all.
Then it occurred to me that as it would drop out when the mains went off
it could also be used to more rapidly discharge the HT smoothing caps
than by using a simple bleed resistor (I reckon there's abnout 150
joules of energy stored in them)
Once the relay is in there, it is then simple to arrange that it also
drops out if someone pulls the HT cable.
The PIC is an 8 pin DIL, costs a dollar and allows me to light a couple
of LEDs to show the HT state, handle the cable interlock and drive the
relay. It needs no external clock components and there's even a spare IO
line I could use for a push button emergency off switch.
Cheers
Ian
Phil Allison
"David Looser has Roos loose in his top paddock "
> BTW why do you think PCB relays won't have enough voltage rating?
** The laws of physics ??
> I've just found a relay with a 10kV rating for the contact,
> and that is a PCB mounting type.
** One of these - right ?
http://www.farnell.com/datasheets/56123.pdf
It's a reed relay, so switching is done in a vacuum.
The 50 watt max rating means it will switch 5mA at 10kV.
Or 140mA at 350 VDC .............
Same as many other types mentioned already.
Wot a pathetic bullshit artist you are.
..... Phil
Phil Allison
"Ian Bell"
>
> Once the relay is in there, it is then simple to arrange that it also
> drops out if someone pulls the HT cable.
** Seems we have all be sucked into another of Bell's insane
misunderstandings of electronics.
IF the relay only has to disconnect the HT pin on the PSU connecter
AFTER the plug has been pulled - then it is switching off
NO CURRENT AT ALL !!!
ANY relay at all can be used for that.
.... Phil
Jim Lesurf
<not....@invalid.invalid> wrote:
> On Fri, 25 Dec 2009 09:45:32 +0000, Jim Lesurf wrote:
> <snip>
> >
> > My experience is that almost any type of conventional relay will end
> > up causing problems if the unit is used for many years. Avoid if at
> > all possible. Solid state relays would be much better if you can find
> > one that suits your purpose.
> >
> For goodness sake don't tell the thousands of small generating stations
> that have been using relays for generator control continuously for the
> last 40 years or so... They might get suspicious if they find out that
> their technology is so unreliable. ;-)
> When that is coupled with the very simple plug- n-play maintenance
> (which is often all the systems get) it makes them ideal for use in
> remote generation plant.
The above has just reminded me of the joke in "Only Fools and Horses" where
Trig (IIRC) was proudly telling everyone how long his broom had lasted.
Then to say, "...Of course I have had to replace the head X times... and
the handle Y times. But it is remarkable how long that broom has lasted."
:-)
David Looser
news:7pqar2...@mid.individual.net...
>
> "David Looser has Roos loose in his top paddock "
>
>
>> BTW why do you think PCB relays won't have enough voltage rating?
>
> ** The laws of physics ??
>
>
>> I've just found a relay with a 10kV rating for the contact,
>> and that is a PCB mounting type.
>
>
> ** One of these - right ?
>
> http://www.farnell.com/datasheets/56123.pdf
As it happens no, but that one will do.
>
> It's a reed relay, so switching is done in a vacuum.
Yes, and? Did I say otherwise?
>
> The 50 watt max rating means it will switch 5mA at 10kV.
>
> Or 140mA at 350 VDC .............
Yes, and?
>
> Same as many other types mentioned already.
>
It's a high voltage relay that is PC mounted, which is what I said. Whether
it is a reed-relay or not, or what its current rating is is irrelevant to
the point.
> Wot a pathetic bullshit artist you are.
>
Sorry Phil, the "bullshit artist" around here is you, always has been.
David.
Phil Allison
"David Loser has Roos loose in his top paddock "
> BTW why do you think PCB relays won't have enough voltage rating?
** The laws of physics ??
> I've just found a relay with a 10kV rating for the contact,
> and that is a PCB mounting type.
** One of these - right ?
http://www.farnell.com/datasheets/56123.pdf
It's a reed relay, so switching is done in a vacuum.
The 50 watt max rating means it will switch 5mA at 10kV.
Or 140mA at 350 VDC .............
Same as many other types mentioned already.
Wot a pathetic fuckimg bullshit artist you are.
FOAD.
..... Phil
David Looser
>
> It's a reed relay, so switching is done in a vacuum.
>
> The 50 watt max rating means it will switch 5mA at 10kV.
>
> Or 140mA at 350 VDC .............
>
> Same as many other types mentioned already.
>
Why re-post stuff which I have already explained is irrelevant?
Because you simply cannot bear to lose an argument, even when you are
talking complete bollocks.
Goodbye Phil..
David.
Jim Lesurf
<ruffr...@yahoo.com> wrote:
> David Looser wrote:
> > "Ian Bell" <ruffr...@yahoo.com> wrote
> >
> >> PIC that decides when the relay should operate.
> >
> > A PIC? good lord you are making a meal out of this one aren't you?
> No, the initial reason for the relay was to delay the switch on of the
> HT by 30 seconds or so to allow the heaters to warm up. That was all.
During the first years of manufacture the ampliers/recievers in the
Armstrong 600 range used a thermal delay relay to avoid a surge when the
unit was switched on.
These were expensive. And they were one of the main failure modes which
brought sets back for repair. One of the reasons I avoid physical relays. I
replaced them in later sets with a better PSU that simply didn't need a
mechanical relay. Over the following few years it was quite clear that this
had a big impact on the reliability of the sets.
Jim Lesurf
<david....@btinternet.com> wrote:
> Had you thought about simply hard-wiring the two units together? then
> the problem vanishes. Or if you *must* use a plug & socket make it
> impossible to remove it without unplugging the mains first. I really do
> think that you are likely to end up with an electrical interlock system
> that is so complex, and based on potentially unreliable components, as
> to fail to provide the safety that is supposedly your aim.
I also worry about that. The more complex the system is - particularly when
physical items like relays and plugs/sockets are involved - the more
failure modes and sources of failure there will be.
TBH it still seems safer as well as simpler (and cheaper) to me to just fit
appropriate forms of connectors and use a protection diode and bleeds to
the destination. Safer because the number of items present has been
reduced, and no mechanical sensing is involved.
David Looser
> During the first years of manufacture the ampliers/recievers in the
> Armstrong 600 range used a thermal delay relay to avoid a surge when the
> unit was switched on.
>
I remember it well as I had a 600 with one in. To me it appeared to be a
cheap & nasty little thing that failed very early on. I simply shorted it
out and the receiver continued to work for many years without it.
> These were expensive.
Maybe, but it was still cheap & nasty.
> And they were one of the main failure modes which
> brought sets back for repair. One of the reasons I avoid physical relays.
But a thermal relay is a totally different animal from a conventional
electromagnetic relay. They have very slow make & break of the contacts, and
the characteristics of the bi-metal strip changes with age. Having said that
I've known *much* better and more reliable thermal relays than the one you
used.
I> replaced them in later sets with a better PSU that simply didn't need a
> mechanical relay.
Your original PSU didn't need it either IME. :-)
David.
Phil Allison
"David Loser is a DAMN LIAR "
> Why re-post stuff which I have already explained is irrelevant?
** Because you " explained " no such fucking thing.
You are nothing but a context manipulating pile of autistic garbage.
BTW:
Autism is 100% genetic.
So your parents and any children have it too.
A pox on the lot of them.
.... Phil
Phil Allison
"Jim Lesurf"
> During the first years of manufacture the ampliers/recievers in the
> Armstrong 600 range used a thermal delay relay to avoid a surge when the
> unit was switched on.
>
> These were expensive. And they were one of the main failure modes which
> brought sets back for repair. One of the reasons I avoid physical relays.
> I
> replaced them in later sets with a better PSU that simply didn't need a
> mechanical relay. Over the following few years it was quite clear that
> this
> had a big impact on the reliability of the sets.
** So the pig ignorant pommy fuckwit was so stupid he used one of these ?
http://www.tpub.com/content/neets/14175/img/14175_153_1.jpg
The kind on the left - I bet.
ROTFLMAO !!!!!!!!!!
Wot a fucking MORON !!!
...... Phil
Jim Lesurf
> > During the first years of manufacture the ampliers/recievers in the
> > Armstrong 600 range used a thermal delay relay to avoid a surge when
> > the unit was switched on.
> >
> I remember it well as I had a 600 with one in. To me it appeared to be a
> cheap & nasty little thing that failed very early on. I simply shorted
> it out and the receiver continued to work for many years without it.
In that case you were fortunate to get away with not realising what other
changes needed to be made! Sometimes the Ghods forgive those who make
changes in ignorance of the likely consequences. The main point of the
delay relay was that the bridge diodes that had been used had too low a
surge current rating. So bypassing the relay without also changing to new
diodes was increasing the chance of PSU failure.
> > These were expensive.
> Maybe, but it was still cheap & nasty.
Odd that you haven't noticed that 'expensive' and 'cheap' clash here. :-)
However I tested a number of them over a period of time, and compared them
with alternatives. I note your opinion based - presumably - on appearance,
though. :-)
> > And they were one of the main failure modes which brought sets back
> > for repair. One of the reasons I avoid physical relays.
> But a thermal relay is a totally different animal from a conventional
> electromagnetic relay. They have very slow make & break of the contacts,
> and the characteristics of the bi-metal strip changes with age.
You may need to think more carefully about the application in the example
before assuming that is relevant. You could also check to see if the relay
was such that it flipped state with some hysteresis.
> Your original PSU didn't need it either IME. :-)
As I said, you were lucky. I probably saw more of the sets than you did.
:-) Without the relay the surge peak applied was well above the rating of
the original bridge diodes. ...And the orginal PSU wasn't actually mine. It
was designed by Ted Rule. I just changed it a few years later to be cheaper
and far more reliable.
The particular problem was easily solved simply by using high surge rated
diodes. So far as I was concerned any mechanical relay at all was expensive
and nasty in such a situation, and easily avoided with far more reliable
solid state devices. Hence my mentioning it in this thread.
Jim Lesurf
<phi...@tpg.com.au> wrote:
> "Jim Lesurf"
> http://www.tpub.com/content/neets/14175/img/14175_153_1.jpg
You lose you bet on both counts. :-)
I did not use them. I replaced them with something else.
Nor was that type used.
Phil Allison
>
>> > During the first years of manufacture the ampliers/recievers in the
>> > Armstrong 600 range used a thermal delay relay to avoid a surge when
>> > the unit was switched on.
** All you needed was a simple NTC thermistor.
You wanking IMBECILE !!!!!!!
>> > These were expensive. And they were one of the main failure modes
>> > which brought sets back for repair. One of the reasons I avoid
>> > physical relays. I replaced them in later sets with a better PSU that
>> > simply didn't need a mechanical relay. Over the following few years it
>> > was quite clear that this had a big impact on the reliability of the
>> > sets.
>>
> >
>> ** So the pig ignorant pommy fuckwit was so stupid he used one of these ?
>
>> http://www.tpub.com/content/neets/14175/img/14175_153_1.jpg
>
>> The kind on the left - I bet.
>
> You lose you bet on both counts. :-)
>
> I did not use them. I replaced them with something else.
>
> Nor was that type used.
** Really ?????
How fucking typical of a grossly autistic CUNT like Jim Lesurf to only
ever ALLUDE to something while never saying what it is, so forcing folk to
make guesses.
Then he still refuses to be specific.
It must been an even stupider choice than the one above.
Hope the lying pommy cunt gets cancer - ASAP.
..... Phil
David Looser
news:50d0c0d...@audiomisc.co.uk...
> In article <7prk2m...@mid.individual.net>, David Looser
> <david....@btinternet.com> wrote:
>> "Jim Lesurf" <no...@audiomisc.co.uk> wrote in
>
>> > During the first years of manufacture the ampliers/recievers in the
>> > Armstrong 600 range used a thermal delay relay to avoid a surge when
>> > the unit was switched on.
>> >
>
>> I remember it well as I had a 600 with one in. To me it appeared to be a
>> cheap & nasty little thing that failed very early on. I simply shorted
>> it out and the receiver continued to work for many years without it.
>
> In that case you were fortunate to get away with not realising what other
> changes needed to be made! Sometimes the Ghods forgive those who make
> changes in ignorance of the likely consequences. The main point of the
> delay relay was that the bridge diodes that had been used had too low a
> surge current rating. So bypassing the relay without also changing to new
> diodes was increasing the chance of PSU failure.
>
simply have replaced them by more substantial types. But that didn't prove
necessary.
>> > These were expensive.
>
>> Maybe, but it was still cheap & nasty.
>
> Odd that you haven't noticed that 'expensive' and 'cheap' clash here. :-)
They do not necessarily clash. By what criterion were they "expensive"?
>
> However I tested a number of them over a period of time, and compared them
> with alternatives. I note your opinion based - presumably - on appearance,
> though. :-)
Indeed, one can often get a fair idea of the quality of an item, and it's
likely reliability, from a physical inspection. When you say you compared
them with alternatives, do you mean alternative thermal relays?, or
alternative methods of providing a soft-start?
>
>> > And they were one of the main failure modes which brought sets back
>> > for repair. One of the reasons I avoid physical relays.
>
>> But a thermal relay is a totally different animal from a conventional
>> electromagnetic relay. They have very slow make & break of the contacts,
>> and the characteristics of the bi-metal strip changes with age.
>
> You may need to think more carefully about the application in the example
> before assuming that is relevant. You could also check to see if the relay
> was such that it flipped state with some hysteresis.
Actually I'm suggesting that *any* thermal relay was inappropriate in that
application, or leastways used as you used it. By the standards of
electromagnetic relays contact pressures are low and switching times slow
(even with hysteresis). When thermal relays were commonly used as delay
timers it was normal practice to use their contact to operate a conventional
relay, which then did the real work.
>
>
> The particular problem was easily solved simply by using high surge rated
> diodes. So far as I was concerned any mechanical relay at all was
> expensive
> and nasty in such a situation, and easily avoided with far more reliable
> solid state devices. Hence my mentioning it in this thread.
>
Experience with thermal relays is irrelevant when talking about
electromagnetic relays, the two are chalk and cheese. You might as well make
judgements on the reliability of polypropylene capacitors in fast rise-time
pulse circuits based on your experience with electrolytics.
David.
Geoff Mackenzie
>
> The above has just reminded me of the joke in "Only Fools and Horses"
> where
> Trig (IIRC) was proudly telling everyone how long his broom had lasted.
> Then to say, "...Of course I have had to replace the head X times... and
> the handle Y times. But it is remarkable how long that broom has lasted."
> :-)
>
> Slainte,
>
> Jim
>
Oh, c'mon! That joke started with George Washington's axe, that he
allegedly used to fell a cherry tree....
Regards,
GMacK
Jim Lesurf
<david....@btinternet.com> wrote:
> "Jim Lesurf" <no...@audiomisc.co.uk> wrote in message
> news:50d0c0d...@audiomisc.co.uk...
>
> >
> > In that case you were fortunate to get away with not realising what
> > other changes needed to be made! Sometimes the Ghods forgive those who
> > make changes in ignorance of the likely consequences. The main point
> > of the delay relay was that the bridge diodes that had been used had
> > too low a surge current rating. So bypassing the relay without also
> > changing to new diodes was increasing the chance of PSU failure.
> >
> Maybe, but in my case it worked. Had the diodes subsequently failed I'd
> simply have replaced them by more substantial types. But that didn't
> prove necessary.
Alas, commercial makers of domestic kit can't rely on all their customers
taking such an attitude. :-)
> >> > These were expensive.
> >
> >> Maybe, but it was still cheap & nasty.
> >
> > Odd that you haven't noticed that 'expensive' and 'cheap' clash here.
> > :-)
> They do not necessarily clash. By what criterion were they "expensive"?
The thermal delay relays used were the order of a couple of quid *each* in
quantity, back in the 1970s. Compare that with the few pence per diode
bridge for better diodes that didn't need the delay. Plus of course the
savings to all involved when the change had a quite marked effect in
lowering the numbers being returned for repair because the thermal delay
was unreliable.
> >
> > However I tested a number of them over a period of time, and compared
> > them with alternatives. I note your opinion based - presumably - on
> > appearance, though. :-)
> Indeed, one can often get a fair idea of the quality of an item, and
> it's likely reliability, from a physical inspection. When you say you
> compared them with alternatives, do you mean alternative thermal
> relays?, or alternative methods of providing a soft-start?
Both. And with the alternative of the kind I chose. Simply modifying the
design so as to make the component redundant.
As per the suggestions I made to Ian - using a diode as a 'one way'
connector, etc, so as to avoid needing any mechanical relays. Why design in
future problems you can avoid?
> >
> >> > And they were one of the main failure modes which brought sets back
> >> > for repair. One of the reasons I avoid physical relays.
> >
> >> But a thermal relay is a totally different animal from a conventional
> >> electromagnetic relay. They have very slow make & break of the
> >> contacts, and the characteristics of the bi-metal strip changes with
> >> age.
> >
> > You may need to think more carefully about the application in the
> > example before assuming that is relevant. You could also check to see
> > if the relay was such that it flipped state with some hysteresis.
> Actually I'm suggesting that *any* thermal relay was inappropriate in
> that application, or leastways used as you used it.
I'm agreeing with you. But also pointing out that experience with this
relay being unreliable was one of the examples I've encountered where
physical relays were an expensive way to design in increased unreliability.
:-)
I also pointed out that I didn't 'use it' that way. I found it was being so
used, and designed it out.
> By the standards of electromagnetic relays contact pressures are low and
> switching times slow (even with hysteresis).
What measurements did you do on the delay unit we are discussing? And how
many of them did you check, and over what period of time?
> When thermal relays were commonly used as delay timers it was normal
> practice to use their contact to operate a conventional relay, which
> then did the real work.
In this application the need was to close a switch when the current level
was generally low. Not to start high currents. Nor to break a current.
I did try other mechanical types/arrangements at the time but decided they
simply weren't worth the cost and reliability worries. So fixed the problem
in another way.
This meant that repairs to old sets actually got cheaper as well since the
offending item was removed/bypassed and new diodes fitted.
> >
> >
> > The particular problem was easily solved simply by using high surge
> > rated diodes. So far as I was concerned any mechanical relay at all
> > was expensive and nasty in such a situation, and easily avoided with
> > far more reliable solid state devices. Hence my mentioning it in this
> > thread.
> >
> Experience with thermal relays is irrelevant when talking about
> electromagnetic relays, the two are chalk and cheese. You might as well
> make judgements on the reliability of polypropylene capacitors in fast
> rise-time pulse circuits based on your experience with electrolytics.
I note your personal opinions. Perhaps you missed some of the things I
wrote. :-)
David Looser
> In article <7prurg...@mid.individual.net>, David Looser
> <david....@btinternet.com> wrote:
>
>> They do not necessarily clash. By what criterion were they "expensive"?
>
> The thermal delay relays used were the order of a couple of quid *each* in
> quantity, back in the 1970s. Compare that with the few pence per diode
> bridge for better diodes that didn't need the delay. Plus of course the
> savings to all involved when the change had a quite marked effect in
> lowering the numbers being returned for repair because the thermal delay
> was unreliable.
You'd expect thermal delay units to be expensive compared to diodes. But a
couple of quid each is cheap for thermal relays.
>
> > What measurements did you do on the delay unit we are discussing? And
> > how
> many of them did you check, and over what period of time?
>
Well clearly I made no measurements. I only had one unit to examine and it
had already failed by the time I examined it. But I do understand how the
things work, and have met a number used in different bits of kit over the
years. So I do know the limitations of the device.
> I note your personal opinions. Perhaps you missed some of the things I
> wrote. :-)
>
I don't think I missed any, though you seem to have missed the point that
making judgments on the reliability of electromagnetic relays based on your
experience of an *entirely different* device seems, at best, hasty!
David.
mick
<snip>
>
> The PIC is an 8 pin DIL, costs a dollar and allows me to light a couple
> of LEDs to show the HT state, handle the cable interlock and drive the
> relay. It needs no external clock components and there's even a spare IO
> line I could use for a push button emergency off switch.
>
<snip>
I tried something similar to this - using a PIC - a while ago, but never
actually put it into practice. The idea was to drive 2 relays. One
switched the HT on and the other shorted out a series resistor in the HT
line after a few seconds to give a soft-on characteristic. A single
button was used to control everything and a 2-colour LED for Off/Standby/
On indication (provisionally). Software was written in GCBASIC just
because it was a quick-n-dirty way to do it! The eventual idea was to
also monitor the cathode current of the output valve & shut down with an
error indication if it got too high. I never got to that bit because I
couldn't arrange a grounded DC supply for the PIC. The PIC is still
floating around somewhere... I like PICs... :-)
You must *never* depend on software for safety interlocks by the way.
It's very bad design.
mick
You're correct that 5v coils are common on PCB relays, but there aren't
all that many PCB relays that could switch the HT line of an amplifier.
350vDC at 200mA can be difficult to break (and make). I originally
suggested plug-in devices because they come ready insulated and are easy
to use (the 2-pole ones fit a standard octal socket). Unfortunately I
don't think any manufacturer makes these with a 5v coil.
Having said all that, I've just found this: Tyco Electronics RT424005
(Farnell 162-9052). It's a 2-pole c/o PCB-mounting relay with a 5vdc
coil. Both n.o. contacts in series would switch about 400mA at 350v
(175vDC each). Using a single n.c. contact for the discharge function
would mean limiting the current to 100mA or so. It's a meaty beast and
it's cheap too! :-) Now, whether Ian would be happy with 350v floating
round these pins is something else.
A note on Isolation: For those considering using a solid-state device to
switch *off* the HT - you can't. SSRs, triacs etc. are not counted as
isolation devices. You should always have at least 2 air gaps, either as
2 "switch-type" contacts in series *on the same relay armature* or a
single "double-break" type contact. You can, of course, switch *on* the
HT using solid-state.
Someone suggested that a contact isn't needed. That's not strictly right
if you are protecting a plug and socket, unless the design is such that
the HT lead breaks cleanly before the earth connection and the HT socket
is suitably touch protected both during and after unplugging. You can
protect the user from a back-feed from the amp using a suitable diode, as
Jim has already mentioned. Protecting from the PSU is a bit harder. I fed
the heater supply back from the amp to the PSU and used that to close the
HT control relay. That way the plug has to be in to get any HT. I
switched that feed off on the amp using a miniature toggle switch (only
switching low voltage & current) to put it into standby.
David Looser
news:006f2ab7$0$21692$c3e...@news.astraweb.com...
> On Sun, 27 Dec 2009 23:16:05 +0000, David Looser wrote:
>
>> "mick" <not....@invalid.invalid> wrote
>>>
>>> By the way, you may not find it too easy getting hold of a suitable
>>> relay with a 5v coil. Manufacturers often list them down to 6v but
>>> nobody stocks them! The nearest that you will get easily will be 12vdc
>>> and the next one up is 24vdc. 5v coils are usually only on small pcb
>>> relays, which don't have enough voltage rating on the contacts, even in
>>> series (if you can ever find a complete data sheet!).
>>>
>>>
>> ISTM that 5V coils are now rather more common than 6V ones, and 5V coils
>> are just as likely to be available on power relays as any other coil
>> voltage. BTW why do you think PCB relays won't have enough voltage
>> rating?, I've just found a relay with a 10kV rating for the contact, and
>> that is a PCB mounting type.
>>
>
>
> You're correct that 5v coils are common on PCB relays,
I didn't say that. I said that 5V was more common than 6V on ALL relays.
> but there aren't
> all that many PCB relays that could switch the HT line of an amplifier.
> 350vDC at 200mA can be difficult to break (and make). I originally
> suggested plug-in devices because they come ready insulated and are easy
> to use (the 2-pole ones fit a standard octal socket). Unfortunately I
> don't think any manufacturer makes these with a 5v coil.
The octal plug in ones mostly seem to use mains voltage coils. But most
relays these days, the vast majority, are PCB mount types, their voltage and
current ratings are well in excess of what might be needed for this
application (and no worse than those of the octal plug-in jobs)
>
> Having said all that, I've just found this: Tyco Electronics RT424005
> (Farnell 162-9052).
Or this one?
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=1986955
>
> Someone suggested that a contact isn't needed.
I suggested that. It isn't, not if you don't unplug the PSU from the load
when it's powered up! IMO by far the best solution is to NOT do that. Either
keep people stupid enough to try doing so away from the kit, or if you can't
do that make sure that the mains has to be disconnected first. Of course
the "PSU" is only the transformer, there is no point in having the rectifier
and reservoir capacitors remote from the load, so as soon as the mains goes
off the PSU output is safe.
David.
Jim Lesurf
<gm...@chapterfive.org.uk> wrote:
> >
> > The above has just reminded me of the joke in "Only Fools and Horses"
> > where Trig (IIRC) was proudly telling everyone how long his broom had
> > lasted. Then to say, "...Of course I have had to replace the head X
> > times... and the handle Y times. But it is remarkable how long that
> > broom has lasted."
> > :-)
> Oh, c'mon! That joke started with George Washington's axe, that he
> allegedly used to fell a cherry tree....
Jings! I'm younger than someone else here. :-)
mick
> "mick" <not....@invalid.invalid> wrote in message
> news:006f2ab7$0$21692$c3e...@news.astraweb.com...
>> On Sun, 27 Dec 2009 23:16:05 +0000, David Looser wrote:
>>
>>> "mick" <not....@invalid.invalid> wrote
>>>>
>>>> By the way, you may not find it too easy getting hold of a suitable
>>>> relay with a 5v coil. Manufacturers often list them down to 6v but
>>>> nobody stocks them! The nearest that you will get easily will be
>>>> 12vdc and the next one up is 24vdc. 5v coils are usually only on
>>>> small pcb relays, which don't have enough voltage rating on the
>>>> contacts, even in series (if you can ever find a complete data
>>>> sheet!).
>>>>
>>>>
>>> ISTM that 5V coils are now rather more common than 6V ones, and 5V
>>> coils are just as likely to be available on power relays as any other
>>> coil voltage. BTW why do you think PCB relays won't have enough
>>> voltage rating?, I've just found a relay with a 10kV rating for the
>>> contact, and that is a PCB mounting type.
>>>
>>>
>>
>> You're correct that 5v coils are common on PCB relays,
>
> I didn't say that. I said that 5V was more common than 6V on ALL relays.
>
Probably 90% of plug-in relays used for general purpose control are 24v
(AC & DC), 110vAC and 220/230vAC. You also find 12vDC used occasionally,
particularly in fire alarm applications. You won't often find 5v relays
with 3 or more contacts either.
>> but there aren't
>> all that many PCB relays that could switch the HT line of an amplifier.
>> 350vDC at 200mA can be difficult to break (and make). I originally
>> suggested plug-in devices because they come ready insulated and are
>> easy to use (the 2-pole ones fit a standard octal socket).
>> Unfortunately I don't think any manufacturer makes these with a 5v
>> coil.
>
> The octal plug in ones mostly seem to use mains voltage coils. But most
> relays these days, the vast majority, are PCB mount types, their voltage
> and current ratings are well in excess of what might be needed for this
> application (and no worse than those of the octal plug-in jobs)
I wouldn't like to say "the vast majority, are PCB mount types". I'd
agree that there is more choice of PCB types though (there are really
only 3 or 4 plug-in base-styles in common use). Likewise I'd disagree
with "their voltage and current ratings are well in excess of what might
be needed for this application (and no worse than those of the octal plug-
in jobs)" as this implies that they are suitable for DC switching above
30vDC, which is almost always not the case. There quite simply isn't
enough air gap between the contacts in most miniature packages. Many of
them are only rated up to 125vAC or less. There's a good reason for that.
Most control circuits used to be 48vDC (mostly telecoms), 110vDC or
240vAC. 48v and 110v were from batteries. Nowadays most control circuits
are at low voltage (24v max) and current, so there is far less need to
use such large relays. Generally, where AC switching is needed, relays
aren't used at all now of course.
>>
>> Having said all that, I've just found this: Tyco Electronics RT424005
>> (Farnell 162-9052).
>
> Or this one?
>
> http://uk.rs-online.com/web/search/searchBrowseAction.html?
method=getProduct&R=1986955
>
That might be ok. Running it with 350v takes it out of it's DC rating,
but you'ld probably get away with it if the load is about 200mA. It does
break the isolation rule of having 2 breaks in series though, as it's
only a single pole relay.
>
>> Someone suggested that a contact isn't needed.
>
> I suggested that. It isn't, not if you don't unplug the PSU from the
> load when it's powered up! IMO by far the best solution is to NOT do
> that. Either keep people stupid enough to try doing so away from the
> kit, or if you can't do that make sure that the mains has to be
> disconnected first. Of course the "PSU" is only the transformer, there
> is no point in having the rectifier and reservoir capacitors remote from
> the load, so as soon as the mains goes off the PSU output is safe.
>
That's fair enough. I'd originally understood (perhaps wrongly) that the
PSU had the HT going through the plug. That's how I built mine because I
wanted a modular PSU that I could use with alternative amplifier chassis.
Perhaps a better way still is to have a bracket holding the plug in. The
bracket has to be removed by undoing a screw. That would be ok even for
"stupid people". :-) It always annoys me that we can install a lump of
live copperwork in a steel cupboard, stick big warning notices all over
the door and yet still have to shroud the copper to IP2x (with more
warning notices) just in case someone is daft enough to ignore the
notices, open the door and stick their hand in without looking...
>>sizzle<<
Jim Lesurf
<david....@btinternet.com> wrote:
> "Jim Lesurf" <no...@audiomisc.co.uk> wrote in message
> news:50d0cbf...@audiomisc.co.uk...
> > In article <7prurg...@mid.individual.net>, David Looser
> > <david....@btinternet.com> wrote:
> >
> >> They do not necessarily clash. By what criterion were they
> >> "expensive"?
> >
> > The thermal delay relays used were the order of a couple of quid
> > *each* in quantity, back in the 1970s. Compare that with the few pence
> > per diode bridge for better diodes that didn't need the delay. Plus of
> > course the savings to all involved when the change had a quite marked
> > effect in lowering the numbers being returned for repair because the
> > thermal delay was unreliable.
> You'd expect thermal delay units to be expensive compared to diodes. But
> a couple of quid each is cheap for thermal relays.
But still expensive when the - far more reliable - alternative is a few
pence for diodes.
> >
> > > What measurements did you do on the delay unit we are discussing?
> > > And how
> > many of them did you check, and over what period of time?
> >
> Well clearly I made no measurements. I only had one unit to examine and
> it had already failed by the time I examined it. But I do understand
> how the things work, and have met a number used in different bits of
> kit over the years.
> So I do know the limitations of the device.
Actually, no, you don't. You saw one and from your own admission judged it
simply on appearance and your general theories about the class of such
devices.
Whereas I at the time had experience of many of the specific devices *and*
did tests to compare them with alternatives, of a range of types/methods.
However I agree with you that such devices do tend to be far less reliable
than designing them out of the system. That is my experimental experience
for a range of types of mechanical switches. Mostly conventional relays,
but also thermal and other forms of sensor driven types. Hence my quoting
the delay as an example of a device relying on mechanical contacts that was
unreliable - and easily replaced by something cheaper and better.
> > I note your personal opinions. Perhaps you missed some of the things I
> > wrote. :-)
> >
> I don't think I missed any, though you seem to have missed the point
> that making judgments on the reliability of electromagnetic relays
> based on your experience of an *entirely different* device seems, at
> best, hasty!
I agree. Hence my noting that you made no measurements on the specific
device I used as an example, and just judged it on the basis of appearance
and your own general opinions. :-)
You may however have still missed the point that I have also experience
with conventional electrically operated relays in a range of applications.
So I'll stay with my views based on a mix of my experimental experience and
having tried various alternatives. if you prefer electromechanical switches
to solid state alternatives you are welcome to do so. :-)
David Looser
>
> Probably 90% of plug-in relays used for general purpose control are 24v
> (AC & DC), 110vAC and 220/230vAC. You also find 12vDC used occasionally,
> particularly in fire alarm applications. You won't often find 5v relays
> with 3 or more contacts either.
>
Most relays don't plug in, though clearly they are the type you are most
familiar with.
>
> I wouldn't like to say "the vast majority, are PCB mount types".
If you look in the catalogues you'll find many PCB relays for every plug-in
one..
> Likewise I'd disagree
> with "their voltage and current ratings are well in excess of what might
> be needed for this application
Yes, I was wrong to say voltage. Few currently available GP relays have an
adequate voltage rating for this purpose. However PCB relays are no worse
than plug-in types in this respect.
>
> That might be ok. Running it with 350v takes it out of it's DC rating,
> but you'ld probably get away with it if the load is about 200mA. It does
> break the isolation rule of having 2 breaks in series though, as it's
> only a single pole relay.
>
Is it single pole?, I didn't read the write up very carefully I'm afraid. I
just noticed that it had a 440V ac rating, wheras most relays have only a
250V ac rating.
> >
> That's fair enough. I'd originally understood (perhaps wrongly) that the
> PSU had the HT going through the plug.
That was my understanding too. But from a safety POV it's the wrong approach
IMO.
> That's how I built mine because I
> wanted a modular PSU that I could use with alternative amplifier chassis.
>
> Perhaps a better way still is to have a bracket holding the plug in. The
> bracket has to be removed by undoing a screw. That would be ok even for
> "stupid people". :-) It always annoys me that we can install a lump of
> live copperwork in a steel cupboard, stick big warning notices all over
> the door and yet still have to shroud the copper to IP2x (with more
> warning notices) just in case someone is daft enough to ignore the
> notices, open the door and stick their hand in without looking...
>>>sizzle<<
>
Of course if you are building an amp and separate PSU for your own use you
don't *have* to do anything in particular in that respect. Just bear in mind
that you are responsible if anybody gets hurt.
David.
David Looser
>
>> So I do know the limitations of the device.
>
> Actually, no, you don't. You saw one and from your own admission judged it
> simply on appearance and your general theories about the class of such
> devices.
There's something odd going on here. :-) *You* can group all
electromechanical devices together and damn the lot of them, but if I try to
talk about a class of devices known as "thermal relays" you instantly try to
restrict the discussion to the particular unit that you used on the 600. I
wonder why that is? Just to make it clear, by "the device" I mean the device
known as a thermal relay, not specifically the particular one you chose to
use.
> However I agree with you that such devices do tend to be far less reliable
> than designing them out of the system.
Designing out any unecessary complexity improves reliability, and that
includes semiconductor devices. High-voltage semiconductors, including SSRs,
do not have a particularly good reliability record.
>> >
>> I don't think I missed any, though you seem to have missed the point
>> that making judgments on the reliability of electromagnetic relays
>> based on your experience of an *entirely different* device seems, at
>> best, hasty!
>
> I agree. Hence my noting that you made no measurements on the specific
> device I used as an example, and just judged it on the basis of appearance
> and your own general opinions. :-)
You are so keen to keep coming back to my judgement of that specific device
aren't you? I have extensive experience of relays and their uses. Whilst
electromagnetic relays can be very reliable the thermal relay isn't (That's
thermal relays in general, not just your specific one!)
>
> You may however have still missed the point that I have also experience
> with conventional electrically operated relays in a range of applications.
>
And so do I!
> So I'll stay with my views based on a mix of my experimental experience
> and
> having tried various alternatives. if you prefer electromechanical
> switches
> to solid state alternatives you are welcome to do so. :-)
>
That entirely depends on the application. For signal switching, particularly
low-level unwetted analogue audio/video I'd usually prefer electronic
switching. Loudspeaker relays in high-powered amps can be a problem too
because the contact material needs to be able to handle high currents yet
also work well with very low signal currents. But then SSRs are no use in
that application either.
But the particular virtues of the relay - very low on resistance,
negligable off-state leakage, ability to offer a wide range of contact types
and number, galvanic isolation between coil and contacts etc. can make it a
very attractive option in many applications.
At it's peak in the 1960s the Strowger-based PSTN network in the UK had in
excess of 10 million relays in use, and by then many of them were over 30
years old. Several hundred might well be involved in any one call. Unlike
selector mechanisms relays generally did not receive routine maintenance,
nor were they (or selectors for that matter) replaced as a standard repair
technique. Individual relays were wired into relay sets and replacing them
was a fiddly job and rarely done. When exchanges were retired after a
typical life of 40 years almost all of the original relays and selectors
would still be in use. Yet relays were rarely the cause of faults, selector
mechanisms, contact banks, selector jacks or indeed shelf wiring were in
fact the usual cause of faults in Strowger exchanges. Of course these were
mostly PO3000 type relays, bulky and expensive by modern standards, but
very reliable.
David.
John Stumbles
> I am trying to select a relay for a delayed HT switch (which will also
> discharge the HT when off). Most relays I can find have contacts rated
> at 250VAC which translates into a peak of about 350V. However, data is
> scarce on what dc voltage these relays can switch. So far I have found
> only one that gives a dc current versus voltage curve and that stops at
> 210V dc (and 200mA) and I really want to be able to switch up to 350V at
> up to 200mA. The rest just give a dc voltage at max current value.
The reason DC switching ratings are lower than AC ratings is that, when
breaking a circuit, AC limits the duration of arcing as the current
through the contacts drops to zero twice a cycle thus helping to
extinguish the arc. With DC the contacts have to open wide enough to
extinguish the arc on their own.
You say:
> The PSU is remote and the relay has an interlock to turn off the HT if
> the PSU HT output lead is disconnected. I need to either disconnect the
> HT or bleed it very quickly to avoid a possible shock hazard.
I assume we have the following setup in the PSU:
relay
HT+
to <-------------o COM
LOAD /
o o----------- HT+ SUPPLY
NC | NO
|
-----
| R | <- discharge resistor
-----
|
GROUND <-----------+---------------- GROUND
The relay contacts change from NC to NO a little while after mains has
been applied to the PSU (allowing valves time to warm up) and from NO
back to NC when either the system is switched off or the HT output lead
is disconnected.
In the first instance there is no current flowing until the contacts
close, so no arcing to bother with and I wouldn't worry about the relay's
DC switching rating being lower than the HT voltage: as long as the relay
is capable of holding off the voltage when the contacts are open and no
current is flowing (and of handling the expected supply current) it
should be OK.
In the second and third cases the HT current will try to arc across from
the NO to the COM contact until COM closes on NC at which point the
discharge resistor should attenuate the voltage from the supply and
extinguish the arc. In the second case - when the power to the PSU is
switched off normally - if you can allow a certain delay before operating
the relay then most the the HT energy should have been dissipated in the
load electronics, so the relay should have little current-breaking work
to do and the contacts would not be strained.
The third case - when the HT lead is disconnected - seems to pose the
most stressful conditions. The interlock that operates the relay must
also cut mains power into the PSU, and it is then a case of whether the
relay can handle the energy stored in the transformer and smoothing
capacitor(s) at the moment of switching. However this situation should be
relatively infrequent so as long as the relay contacts don't melt down
instantaneously it shouldn't be a problem.
I think your plan to choose a relay with contacts rated at higher current
than that you expect to switch is good: the relay will in any case need
to be rated to supply the current from the supply (with fully charged HT
smoothing capacitor(s)) into the discharge resistor. A bit of suck it and
see with deliberate disconnection of the PSU from the load and
observation of the relay as it operates (assuming it has a transparent
cover) would be the order of the day.
--
John Stumbles
When I die, I want to go peacefully in my sleep like my father did,
not screaming in terror like his passengers.
Ian Bell
> On Thu, 24 Dec 2009 14:43:30 +0000, Ian Bell wrote:
>
>> I am trying to select a relay for a delayed HT switch (which will also
>> discharge the HT when off). Most relays I can find have contacts rated
>> at 250VAC which translates into a peak of about 350V. However, data is
>> scarce on what dc voltage these relays can switch. So far I have found
>> only one that gives a dc current versus voltage curve and that stops at
>> 210V dc (and 200mA) and I really want to be able to switch up to 350V at
>> up to 200mA. The rest just give a dc voltage at max current value.
>
> The reason DC switching ratings are lower than AC ratings is that, when
> breaking a circuit, AC limits the duration of arcing as the current
> through the contacts drops to zero twice a cycle thus helping to
> extinguish the arc. With DC the contacts have to open wide enough to
> extinguish the arc on their own.
>
Indeed, I have found a very informative application note by Tyco that
explains wvery well what happens when contacts make and when they break.
> You say:
>
>> The PSU is remote and the relay has an interlock to turn off the HT if
>> the PSU HT output lead is disconnected. I need to either disconnect the
>> HT or bleed it very quickly to avoid a possible shock hazard.
>
> I assume we have the following setup in the PSU:
>
> relay
> HT+
> to <-------------o COM
> LOAD /
> o o----------- HT+ SUPPLY
> NC | NO
> |
> -----
> | R | <- discharge resistor
> -----
> |
> GROUND <-----------+---------------- GROUND
Not quite, swap the LOAD and SUPPLY and it is correct - in other words
the HT supply goes to the common and is switched either to the load or
the discharge resistor because nearly all the charge storage is in the
HT supply not the load.
That somewhat negates the discussion below (which is still useful) so
perhaps you would be kind enough to repeat it with the actual circuit??
Cheers
Ian
mick
Nice ASCII art John!
<snip>
>>
>> relay
>> HT+
>> to <-------------o COM
>> LOAD /
>> o o----------- HT+ SUPPLY
>> NC | NO
>> |
>> -----
>> | R | <- discharge resistor
>> -----
>> |
>> GROUND <-----------+---------------- GROUND
>
> Not quite, swap the LOAD and SUPPLY and it is correct - in other words
> the HT supply goes to the common and is switched either to the load or
> the discharge resistor because nearly all the charge storage is in the
> HT supply not the load.
>
Ah. Just as I thought.
As David has pointed out though, moving the rectifier(s) and reservoir
caps etc. onto the amp chassis removes most of the problem. It has
several advantages:
You don't need a relay.
Only AC goes through the plug & socket.
The rectifier protects the socket against a back feed from the HT.
The plug & socket are "dead" as soon as the mains is switched off.
All you need to do is to make sure that the plug & socket can't be
disconnected while the mains is on, or rig up something so that a tool is
needed.
Incidentally, I used a flying lead from the PSU and a socket on the amp.
Fixed socket: RS 487-378
Cable plug: RS 487-384
These have a wider pin spacing than octal, so I didn't have any qualms
about using them, even though Bulgin don't give a DC rating for them.
The HT was switched by a relay as you described above, with an electrical
interlock so that the relay couldn't pull in until the plug & socket had
made.
Ian Bell
It also has some serious drawbacks.
1. Some god awful current spikes going up and down a longish cable
radiating crap all over the place.
2. AC inside a box where I am trying to keep it out by having an
external PSU
> Incidentally, I used a flying lead from the PSU and a socket on the amp.
> Fixed socket: RS 487-378
> Cable plug: RS 487-384
> These have a wider pin spacing than octal, so I didn't have any qualms
> about using them, even though Bulgin don't give a DC rating for them.
> The HT was switched by a relay as you described above, with an electrical
> interlock so that the relay couldn't pull in until the plug & socket had
> made.
>
Funnily enough, those are precisely what I was planning to use and I
even bought a few pairs. Trouble is they do not lock and can very easily
be pulled out.
I am now planning on using an 8 way Speakon locking connector - a tad
more expensive but you won't be able to pull it out.
Cheers
Ian
mick
<snip>
> It also has some serious drawbacks.
>
> 1. Some god awful current spikes going up and down a longish cable
> radiating crap all over the place.
>
I hadn't considered that - good point.
> 2. AC inside a box where I am trying to keep it out by having an
> external PSU
>
That makes sense too.
>> Incidentally, I used a flying lead from the PSU and a socket on the
>> amp. Fixed socket: RS 487-378
>> Cable plug: RS 487-384
>> These have a wider pin spacing than octal, so I didn't have any qualms
>> about using them, even though Bulgin don't give a DC rating for them.
>> The HT was switched by a relay as you described above, with an
>> electrical interlock so that the relay couldn't pull in until the plug
>> & socket had made.
>>
>>
> Funnily enough, those are precisely what I was planning to use and I
> even bought a few pairs. Trouble is they do not lock and can very easily
> be pulled out.
>
> I am now planning on using an 8 way Speakon locking connector - a tad
> more expensive but you won't be able to pull it out.
>
I just relied on the fact that I was going to be in the room while it was
switched on. :-) At one point I even had bare HT terminals on the OPTs -
just for the melodrama, you understand... "It's got lit valves - dare I
touch it?" :-D I put the connector on top of the chassis, with the
umbilical going off backwards. It's easy to see and doesn't pull out
easily at all, as it requires pulling the plug upward. I wonder if you
could take out the centre pin and fit a retaining screw in that position
somehow? That would be neat.
The Speakons would be fine, but you can still undo them without a tool.
David Looser
>
> It also has some serious drawbacks.
>
> 1. Some god awful current spikes going up and down a longish cable
> radiating crap all over the place.
There shouldn't be "god awful current spikes" unless the conduction angle is
excessively short, which isn't good for the transformer, the rectifiers or
the capacitors. Since both the a.c. conductors are, presumably, co-routed,
the magnetic fields will very largely cancel anyway.
>
> 2. AC inside a box where I am trying to keep it out by having an external
> PSU
>
Are you using a.c. for the heaters? that has to go right up and into to the
valves, whilst the a.c. for the HT need go nowhere near them or any audio
wiring.
The reasons for keeping the transformer separate are to lose it's
weight/bulk and to remove leakage magnetic fields from the amp.
David.
Ian Bell
Yes, Speakons have a little slide thingy on top to release them so you
can twist and release them. It is not easy to do (good) but it can be
done (good).
When I was a lot younger (40+ years ago) I had built so many valve
projects that I was just about immune to 300V dc shocks. However, on day
our old B&W TV broke down so I took the back off to have a look.
Unfortunately the EHT had not completely discharged and I got a 12KV
belt that shot me across the room.
Cheers
ian
Ian Bell
> "Ian Bell" <ruffr...@yahoo.com> wrote
>> It also has some serious drawbacks.
>>
>> 1. Some god awful current spikes going up and down a longish cable
>> radiating crap all over the place.
>
> There shouldn't be "god awful current spikes" unless the conduction angle is
> excessively short, which isn't good for the transformer, the rectifiers or
> the capacitors. Since both the a.c. conductors are, presumably, co-routed,
> the magnetic fields will very largely cancel anyway.
Well, if you aim initially for 10% ripple then that will be the
conduction angle to a first approximation and the current spikes will be
10 times the load current.
>> 2. AC inside a box where I am trying to keep it out by having an external
>> PSU
>>
> Are you using a.c. for the heaters? that has to go right up and into to the
> valves, whilst the a.c. for the HT need go nowhere near them or any audio
> wiring.
>
No, the heaters are dc.
> The reasons for keeping the transformer separate are to lose it's
> weight/bulk and to remove leakage magnetic fields from the amp.
That's a couple of them, yes.
Cheers
ian
>
> David.
>
>
David Looser
news:hhkrbt$evm$2...@localhost.localdomain...
> David Looser wrote:
>>
>> There shouldn't be "god awful current spikes" unless the conduction angle
>> is excessively short, which isn't good for the transformer, the
>> rectifiers or the capacitors. Since both the a.c. conductors are,
>> presumably, co-routed, the magnetic fields will very largely cancel
>> anyway.
>
> Well, if you aim initially for 10% ripple then that will be the conduction
> angle to a first approximation and the current spikes will be 10 times the
> load current.
>
You were talking about a 200mA DC load, so how big are these spikes then? 3A
peak maybe?. hardly sounds like "god awful" to me. And you can use a twisted
pair for the ac so that the magnetic fields cancel out. I presume you are
not running the power cable close to the audio wiring? Are you using a
low-impedance balanced audio input to this amp of yours? if not I suggest
that doing that will make far more difference to your hum levels than
worrying about ac in the power cable.
David.
John Stumbles
Could you not arrange it as above? I mean that all the above components
are in the PSU box and the node marked 'HT+ to LOAD' is the pole on the
output connector of the PSU to which the amplifier/load is connected. Thus
as soon as the relay falls back to its rest position the output lead is
connected via the discharge resistor to ground (discharging any residual
capacitance in the lead electronics) and the internal HT smoothing
capacitors within the PSU can be allowed to discharge under their own
steam, or with a small, high-value bleed resistor.
Doesn't that do what you want?
--
John Stumbles
Procrastinate now!
John Stumbles
> Legacy crowbar - fire a thyratron.
If you can guarantee 99.9999% that it will fire whenever you want it to.
BTDTGTTS
--
John Stumbles
Men are from Mars, Women are from Venus and Pop Psychologists are from Uranus
Arny Krueger
message news:7qhlbs...@mid.individual.net
> On Fri, 25 Dec 2009 09:03:41 -0500, Arny Krueger wrote:
>
>> Legacy crowbar - fire a thyratron.
>
> If you can guarantee 99.9999% that it will fire whenever
> you want it to.
>
> BTDTGTTS
If its made with tubes, we can't even guarantee that the entire piece of
equipment will work 99.9999% of the time.
IOW the criteria of 99.9999% is unrealistically high.
Ian Bell
All those components will be in the power supply and the connection to
the electronics made as you state.
Thus
> as soon as the relay falls back to its rest position the output lead is
> connected via the discharge resistor to ground (discharging any residual
> capacitance in the lead electronics) and the internal HT smoothing
> capacitors within the PSU can be allowed to discharge under their own
> steam, or with a small, high-value bleed resistor.
>
> Doesn't that do what you want?
>
Pretty much but I was trying to avoid having a bleed resistor
permanently across the supply. The PSU has a lot of stored charge and a
bleed resistor that takes only a nominal 1% of the load current would
take about 5 minutes to discharge it. That in itself is not a problem as
the PSU is in an enclosure but when I am testing it, it is a pain to
have to wait that long.
Cheers
Ian
John Stumbles
> Pretty much but I was trying to avoid having a bleed resistor
> permanently across the supply. The PSU has a lot of stored charge and a
> bleed resistor that takes only a nominal 1% of the load current would
> take about 5 minutes to discharge it. That in itself is not a problem as
> the PSU is in an enclosure but when I am testing it, it is a pain to
> have to wait that long.
You seem to be changing the design criteria. As I understood it the design
brief (if you like) was that the connections on the outside of the box had
to assume safe potentials in the time it took a British Standard Idiot to
unplug the connector from the power amp and plug his finger in there
instead. Now you seem to be saying you want the potentials within the
PSU box to drop to safe levels in some arbitrarily short time so you can
stick your finger on conductors inside the PSU box without harm. That's a
different problem. I think what I was proposing addresses the original
problem, but if you want to make it safe for yourself to prod around in
the PSU arbitrarily I suspect that's going to be a non-trivial problem to
solve.
--
John Stumbles
Time flies like an arrow
Fruit flies like a banana
Tits like coconuts
Jim Lesurf
> > Pretty much but I was trying to avoid having a bleed resistor
> > permanently across the supply. The PSU has a lot of stored charge and
> > a bleed resistor that takes only a nominal 1% of the load current
> > would take about 5 minutes to discharge it. That in itself is not a
> > problem as the PSU is in an enclosure but when I am testing it, it is
> > a pain to have to wait that long.
> That's a different problem. I think what I was proposing
> addresses the original problem, but if you want to make it safe for
> yourself to prod around in the PSU arbitrarily I suspect that's going to
> be a non-trivial problem to solve.
The simplest solution is to have an additional, low value, bleed resistor
soldered across the caps during the period of development. Then clip them
off when finished, check the behaviour is still OK, and stop.
But as you say, your situation as someone developing and testing a circuit
is quite different to that of the end user. So the primary requirement is
to take due care and use whatever extra safety methods seem relevant during
development.
David Looser
>
> Pretty much but I was trying to avoid having a bleed resistor permanently
> across the supply. The PSU has a lot of stored charge and a bleed resistor
> that takes only a nominal 1% of the load current would take about 5
> minutes to discharge it. That in itself is not a problem as the PSU is in
> an enclosure but when I am testing it, it is a pain to have to wait that
> long.
>
If you switch off the supply to the HT rectifier before the heaters go off
then the load current itself will discharge the smoothing capacitors.
David.
Ian Bell
I have not changed the brief. Originally it was simply to delay the
switch on of HT until the heaters had warmed up; that and nothing more.
It then occurred to me that when the relay dropped out a contact could
be used to discharge the PSU caps more rapidly than a bleed resistor. It
also occurred to me that I could include the relay coil in a loop in the
dc power cable so if it were disconnected the HT would be disconnected too.
The delay of the HT until heaters warm up is the requirement. The other
'features' came out as possibilities in using a relay to meet the
requirement.
Hope that clarifies things.
Cheers
ian
Ian Bell
Normally that would be the case but this power supply has a lot more
capacitive smoothing than the average and there is still significant
stored charge once the heaters have cooled.
Cheers
Ian
> David.
>
>
David Looser
I think you missed my point. Switch off the supply to the HT rectifier
*before* you switch off the heaters. You simply leave the heaters on until
the HT capacitors have discharged. Having separate switches for LT and HT
also has the distinct advantage that you can leave the heaters on whilst
making modifications etc. No need to wait for the valves to warm up again
for the next test.
I'm currently restoring a mid '50s vintage broadcast TV transmitter. It has
a 4-stage power-up sequence:-
1/ Valve heaters and cooling fans,
2/ Auxiliary (350V and 560V) HT and bias supplies
3/ Main HT low
4/ Main HT full (1750V)
(Low main HT is obtained by connecting a bloody great resistor in series
with the primary of the Main HT transformer!)
The principle reason for this sequence, of course, was to ensure that valves
were fully up to temperature and that bias supplies were present and correct
before HT was applied during normal service switch-on, but being able to
switch off the HT supplies without having to switch off the heaters whilst
making adjustments is a real boon.
David.
Jim Lesurf
Must admit to being surprised by that. How long do the cathodes take to
cool down?
Ian Bell
> "Ian Bell" <ruffr...@yahoo.com> wrote in message
> news:hi4e2t$gpr$2...@localhost.localdomain...
>> David Looser wrote:
>>> "Ian Bell" <ruffr...@yahoo.com> wrote in message
>>> news:hi2f97$umm$1...@localhost.localdomain...
>>>> Pretty much but I was trying to avoid having a bleed resistor
>>>> permanently across the supply. The PSU has a lot of stored charge and a
>>>> bleed resistor that takes only a nominal 1% of the load current would
>>>> take about 5 minutes to discharge it. That in itself is not a problem as
>>>> the PSU is in an enclosure but when I am testing it, it is a pain to
>>>> have to wait that long.
>>>>
>>> If you switch off the supply to the HT rectifier before the heaters go
>>> off then the load current itself will discharge the smoothing capacitors.
>>>
>>
>> Normally that would be the case but this power supply has a lot more
>> capacitive smoothing than the average and there is still significant
>> stored charge once the heaters have cooled.
>>
>
> I think you missed my point. Switch off the supply to the HT rectifier
> *before* you switch off the heaters. You simply leave the heaters on until
> the HT capacitors have discharged. Having separate switches for LT and HT
> also has the distinct advantage that you can leave the heaters on whilst
> making modifications etc. No need to wait for the valves to warm up again
> for the next test.
>
You are right, I had misunderstood. I now see what you meant.
> I'm currently restoring a mid '50s vintage broadcast TV transmitter. It has
> a 4-stage power-up sequence:-
>
> 1/ Valve heaters and cooling fans,
> 2/ Auxiliary (350V and 560V) HT and bias supplies
> 3/ Main HT low
> 4/ Main HT full (1750V)
>
> (Low main HT is obtained by connecting a bloody great resistor in series
> with the primary of the Main HT transformer!)
>
I think I have seen that used elsewhere with a relay across the resistor.
> The principle reason for this sequence, of course, was to ensure that valves
> were fully up to temperature and that bias supplies were present and correct
> before HT was applied during normal service switch-on, but being able to
> switch off the HT supplies without having to switch off the heaters whilst
> making adjustments is a real boon.
And what do they use to switch the HT?
Cheers
ian
>
> David.
>
>
Ian Bell
> In article <hi4e2t$gpr$2...@localhost.localdomain>, Ian Bell
> <ruffr...@yahoo.com> wrote:
>> David Looser wrote:
>>> "Ian Bell" <ruffr...@yahoo.com> wrote in message
>>> news:hi2f97$umm$1...@localhost.localdomain...
>>>> Pretty much but I was trying to avoid having a bleed resistor
>>>> permanently across the supply. The PSU has a lot of stored charge
>>>> and a bleed resistor that takes only a nominal 1% of the load
>>>> current would take about 5 minutes to discharge it. That in itself
>>>> is not a problem as the PSU is in an enclosure but when I am testing
>>>> it, it is a pain to have to wait that long.
>>>>
>>> If you switch off the supply to the HT rectifier before the heaters go
>>> off then the load current itself will discharge the smoothing
>>> capacitors.
>>>
>
>
>> Normally that would be the case but this power supply has a lot more
>> capacitive smoothing than the average and there is still significant
>> stored charge once the heaters have cooled.
>
> Must admit to being surprised by that. How long do the cathodes take to
> cool down?
>
30 seconds or thereabouts.
Cheers
Ian
> Slainte,
>
> Jim
>
Jim Lesurf
<ruffr...@yahoo.com> wrote:
> Jim Lesurf wrote:
> > In article <hi4e2t$gpr$2...@localhost.localdomain>, Ian Bell
> >
> >> Normally that would be the case but this power supply has a lot more
> >> capacitive smoothing than the average and there is still significant
> >> stored charge once the heaters have cooled.
> >
> > Must admit to being surprised by that. How long do the cathodes take
> > to cool down?
> >
> 30 seconds or thereabouts.
OK, that sounds reasonably long. So it makes me curious as to what fraction
of the cap voltage remains after 30 sec or 1min. And why you need so much
capacitance, I guess...
FWIW One of the advantages of 'capacity multiplier' PSU methods is that
the apparent working capacitance isn't the real storage. So although
you get the multiplied value for smoothing and output impedance
purposes when powered, the supply dies quickly when switched off as
the charge is soon sucked out when the multiplier effect is lost.
That said, if you are still a fan of relays you could use another
to drop another bleed resistor across when the input rectified
power vanishes. So cutting down the HT quickly when the mains
is removed.
David Looser
>
> And what do they use to switch the HT?
>
The switching is all done in the primaries of the various mains
transformers, of which there are about 20 altogether. As originally designed
in the 1950s manual switches were used for steps 1, 2 and 4, and a relay
operated by a push-button for step 3. The relay could not be operated until
the bias supplies were present, and would be released if the main HT current
exceed a set level. Then in the late 1960s it was modified to allow the
transmitter to be controlled from a remote location.
In the modified system relays are used for all 4 steps and a logic board
using Mullard "Norbit" logic modules was added. In 'remote' operation this
steps through the four steps in turn allowing sufficient time between each
step for valves to warm up and H.T. supplies to settle in response to a
remote 'on' command.
In local operation a rotary switch enables each step in turn, though the
logic will not allow the relays to be operated before a minimum time has
elapsed since the step before. Again step 3 is conditional on bias supplies
being present, but the HT overload system is more complex than before. An
overload sets the logic back to step 2 for three seconds, after which steps
3 and then 4 are enabled as normal. If, however, three overloads occur
within a space of 30 seconds the transmitter is powered down completely and
requires manual intervention to restart it.
David.
Ian Bell
> In article <hi5178$me3$2...@localhost.localdomain>, Ian Bell
> <ruffr...@yahoo.com> wrote:
>> Jim Lesurf wrote:
>>> In article <hi4e2t$gpr$2...@localhost.localdomain>, Ian Bell
>
>>>> Normally that would be the case but this power supply has a lot more
>>>> capacitive smoothing than the average and there is still significant
>>>> stored charge once the heaters have cooled.
>>> Must admit to being surprised by that. How long do the cathodes take
>>> to cool down?
>>>
>
>> 30 seconds or thereabouts.
>
> OK, that sounds reasonably long.
Basically if you drive the circuit fairly hard (20V rms output),leave
the HT on and disconnect power to the heaters, the signal remains at the
set level for nearly 10 seconds and then gradually decays over the next 20.
So it makes me curious as to what fraction
> of the cap voltage remains after 30 sec or 1min. And why you need so much
> capacitance, I guess...
>
The time constant of a 1% load bleed resistor is about 5 minutes. There
is a total nearly 2000uF of smoothing capacitance. Yes it is a lot and
may be more than I need. The application is an all valve mixer which
essentially contains 40 class A stages each drawing about 5 mA at 350V
i.e about 70W. As many of these are sensitive microphone pre amps I
cannot afford the kind of ripple you would tolerate on the HT supply of
a 50W push pull power amp for example.
I am using a smoothing technique first described by Scroggie around 60
years ago where you replace a single R C filter with N stages in series
each of R/N and C/N so you have the same series resistance (and voltage
drop) and total capacitance. However, you get orders of magnitude
improvement in ripple reduction. The four resistors and four 470uF caps
I am using give 60dB more ripple reduction than a single resistor and
2000uF capacitance. In total I get nearly 120dB of 100Hz attenuation
which means 10V ripple at the input capacitor is 10uV on the HT line. As
the pre amp stages have a PSRR of about 20dB there should be about 1uV
residual 100Hz hum at the stage output i.e -120dBV which is comfortably
below the pre amps own output noise of around -85dBV. I could get away
with the ripple being an order of magnitude higher but that translates
only into a small change in capacitance.
> FWIW One of the advantages of 'capacity multiplier' PSU methods is that
> the apparent working capacitance isn't the real storage. So although
> you get the multiplied value for smoothing and output impedance
> purposes when powered, the supply dies quickly when switched off as
> the charge is soon sucked out when the multiplier effect is lost.
>
> That said, if you are still a fan of relays you could use another
> to drop another bleed resistor across when the input rectified
> power vanishes. So cutting down the HT quickly when the mains
> is removed.
>
That was pretty much the plan - the NC contact of the relay connects to
a bleed resistor equivalent to say 20% of the load, the NO contact is
the output and the common goes to the supply.
Cheers
Ian
> Slainte,
>
> Jim
>
John Stumbles
> I have not changed the brief. Originally it was simply to delay the
> switch on of HT until the heaters had warmed up; that and nothing more.
> It then occurred to me that when the relay dropped out a contact could
> be used to discharge the PSU caps more rapidly than a bleed resistor. It
> also occurred to me that I could include the relay coil in a loop in the
> dc power cable so if it were disconnected the HT would be disconnected too.
With respect, Ian, although your original post talked about discharging
the HT, later the same day you said "The PSU is remote and the relay has an
interlock to turn off the HT if the PSU HT output lead is disconnected. I
need to either disconnect the HT or bleed it very quickly to avoid a
possible shock hazard." What I proposed would seem to satisfy the
requirement by implementing the first option: "disconnect the HT".
--
John Stumbles
I am neither for nor against apathy
John Stumbles
> I am using a smoothing technique first described by Scroggie
Not in Foundations of Wireless, IIRC. Second Thoughts, perhaps? I never
really got my brain round that one as well as I did the former (which was
a pressie to me when I was a spotty 'erbert from an uncle who worked in the
beeb).
> That was pretty much the plan - the NC contact of the relay connects to
> a bleed resistor equivalent to say 20% of the load, the NO contact is
> the output and the common goes to the supply.
Well even with that arrangement any arc drawn as the contacts interrupt
the 'normal' load current would then have somewhere to go - into the bleed
resistance - so shouldn't be as bad for them as trying to break the
current to nothing. Why would the bleed resistor be sized to draw only 20%
of the load current? Why not all of it? (assuming the mains input to the
PSU can be guaranteed to be off at this point and so the bleed R isn't
being asked to dissipate the 70W load continuously).
--
John Stumbles
Never believe anyone who claims to be a liar
Ian Bell
Yes, you are quite right. My thoughts were developing during that day.
IIRC someone said 'why not switch the ac' and perhaps I should have said
I would like the 'option' to disconnect or quickly bleed the HT - just
trying to keep as many design option doors open as I can.
My primary concern was, and still is, dc switching of high voltages with
relays. The specs as published gave me cause for concern, hence my
original post. As a result of several replies, yours included, and
reading some articles by relay manufacturers, it is clear to me that
managing the voltage across the contacts, especially when breaking them,
is the key.
Many thanks for your input.
Cheers
Ian
Ian Bell
> On Thu, 07 Jan 2010 21:07:00 +0000, Ian Bell wrote:
>> I am using a smoothing technique first described by Scroggie
>
> Not in Foundations of Wireless, IIRC. Second Thoughts, perhaps? I never
> really got my brain round that one as well as I did the former (which was
> a pressie to me when I was a spotty 'erbert from an uncle who worked in the
> beeb).
>
I think I have in in a Wireless World article form sometime during or
soon after WWII.
Ian Bell
> On Thu, 07 Jan 2010 21:07:00 +0000, Ian Bell wrote:
>> I am using a smoothing technique first described by Scroggie
>
> Not in Foundations of Wireless, IIRC. Second Thoughts, perhaps? I never
> really got my brain round that one as well as I did the former (which was
> a pressie to me when I was a spotty 'erbert from an uncle who worked in the
> beeb).
>
I think I have it in a Wireless World from during or shortly after WWII.
>
>> That was pretty much the plan - the NC contact of the relay connects to
>> a bleed resistor equivalent to say 20% of the load, the NO contact is
>> the output and the common goes to the supply.
>
> Well even with that arrangement any arc drawn as the contacts interrupt
> the 'normal' load current would then have somewhere to go - into the bleed
> resistance - so shouldn't be as bad for them as trying to break the
> current to nothing. Why would the bleed resistor be sized to draw only 20%
> of the load current? Why not all of it? (assuming the mains input to the
> PSU can be guaranteed to be off at this point and so the bleed R isn't
> being asked to dissipate the 70W load continuously).
>
>
Whoops, sent the last reply without finishing.
Because , if I use an interlock to detect if someone removes the dc
cable then the bleed resistor could be asked to dissipate 70W continuously.
Cheers
Ian
David Looser
>
> Because , if I use an interlock to detect if someone removes the dc cable
> then the bleed resistor could be asked to dissipate 70W continuously.
>
Though if the relay also disconnects the a.c. it won't.
I recommend adding a snubber - say 100nF + 100R. Actually you'll want two,
one from each of the NO and NC contacts.
David.
Ian Bell
> "Ian Bell" <ruffr...@yahoo.com> wrote
>> Because , if I use an interlock to detect if someone removes the dc cable
>> then the bleed resistor could be asked to dissipate 70W continuously.
>>
>
> Though if the relay also disconnects the a.c. it won't.
Indeed, but that requires a separate supply for the relay - which may
not be a problem anyway. Nice idea, thanks.
Cheers
Ian
John Stumbles
> Because , if I use an interlock to detect if someone removes the dc
> cable then the bleed resistor could be asked to dissipate 70W continuously.
Ah, gotcha.
I know, since you're into thermionics how about using one of those
thermionic LEDs as bleed resistor? Cheap, readily available (give or take
recent EU rulings), nice glass enclosure, bright visible indication to
the user that the PSU isn't happy with what they've done ... ;-)
--
John Stumbles
Many hands make light work. Too many cooks spoil the broth.
Ian Bell
> On Fri, 08 Jan 2010 10:43:05 +0000, Ian Bell wrote:
>
>> Because , if I use an interlock to detect if someone removes the dc
>> cable then the bleed resistor could be asked to dissipate 70W continuously.
>
> Ah, gotcha.
>
> I know, since you're into thermionics how about using one of those
> thermionic LEDs as bleed resistor? Cheap, readily available (give or take
> recent EU rulings), nice glass enclosure, bright visible indication to
> the user that the PSU isn't happy with what they've done ... ;-)
>
>
LOL, that's a very good idea! I think I have some very nice candle
shaped ones somewhere.
Cheers
Ian
Jim Lesurf
An additional advantage of using an old fashioned 'light bulb' is that its
resistance reduces as the applied voltage is reduced. So it may actually
have a lower value in normal use to when it is removing the charge as the
cap voltage falls.
Given what you've said previously about the long time required for the
charge to be removed using a bleed resistor that may actually help.
Jim Lesurf
> So it may actually have a lower value in normal use to when it is
> removing the charge as the cap voltage falls.
Oops. Should be "higher value".