Driving a CRT monitor flyback transformer with squarewave
Wiebe Cazemier
I'm working on designing a flyback transformer driver and I have a few
problems. It's been a while since I've been in sci.electronics.* and I've
come to ask for the expertise here again :)
My driver is an Atmega 168 mictrocontroler, driving a IRF840 (500V 8A)
MOSFET through two stages of BC547/BC557 transistors. It drives the flyback
in pure flyback mode (so no resonant circuits with capacitors and such).
My problem is dealing with the back-EMF. Right now, I have 150R 15W
resistance with a UF4007 75 ns diode. With this, not only does it create an
enormous amount of current in the back-EMF ballast when the secondary
winding is open, it also creates high voltage spikes, often destroying my
500V 8A MOSFET. And, I feel that it is severly limiting the output of the
secondary winding. My squarewave is from 0 to 50V 90% dutycycle, yet
breakdown distance of the transformer's output is about 1.5 cm. When the
flyback was still in the CRT, it was about 2 to 2.5 cm, and that is with a
sawtooth drive signal, putting less energy in the core than a squarewave.
In an old style CRT TV, the flyback driver circuit does not just generate
high voltage by collapsing the magnetic field, but it creates a resonance
circuit with the retrace timing capacitor (also called safety capacitor).
This prevents having large back-EMF problems.
My first question is: how does a multiscan CRT monitor (any not-very-old VGA
and better) drive its flyback transformer? These things scan at 30-150 kHz,
so it's kind of difficult to make a resonant circuit with a capacitor,
because that would only work at one frequency.
My second question is dependent on the answer to the first, I guess, namely:
how do I handle the back-EMF efficiently? Can I do it like a CRT monitor
does? I'm going to try 'transorbs' (I've ordered some). These are basically
very fast bipolar zener diodes, designed to suppress transients. Perhaps if
I use a 400V version to shunt the back-EMF (meaning limit it to 400 volts),
there will still be enough energy to give my decent secondary winding
output. I don't know whether I still need resistance in the back-EMF path,
though.
Any insight is welcome :)
whit3rd
> Hi,
>
> I'm working on designing a flyback transformer driver and I have a few
> My driver is an Atmega 168 mictrocontroler, driving a IRF840 (500V 8A)
> MOSFET through two stages of BC547/BC557 transistors. It drives the flyback
> in pure flyback mode (so no resonant circuits with capacitors and such).
>
> My problem is dealing with the back-EMF. Right now, I have 150R 15W
> resistance with a UF4007 75 ns diode. With this, not only does it create an
> enormous amount of current in the back-EMF ballast when the secondary
> winding is open,
I think that's your problem. Don't leave the secondary winding open,
at least put a spark gap (an auto spark plug will do) on the secondary.
cbarn...@aol.com
reduce the duty cycle to 50% max.
Wiebe Cazemier
Most people building these things, mostly based on 555 timers, use
dutycycles of 90%. With 50%, it's impossible to get any kind of spark out of
it.
Wiebe Cazemier
That would be part of the problem. There's still the problem that the back-
EMF suppressor also limits output voltage.
Michael A. Terrell
The flyback isn't used to drive the yoke in multiple 'H' frequency
monitors.
--
You can't fix stupid. You can't even put a Band-Aid™ on it, because it's
Teflon coated.
Wiebe Cazemier
>
> The flyback isn't used to drive the yoke in multiple 'H' frequency
> monitors.
>
>
But the flyback is driven by the horizontal output stage, so the flyback
itself operates at different frequencies. So, what driving method is used
and how is back-EMF handled?
petrus bitbyter
"Wiebe Cazemier" <wi...@halfgaar.net> schreef in bericht
news:4d961ef2$0$309$703f...@news.kpn.nl...
In the old CRT TVs/monitors the LOPT is not driven by a sawtouth, it
produces the sawtouth current for the horizontol yoke. This is mainly done
by tuning the (complex) load so the current in the yoke raises lineair
during the ramp. At the end of the ramp, the primary supply is switched off
by a short pulse to retrace the line. The collapsing field produces a
back-emf which is used to produce the high voltage, roughly 10kV to 25kV.
Though the current provided is small, there's a real load. Often some other,
less high voltages are also produced by the LOPT causing a real load.
Together they prevent the back-emf from rising indefinitely. Nevertheless,
when facing a failing CRT, the line output transistor is a primary suspect.
To protect your switching transistor, you can make an artificial load that
only comes in when the collector voltage becomes too high. See below using a
fixed font.
--->|-----+----+-----+
fast | | |
high | | -
voltage | .-. / zener diode
diode --- | | - p.e. 150V
--- | | A
| '-' -
| | |
| | |
+----++----+
| | |
| | |
| | -
| .-. /
--- | | -
--- | | A
| '-' -
| | |
| | |
+----++----+
| | |
| | |
|
repeat as often
as required.
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
petrus bitbyter
Sylvia Else
It's not clear to me what you're trying to achieve, but if by "decent
output" you mean a high voltage, I think you have a fundamental problem.
In an ideal transformer, all the flux links both windings, and the ratio
of the emfs in the two windings has, at all times, to equal the turn
ratio. So you cannot get a high voltage on the secondary without having
that same voltage divided by the turns ratio on the primary. Real
transformers, being less than ideal, will simply make the situation worse.
With the ballast resistor in place, and the secondary open, the voltage
is going to rise initially to a level equal to the current that was
passing through the primary immediately before switching multiplied by
the ballast resistance. In your case, if you were actually driving 8A,
then the voltage would rise to 1200V, and it would be no surprise if
that killed your transistor.
Putting in a zener diode to protect the transistor just has the effect
of throwing away some of the energy that was stored in the magnetic
field in the transformer, which might as well not have been stored there
in the first place (i.e. by using a lower current).
Sylvia.
cbarn...@aol.com
>
> - Show quoted text -
I thought you were trying to protect the mosfet. The output voltage is
related to the back emf, ie. if you reduce one then you reduce the
other by the same proportion. You also need to worry about spikes
caused by the leakage of the "transformer" you need a clipping circuit
for that, typicaly a diode connected to a high voltage supply/sink
whit3rd
> But the flyback is driven by the horizontal output stage, so the flyback
> itself operates at different frequencies. So, what driving method is used
> and how is back-EMF handled?
There's always a load on the flyback (the rectifier), and the switch
transistor used is good for circa 1000V when it's turned OFF.
Either a breakdown device on the secondary coil (the HV side) or
a clamp on the primary (the drive voltage side) is required.
The transformer coupling makes either sufficient for your needs.
For HV, it'll just take a simple spark gap.
Jamie
Saw tooth wave with a damping component? (Diode)?
Jamie
Michael A. Terrell
They are separate circuits in modern monitors, so the flyback
operates at a fixed frequency. Other than that, it's a traditional
TV/monitor HV supply.
Grant
>
>Wiebe Cazemier wrote:
>>
>> Michael A. Terrell wrote:
>>
>> >
>> > The flyback isn't used to drive the yoke in multiple 'H' frequency
>> > monitors.
>> >
>> >
>>
>> But the flyback is driven by the horizontal output stage, so the flyback
>> itself operates at different frequencies. So, what driving method is used
>> and how is back-EMF handled?
>
>
> They are separate circuits in modern monitors, so the flyback
>operates at a fixed frequency. Other than that, it's a traditional
>TV/monitor HV supply.
Ya think? Not in the dozen or so monitors I dismantled in the last six months,
fair bit of MOSFET power switched capacitors around that HO section to tune
it to selected rate. Some monitors you can hear relays click when they go high
speed. But the ones I had were about a decade old.
I saved some of the triplers, maybe build a Jacob's ladder or so.
Grant.
Wiebe Cazemier
I'm trying to achieve about 20 kV at as much current I can get out of the
flyback. In the device that I'm going to power, there will be a spark gap,
but not to protect, but to give me sudden discharge of high voltage
capacitors (long story).
Anyway, at 20V, it draws about 5 amps at 90% duty cycle. So with 150R
ballast, that gives me 750V.
I hadn't really though about what you're saying about throwing away energy;
I've been increasing the voltage on the primary winding to get higher output
voltage, but I might as well increase the back-EMF ballast to reduce the
amount of energy I throw away.
More testing is indicated :)
Wiebe Cazemier
> months, fair bit of MOSFET power switched capacitors around that HO
> section to tune
> it to selected rate. Some monitors you can hear relays click when they go
> high
> speed. But the ones I had were about a decade old.
>
> I saved some of the triplers, maybe build a Jacob's ladder or so.
>
> Grant.
I'm looking at one of those CRT's right now, that clicks when you switch
some resolutions.
Anyway, could you elaborate on these MOSFET power switched capacitors? Do
you mean that modern CRT's do have circuitry to achieve a resonance?
Sylvia Else
If I'm visualising your circuit correctly, that will push up the voltage
that the transistor sees, and 750V is already way above your
transistor's rating.
Perhaps you should look at voltage multipliers.
Sylvia.
Fredxx
I can't see the original post so replying here.
All flyback transformers are resonant in some way or another through
parasitic capacitance. It is the final current flowing in the transformer
when the MOSFET lets go and the LC resonance which determines the ultimate
flyback voltage and finally current reversal or ringing. Less voltage means
less current and so requires a reduced "on" time of the MOSFET.
I assume by back EMF ballast, do you mean you are trying to limit back EMF
by absorbing most of the energy you've just put into the uinductor? A
circuit diagram might be useful with switching times and details of the
transformer and precisely what you're trying to achieve.
hifi-tek
"Wiebe Cazemier" <wi...@halfgaar.net> wrote in message
news:4d985297$0$9944$703f...@news.kpn.nl...
FWIW, in the past, when I would want to generate HV with current, I would
use an old (50's - 60's) TV flyback with an added primary winding. (Only
open core transformers). Another benefit was that the HV output was AC and
you could do more with it. If this is a one-off project, I would go that
route. Those transformers are still quite common, even NOS. I always used
push-pull driving transistors, with tranzorbs across the CE connections.
Never had a transistor failure, but had to pay special attention to the
unused leads on the flyback to insure against corona arc over.
Tom
Jan Panteltje
<t.ho...@insightbb.com> wrote in
<V8ednV_S8cNtGQXQ...@insightbb.com>:
In my poor school days I used an old tube radio, fed back the speaker output
to the record player input so it ocillated, and then also connected an ignition
coil primary to the speaker output, nice HV for no cost.
Wiebe Cazemier
hope everybody can keep up :)
See my reply below in this post.
Fredxx wrote:
My circuit is basically like this:
http://www.electronics-lab.com/projects/misc/001/index.html
But then the LM555 is replaced by a microcontroller generating a square
wave. I did that so that I could precisely control the frequency and duty
cycle.
Also, 12V hardly does anything for me. I'm using 50V right now, but that
quickly destroys the MOSFET (IRF840).
The back-EMF ballast is the 100R resistor you see in the schematic. But
currently, I have three 1K resistors in parallel, giving me a 30W 330R
ballast. My diode is a UF4007 (instead of UF4008).
I wish I could say anything about my flyback, it doesn't have any model
number on it. I got it out of a 17" HP monitor.
What I'm trying to achieve isn't clear yet. I just need a high voltage, high
current power supply for some experiments. I don't really know yet what kind
of voltage and current we're talking about. The experiment is a Tesla
Hairpin.
Hifi-tek wrote:
> FWIW, in the past, when I would want to generate HV with current, I would
> use an old (50's - 60's) TV flyback with an added primary winding. (Only
> open core transformers). Another benefit was that the HV output was AC
and
> you could do more with it. If this is a one-off project, I would go that
> route. Those transformers are still quite common, even NOS. I always used
> push-pull driving transistors, with tranzorbs across the CE connections.
> Never had a transistor failure, but had to pay special attention to the
> unused leads on the flyback to insure against corona arc over.
> Tom
I don't know where I'm going to find that kind of a flyback, though. Perhaps
someone has one of those wooden TV's for me :)
You're also using transzorbs? They don't limit your output too much?
And if you're using push-pull, does that mean you're also using capacitors
so that the back-EMF is fed back into the flyback?
I've been having corona breakdown on the unused pins of the flyback as well,
BTW.
Wiebe Cazemier
> If I'm visualising your circuit correctly, that will push up the voltage
> that the transistor sees, and 750V is already way above your
> transistor's rating.
>
> Perhaps you should look at voltage multipliers.
>
> Sylvia.
For more information about my schematic, see my reply in another thread. It
was a reply to whit3rd's message, who replied without a reference, so the
message didn't appear in this thread.
Jim Thompson
You're going at it wrong... square wave is wrong.
Turn on, charge to a current limit, dump.
Otherwise you have fry baby, fry ;-)
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Remember: Once you go over the hill, you pick up speed
Sylvia Else
http://www.electronics-lab.com/projects/misc/001/index.html
OK. That's pretty much what I thought.
With that circuit, it seems to me that the maximum output voltage is the
transistor's voltage rating multiplied by the turns ratio of the
transformer.
To get any higher, you need a way of preventing the transistor from
seeing higher voltages from the primary *without sinking current from
the primary*, which is something of an ask.
Sylvia.
Jamie
This maybe true if the input to the coil was a sine wave or some sort of
slow ramp down after the coil is magnetized. Using the square wave the
signal is being turned off immediately and thus the field is allowed to
collapse abruptly. This as you know, is going to generate more voltage
than what you see in the turn ratio.
We have plasma generators at work used to treat the surface of wire
for printing and the coil in those are being driven via a saw tooth,
where output is regulated by monitoring current in the primary side of
the coil. We set current limit to what we need for correct output.
So when viewing this on the scope it may look like a trapezoid of
sorts at times. But the important part is when the driver drops to zero
is when we get our high voltage we need to generate the plasma.
Jamie.
Jamie
Sylvia Else
What I've argued is that the emfs in the primary and secondary are
always in proportion to the turn ratio, on the grounds that the change
in flux linkage is in that proportion.
Attempting to stop the current in the primary abruptly will cause higher
voltages in both primary and secondary than a slower change would, but
it doesn't alter the relationship. So if you want a high voltage in the
secondary, you have to handle a correspondingly high voltage in the primary.
>
> We have plasma generators at work used to treat the surface of wire for
> printing and the coil in those are being driven via a saw tooth, where
> output is regulated by monitoring current in the primary side of the
> coil. We set current limit to what we need for correct output.
>
> So when viewing this on the scope it may look like a trapezoid of sorts
> at times. But the important part is when the driver drops to zero is
> when we get our high voltage we need to generate the plasma.
>
> Jamie.
The OP's problem is not an inability to create high voltages, it's an
inability to cope with the high voltage that appears in the primary,
which destroys his transistor, as I would expect it to.
Sylvia.
Grant
I've only looked at the PCB, not the circuit, so while I see some MOSFET
switched caps, I don't know what they're connected to, but one of the MOSFETs
is mounted on a heatsink. No photos and nothing here at the moment.
Got an old monitor in the car, it's 1995 though, may check it later.
Grant.
Warren
news:qtbhp65rdr58200sj...@4ax.com:
> On Sun, 03 Apr 2011 19:24:10 +0200, Wiebe Cazemier
> <wi...@halfgaar.net> wrote:
..
>>My circuit is basically like this:
>>
>>http://www.electronics-lab.com/projects/misc/001/index.html
..
>>I don't know where I'm going to find that kind of a
>>flyback, though. Perhaps someone has one of those wooden
>>TV's for me :)
All you have to do is to take a drive through your neighbourhood
on garbage day. Any TV set with a picture tube in it, will have
a flyback transformer.
With folks still switching to flat screen TVs, they still show
up on the curb.
Also any old computer CRT monitor would also have the same.
Their natural operating frequencies will be different though.
Be careful disposing of the picture tube-- they implode and
leave a 50 foot radius of dangerous hunks of glass if you
improperly break one. My grandfather once threw a steel pipe at
one in a bushel basket covered with a blanket. It blew the
blanket off and glass was strewn all over the place. Cleaning up
became a hobby for several weeks.
As a young feller, I eventually learned how to difuse them. But
I never got comfortable doing it. It was like difusing a bomb--
filing the glass end and waiting for that sudden pfffffffft!!
Warren
Warren
news:4d985557$0$577$703f...@news.kpn.nl:
>> I saved some of the triplers, maybe build a Jacob's ladder
>> or so.
>>
>> Grant.
The best Jacobs ladder I ever had was using an ignition
transformer from an old furnace oil burner.
I took that thing to high school once and demonstrated it in an
electrical class (motor/generator stuff). I recall holding some
magnets near the arc, to influence the arc. Then I must have got
some charge from being a bit too close because suddenly my pant
legs balooned out from the static charge collected. That
classroom had a nicely varnished wood floor.
Warren
Spehro Pefhany
wrote:
I think for a good Jacobs ladder you need to put lots of power into
the arc to cause enough heating so that the arc rises on the
electrodes.
Jim Thompson
Just get an old neon transformer.
Warren
news:6g0kp65m0qqcfbg5t...@4ax.com:
An oil burning ignition transformer does that very well. I
don't know how that compares to a neon transformer though.
Warren.
Phil Hobbs
> Jim Thompson expounded in
> news:qtbhp65rdr58200sj...@4ax.com:
>
>> On Sun, 03 Apr 2011 19:24:10 +0200, Wiebe Cazemier
>> <wi...@halfgaar.net> wrote:
>>> My circuit is basically like this:
>>>
>>> http://www.electronics-lab.com/projects/misc/001/index.html
>>> I don't know where I'm going to find that kind of a
>>> flyback, though. Perhaps someone has one of those wooden
>>> TV's for me :)
>
> All you have to do is to take a drive through your neighbourhood
> on garbage day. Any TV set with a picture tube in it, will have
> a flyback transformer.
>
> With folks still switching to flat screen TVs, they still show
> up on the curb.
>
> Also any old computer CRT monitor would also have the same.
> Their natural operating frequencies will be different though.
>
> Be careful disposing of the picture tube-- they implode and
> leave a 50 foot radius of dangerous hunks of glass if you
> improperly break one. My grandfather once threw a steel pipe at
> one in a bushel basket covered with a blanket. It blew the
> blanket off and glass was strewn all over the place. Cleaning up
> became a hobby for several weeks.
>
> As a young feller, I eventually learned how to difuse them. But
> I never got comfortable doing it. It was like difusing a bomb--
> filing the glass end and waiting for that sudden pfffffffft!!
>
> Warren
You can also take the base off and crack the evacuation tube in the
middle. When I was a kid, I used to shove them in a plastic garbage can
and shoot them with my slingshot. Fun.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
Jim Thompson
When I was a kid in my dad's TV shop, we had a 55 gallon drum, with a
round hole in the side just big enough to accommodate a crowbar.
Insert picture tube, replace lid, jam crowbar :-)
Warren
news:7ZGdnToPtfDGiQfQ...@supernews.com:
That's exactly where I did the filing (the little glass nib
inside the base). To file anywhere else is not safe, AFAIK.
Even so, I would take precautions with a wood shield. :)
Warren
Spehro Pefhany
wrote:
Mains frequency neon transformers have a magnetic shunt that gives
them a relatively high output impedance (to act as a ballast for the
negative resistance tube). You'll get a much more intense arc out of
an old ignition transformer, but you might toast the xfmr.
Warren
news:nkakp6t03gh4f8lo5...@4ax.com:
It is probably not designed to run continuously but it never
seemed too strained when I used it. If your oil burner has
trouble it can be active for fairly long periods of time
(several repeated startup cycles). Mine never got warm or hot.
But then it was the kind of thing that was never casually left
on anyway.
Warren
Spehro Pefhany
wrote:
>
>It is probably not designed to run continuously but it never
>seemed too strained when I used it. If your oil burner has
>trouble it can be active for fairly long periods of time
>(several repeated startup cycles). Mine never got warm or hot.
>But then it was the kind of thing that was never casually left
>on anyway.
>
>Warren
The one I found at the dump as a teenager weighed about 10lb and was
potted. It didn't get particularly warm, IIRC.
k...@att.bizzzzzzzzzzzz
Then I was in high school I used an oil ignition transformer to drive a Tesla
coil. It was 10KV @10mA. 100W should be enough to get the air moving in a
Jacobs ladder. ;-)
Grant
You're completely missing the point of flyback! It's not about turns ratio
as more about charging energy into the inductor and letting the flyback
generate the high voltage. Others have pointed out that the OP is on the
wrong track driving the thing open loop like that. The web circuit reference
is bad in so many aspects, from burning out pots or the 555 on user adjustment,
to having no safe control of the power in the inductor.
>
>Attempting to stop the current in the primary abruptly will cause higher
>voltages in both primary and secondary than a slower change would, but
>it doesn't alter the relationship. So if you want a high voltage in the
>secondary, you have to handle a correspondingly high voltage in the primary.
Yeah, sorta. Close but not quite...
>>
>> We have plasma generators at work used to treat the surface of wire for
>> printing and the coil in those are being driven via a saw tooth, where
>> output is regulated by monitoring current in the primary side of the
>> coil. We set current limit to what we need for correct output.
>>
>> So when viewing this on the scope it may look like a trapezoid of sorts
>> at times. But the important part is when the driver drops to zero is
>> when we get our high voltage we need to generate the plasma.
>>
>> Jamie.
>
>The OP's problem is not an inability to create high voltages, it's an
>inability to cope with the high voltage that appears in the primary,
>which destroys his transistor, as I would expect it to.
OP does not even have proper snubber network, if he follows that 555
circuit.
If you're not doing the power supply method of applying a voltage until
current rises to a turn off point, then wait for field collapse, you would
need to do a tuned oscillator topology where you add power at just the
right point in the cycle. Sort of saying the same thing in two different
ways, I know. But power circuitry like this is not rocket science.
Says me, has some bits in a box, not yet produced a nice sparking voltage
source -- though I can draw little sparks from an ioniser I built couple
weeks ago to clean the air in here.
Grant.
>
>Sylvia.
Grant
I put leave the tube in the original case, after removing a monitor's PCB,
put the back of case back on properly -- then the thing is as safe as it
ever was.
Grant.
>
>As a young feller, I eventually learned how to difuse them. But
>I never got comfortable doing it. It was like difusing a bomb--
>filing the glass end and waiting for that sudden pfffffffft!!
Someone dropped a large TV off a lookout tower near here a few
years ago, very surprising how thick that CRT glass is!
Grant.
>
>Warren
Grant
>:o)
>
>Warren
Grant
>On Mon, 04 Apr 2011 13:41:30 -0400, Spehro Pefhany
><spef...@interlogDOTyou.knowwhat> wrote:
>
>>On Mon, 4 Apr 2011 17:27:00 +0000 (UTC), Warren <ve3...@gmail.com>
>>wrote:
>>
>>>Wiebe Cazemier expounded in
>>>news:4d985557$0$577$703f...@news.kpn.nl:
>>>
>>>>> I saved some of the triplers, maybe build a Jacob's ladder
>>>>> or so.
>>>>>
>>>>> Grant.
>>>
>>>The best Jacobs ladder I ever had was using an ignition
>>>transformer from an old furnace oil burner.
>>>
>>>I took that thing to high school once and demonstrated it in an
>>>electrical class (motor/generator stuff). I recall holding some
>>>magnets near the arc, to influence the arc. Then I must have got
>>>some charge from being a bit too close because suddenly my pant
>>>legs balooned out from the static charge collected. That
>>>classroom had a nicely varnished wood floor.
>>>
>>>Warren
>>
>>I think for a good Jacobs ladder you need to put lots of power into
>>the arc to cause enough heating so that the arc rises on the
>>electrodes.
>>
>
>Just get an old neon transformer.
Yes, if I needed one for a stage prop, or quickly, but sourcing a s/h
neon xformer in a country town? I dunno where to look...
Grant.
>
> ...Jim Thompson
Grant
Over here we don't have oil furnaces in common usage, and oil heating
virtually disappeared after the '70s oil price rise, in favour of
natural gas heating. Central heating likely gas or electric reverse
cycle airco.
Neon xformer commonly recommended for Jacobs ladder, otherwise it's
an auto ignition coil or old TV tripler with some 12V high current
power driver circuit.
I've got some high voltage gear recovered from laser printers, but the
current used in them there is very small. No good for drawing big
sparks.
Grant.
Grant
Or put the ladder inside an acrylic tube to shield it from the breeze?
Benefit too of limiting ozone generation, or emission, I imagine.
Grant.
Jim Thompson
I have an uncle, he's ~87 now, who would take a three-corner file and
slowly file open the vacuum teat (sis-s-s-s-s-s :-)
Jim Thompson
Most country towns I know have bars :-) Just find an old sign where
the tubing broke.
Sylvia Else
I don't believe I've missed the point at all. The voltage appears
because the current cannot change instantenously. But the ratio of
voltage on the primary and secondary remains.
Others have pointed out that the OP is on the
> wrong track driving the thing open loop like that. The web circuit reference
> is bad in so many aspects, from burning out pots or the 555 on user adjustment,
> to having no safe control of the power in the inductor.
>>
>> Attempting to stop the current in the primary abruptly will cause higher
>> voltages in both primary and secondary than a slower change would, but
>> it doesn't alter the relationship. So if you want a high voltage in the
>> secondary, you have to handle a correspondingly high voltage in the primary.
>
> Yeah, sorta. Close but not quite...
In what respect is it wrong? In particular, is there any conceivable
circuit that allows the secondary to present a high voltage while not
having the primary present that high voltage divided by the turns ratio?
Sylvia.
Grant
Can't imagine getting a file inside the connector (vacuum it inside the
socket?), but why not jam in a screwdriver and break off the tit and let
the vacuum out? I'm not quite sure I want to try that -- perhaps inside
the wheelie bin with protective glasses on in case it goes Bang!
And I have a monitor in the car, awaiting destruction... Something
different today?
Grant.
>
> ...Jim Thompson
Jim Thompson
Back in those days, the CRT's had octal bases. Unsolder and pull it
off, exposing the vacuum teat.
Grant
Michael A. Terrell
Wimp. All you need is a small triangle file or a big pair of
diagonal cutters. I used to have fun filing a notch into the tip where
the air was evacuated and letting them hiss for hours, or days. :)
--
You can't fix stupid. You can't even put a Band-Aid™ on it, because it's
Teflon coated.
Grant
Why? To watch cat or dog turn their head on a cute, angle trying to
figure it out? >:o)
Grant.
Grant
>On 5/04/2011 9:32 AM, Grant wrote:
...
>> You're completely missing the point of flyback! It's not about turns ratio
>> as more about charging energy into the inductor and letting the flyback
>> generate the high voltage.
>
>I don't believe I've missed the point at all. The voltage appears
>because the current cannot change instantenously. But the ratio of
>voltage on the primary and secondary remains.
Yes, but...
>
> Others have pointed out that the OP is on the
>> wrong track driving the thing open loop like that. The web circuit reference
>> is bad in so many aspects, from burning out pots or the 555 on user adjustment,
>> to having no safe control of the power in the inductor.
>>>
>>> Attempting to stop the current in the primary abruptly will cause higher
>>> voltages in both primary and secondary than a slower change would, but
>>> it doesn't alter the relationship. So if you want a high voltage in the
>>> secondary, you have to handle a correspondingly high voltage in the primary.
>>
>> Yeah, sorta. Close but not quite...
>
>In what respect is it wrong? In particular, is there any conceivable
>circuit that allows the secondary to present a high voltage while not
>having the primary present that high voltage divided by the turns ratio?
The other keyword you overlook: tripler.
CRT HO drives that high voltage tripler, what's inside of that? A little more
than the crappy so-called efficient 555 circuit from a web-site that holds one
of the worst collections of circuits I've seen on the Internet. Not a place of
learning.
So the simple steps to driving a CRT are to drive primary to some current limit,
You may need to add your own low voltage primary around the exposed ferrite for
12V, I've seen 12V circuits, so it can be done.
So, turn current old until some current limit before saturation, turn off
current, collapse of magnetic field generates very high voltage, repeat at
some frequency that suits how you see the circuit, watching on a CRO is
good, or simply tuning for minimum input for big output.
The open loop 555 or whatever drive is not do good, as you really do want
to switch off input power before saturation of the ferrite, otherwise you're
simply going to burn something.
You can see saturation as unreasonable current draw, or with a CRO a breakpoint
from gentle linear ramp to steep almost vertical ramp of input current.
Maximum efficiency is charging the ferrite so some value below saturation.
Thus driving the primary open loop from an oscillator is bad. Better is using
a latch that you set at a regular rate, and current limit resets latch, Q drives
the output transistor, and voltage feedback tells you how long before setting
the latch again for variable frequency, ort also clears the latch for fixed
frequency. This is standard power switching topology stuff.
Other way is at a fixed frequency into a tuned circuit which is how the CRT
does it, I think. There it was a controlled oscillator with a tight frequency
spec with a BTW we can also make some EHT here too, due to the large amount
of energy circulating to produce horizontal scan driver to the yoke coils.
This pulse train pumps up the energy through the tripler and makes your high
voltage of 26kV max for low xrays from the CRT in original app.
Efficiency comes from driving a tuned circuit so you need a proper snubber
and/or tuning to work with the primary side issues. Of course adding you own
low voltage primary sidesteps the noticed high voltage seen on existing driver.
But before you argue about the thing cannot work, it did work as a CRT EHT
source until OP pulled the parts from PCB.
I've written the above from memory, so the might be minor errors. But I'm trying
to explain the general picture, not give a precise working guideline -- for that
I'd have to drag out gear I got here and have a play. I have other things on
my todo list right now.
As far as voltage multipliers go, I used them very recently to make a 40V IC
generate over 100V to drive 50 LEDs in series at 20mA, 36V chopped DC stepped
up from 12V though a tripler with over-voltage feedback and LED current regulation.
This stuff is not rocket science. But EHT adds some corona and visible sparks
that very much add to the excitement. I hope the above ramble has some useful
points to help OP.
Grant.
Sylvia Else
> On Tue, 05 Apr 2011 11:11:23 +1000, Sylvia Else<syl...@not.here.invalid> wrote:
>
>> On 5/04/2011 9:32 AM, Grant wrote:
> ...
>>> You're completely missing the point of flyback! It's not about turns ratio
>>> as more about charging energy into the inductor and letting the flyback
>>> generate the high voltage.
>>
>> I don't believe I've missed the point at all. The voltage appears
>> because the current cannot change instantenously. But the ratio of
>> voltage on the primary and secondary remains.
>
> Yes, but...
>>
>> Others have pointed out that the OP is on the
>>> wrong track driving the thing open loop like that. The web circuit reference
>>> is bad in so many aspects, from burning out pots or the 555 on user adjustment,
>>> to having no safe control of the power in the inductor.
>>>>
>>>> Attempting to stop the current in the primary abruptly will cause higher
>>>> voltages in both primary and secondary than a slower change would, but
>>>> it doesn't alter the relationship. So if you want a high voltage in the
>>>> secondary, you have to handle a correspondingly high voltage in the primary.
>>>
>>> Yeah, sorta. Close but not quite...
>>
>> In what respect is it wrong? In particular, is there any conceivable
>> circuit that allows the secondary to present a high voltage while not
>> having the primary present that high voltage divided by the turns ratio?
>
> The other keyword you overlook: tripler.
I've already suggested to the OP that he look at voltage multipliers,
so I cannot be overlooking triplers.
>
> CRT HO drives that high voltage tripler, what's inside of that? A little more
> than the crappy so-called efficient 555 circuit from a web-site that holds one
> of the worst collections of circuits I've seen on the Internet. Not a place of
> learning.
>
> So the simple steps to driving a CRT are to drive primary to some current limit,
> You may need to add your own low voltage primary around the exposed ferrite for
> 12V, I've seen 12V circuits, so it can be done.
The issue is not the voltage driving the circuit, but the voltage
tolerance of the switching transistor.
Specifically, the OP is seeing that when the transistor turns off, the
primary voltage rises to a level where the transistor is destroyed.
The OP is trying to get a particularly high voltage *on the secondary of
the transformer* without the voltage on the primary destroying his
transistor. I'm suggesting that this goal is misconceived (aside from
getting a transistor that tolerates a higher voltage).
Sylvia.
Warren
news:596dnbbEmYPTHgbQ...@earthlink.com:
What do you think I was using?
> or a big
> pair of
> diagonal cutters.
I never got that brave.
> I used to have fun filing a notch into
> the tip where the air was evacuated and letting them hiss
> for hours, or days. :)
I didn't see any point in waiting. The whole point was to be
able to put it out at the curb for safe disposal. Not sure
they would even take that today, they're so fussy now that
they're sanitation "engineers".
Warren
Michael A. Terrell
I'm talking about one from my set of needle files.
> > or a big
> > pair of
> > diagonal cutters.
>
> I never got that brave.
>
> > I used to have fun filing a notch into
> > the tip where the air was evacuated and letting them hiss
> > for hours, or days. :)
>
> I didn't see any point in waiting. The whole point was to be
> able to put it out at the curb for safe disposal. Not sure
> they would even take that today, they're so fussy now that
> they're sanitation "engineers".
The point was that some people wnadered into part of the shop they
were told to stay out of. A hissing CRT scared the crap out of them. :)
John - KD5YI
>
> The point was that some people wnadered into part of the shop they
> were told to stay out of. A hissing CRT scared the crap out of them. :)
Does a leaky air hose do the same?
Grant
The one I did yesterday suddenly gave way (using a big flat file) and the
hissing only lasted maybe ten seconds... Fun?! Don't think I have the
patience to attack the tube with a tiny needle file. I put photos up of
the switched capacitors stuff too, from upthread, on:
Grant.
Michael A. Terrell
Do they implode & sent glass shrapnel all over the room?
Wiebe Cazemier
> The one I did yesterday suddenly gave way (using a big flat file) and the
> hissing only lasted maybe ten seconds... Fun?! Don't think I have the
> patience to attack the tube with a tiny needle file. I put photos up of
> the switched capacitors stuff too, from upthread, on:
>
> http://grrr.id.au/crt/
>
> Grant.
I've been reading up on switched capacitors, and unfortunately, my
electronics knowledge is too limited to design one...
Wiebe Cazemier
Well, if the voltage is equal to what it was in the CRT monitor, I'm happy.
I'm not trying to do anything beyond what the flyback was meant to do.
I think I have that kind of voltage now, but not very efficiently, since my
MOSFET driving the thing does get very hot. Perhaps it's over-saturation. I
will give the scope reading Grant suggested a go, see if I can detect the
saturation point.
One thing I have noticed, is that when I have an arc, the arc gets a bigger
orange glow when I reduce the duty cycle. At 95% for instance, the arc is a
purple line. At 80, it's almost a flame. It also feels hotter (the air above
it)
Jamie
You need to saturate the Mosfet to reduce heat. This also goes to say
that you need to select a mosfet with the lowest turn on R you can find.
You really should be current limiting the primary side. There are a
couple of ways I can think of to do this. One way not to do it is
biasing the transistor, this will generate heat in the Mosfet but can
be done if you so desire.
Years ago I had to make a 20kv dielectric tester to generate 1 ms
transient pulse across mica capacitors. I was able to achieve this
using an automobile coil. I used 2 555's. one was a current regulated
switching supply and the other was for gating that end point supply
to the primary side of the coil. Mosfets were used on the forefront here
since it made perfect sense. I elected to use a low voltage design since
it was easier to find Fets that had much lower Ron and a car coil was a
perfect candidate for the job.
Jamie
Grant
Same time you don't want to saturate the transformer, unless you're making
a self-oscillating LV switcher that uses that magnetic saturation point to
turn off the gate drive. Then you could regulate the output like a CCFL
backlight inverter circuit, there's lots of those on the 'net too. Though
they regulate to constant current, you can see the topology. Some here would
name it, I forgot the name.
>
> You really should be current limiting the primary side. There are a
>couple of ways I can think of to do this. One way not to do it is
>biasing the transistor, this will generate heat in the Mosfet but can
>be done if you so desire.
>
> Years ago I had to make a 20kv dielectric tester to generate 1 ms
>transient pulse across mica capacitors. I was able to achieve this
>using an automobile coil. I used 2 555's. one was a current regulated
> switching supply and the other was for gating that end point supply
>to the primary side of the coil. Mosfets were used on the forefront here
>since it made perfect sense. I elected to use a low voltage design since
> it was easier to find Fets that had much lower Ron and a car coil was a
>perfect candidate for the job.
Yup. TV EHT tripler has the xformer, tripler built in, but I've not
looked at the driving circuit for years. There's space to wind one's
own 12V primary around them. I collected a couple more monitors to
dismantle, see if I find a more recent one.
Grant.
>
>
>
> Jamie
>
Sylvia Else
> Well, if the voltage is equal to what it was in the CRT monitor, I'm happy.
> I'm not trying to do anything beyond what the flyback was meant to do.
The (bipolar in that case) transistor I removed from my own colour TV
recently had a rating in excess of 1000V.
>
> I think I have that kind of voltage now, but not very efficiently, since my
> MOSFET driving the thing does get very hot. Perhaps it's over-saturation. I
> will give the scope reading Grant suggested a go, see if I can detect the
> saturation point.
>
> One thing I have noticed, is that when I have an arc, the arc gets a bigger
> orange glow when I reduce the duty cycle. At 95% for instance, the arc is a
> purple line. At 80, it's almost a flame. It also feels hotter (the air above
> it)
Well, you'll only get a high voltage in the secondary while the
transistor is off (and then only while there is energy remaining in the
field).
At 95% I wouldn't be surprised if the transistor turns on while there is
still energy in the field, which would also reduce the amount of energy
that can be added, and increase the likelihood of driving the
transformer into saturation.
Sylvia.
JW
Message id: <Xns9EBF5EA1B4CBDW...@188.40.43.213>:
>Not sure
>they would even take that today, they're so fussy now that
>they're sanitation "engineers".
So Dimbulb IS an engineer!
josephkk
>Phil Hobbs expounded in
>news:7ZGdnToPtfDGiQfQ...@supernews.com:
>
>> Warren wrote:
>>> Jim Thompson expounded in
>>> news:qtbhp65rdr58200sj...@4ax.com:
>>>
>>>> On Sun, 03 Apr 2011 19:24:10 +0200, Wiebe Cazemier
>>>> <wi...@halfgaar.net> wrote:
>>> ...
>>>>> My circuit is basically like this:
>>>>>
>>>>> http://www.electronics-lab.com/projects/misc/001/index.ht
>>>>> ml
>>> ...
>>>>> I don't know where I'm going to find that kind of a
>>>>> flyback, though. Perhaps someone has one of those wooden
>>>>> TV's for me :)
>>>
>>> All you have to do is to take a drive through your
>>> neighbourhood on garbage day. Any TV set with a picture
>>> tube in it, will have a flyback transformer.
>>>
>>> With folks still switching to flat screen TVs, they still
>>> show up on the curb.
>>>
>>> Also any old computer CRT monitor would also have the
>>> same. Their natural operating frequencies will be
>>> different though.
>>>
>>> Be careful disposing of the picture tube-- they implode
>>> and leave a 50 foot radius of dangerous hunks of glass if
>>> you improperly break one. My grandfather once threw a
>>> steel pipe at one in a bushel basket covered with a
>>> blanket. It blew the blanket off and glass was strewn all
>>> over the place. Cleaning up became a hobby for several
>>> weeks.
>>>
>>> As a young feller, I eventually learned how to difuse
>>> them. But I never got comfortable doing it. It was like
>>> difusing a bomb-- filing the glass end and waiting for
>>> that sudden pfffffffft!!
>>>
>>> Warren
>>
>> You can also take the base off and crack the evacuation
>> tube in the middle. When I was a kid, I used to shove them
>> in a plastic garbage can and shoot them with my slingshot.
>> Fun.
>>
>> Cheers
>>
>> Phil Hobbs
>
>That's exactly where I did the filing (the little glass nib
>inside the base). To file anywhere else is not safe, AFAIK.
>Even so, I would take precautions with a wood shield. :)
>
>Warren
I used to file them on the neck anywhere near the socket. Used a jewelers
file so it was slow, but no implosions ever. Not that i claim that it was
safe.
Wiebe Cazemier
I've determined the inductance value of the primary coil of my flyback. I
charged it up to 20V through 150R (with a squarewave). It took 30uS for the
voltage across the coil to reach about zero. With T=L/R and assuming it
takes 5 time constants to charge it, inductance is 900 uH.
By calculating the back-EMF ballast resistor with the same formula and the
15 uS it took for it to discharge, it gave me 300R, which is about right.
Now on to find the proper value for a capacitor to make a tuned circuit. And
read up on tuned circuits :)
Jamie wrote:
> You need to saturate the Mosfet to reduce heat. This also goes to say
> that you need to select a mosfet with the lowest turn on R you can find.
The IRF840 has Ron of 0.85 Ohm. At 5 A, it would dissipate 21W. I have a
feeling mine does more.
I drive it with a two stage of bc547 and bc557. Those have a maximal current
of 100 mA (200 peak). Perhaps that's not enough to saturate the MOSFET. But
I don't understand this transconductance thing of MOSFETs... BJT's are
easier...
Grant
That's fairly ordinary. But than you don't get high voltage and low Ron so
easy. Just had a look at IRF840, that's too small for this work, IMHO. IR
are very optimistic on their claimed specs, I'd be looking at much higher V
and I ratings for a MOSFET doing 5A high voltage switching.
>
>I drive it with a two stage of bc547 and bc557. Those have a maximal current
>of 100 mA (200 peak). Perhaps that's not enough to saturate the MOSFET. But
>I don't understand this transconductance thing of MOSFETs... BJT's are
>easier...
If you're using the common pnp + npn emitter follower for gate drive, allow
for amp or more drive current, try a pair of 1A transistors?
Grant.
Wiebe Cazemier
>
> That's fairly ordinary. But than you don't get high voltage and low Ron
> so
> easy. Just had a look at IRF840, that's too small for this work, IMHO.
> IR are very optimistic on their claimed specs, I'd be looking at much
> higher V and I ratings for a MOSFET doing 5A high voltage switching.
The problem is, that the high current ones (like 40A) have lower voltage
rating, like 200V. And there is a problem with the IR product selector. I
can't find any >300V MOSFETs with it, even though they exist (like the
IRF840).
>>
>>I drive it with a two stage of bc547 and bc557. Those have a maximal
>>current of 100 mA (200 peak). Perhaps that's not enough to saturate the
>>MOSFET. But I don't understand this transconductance thing of MOSFETs...
>>BJT's are easier...
>
> If you're using the common pnp + npn emitter follower for gate drive,
> allow for amp or more drive current, try a pair of 1A transistors?
I'll see about adding some extra drive transistors.
cbarn...@aol.com
Why not cascade a bjt transitor with your fet? Maybe one of those tv
deflection types might work.
Jamie
Sylvia Else
If the OP tries cascading, then he needs to put resistors across the
drain and source of each transistor, to balance the voltage across them
when they're turned off.
Also need to watch the drain-gate voltages.
Sylvia.
Grant
Look up the switching transistor from a monitor circuit, 1500V peak, 25A
peak (800V open base, 10A continuous, BU2520). What OP is using is nowhere
near that rating. Mind you, with a peak base turn off current at max 6A,
peak base drive 9A, continuous base current 6A, these puppies wont be the
easiest to drive at speed either ;) But they'll do it for years.
MOSFETs not necessarily the best fit.
Grant.
>
>Sylvia.
cbarn...@aol.com
>
> - Show quoted text -
You need to lookup how a cascade circuit works.
Sylvia Else
If you're thinking about cascaded amplifiers, that's not going to help
the OP in the slightest when it comes to managing the high voltage.
If you have something else in mind, post a link to something describing it.
Of course, it's always easy to post vague comments implying ignorance on
the part of some other person, while not saying enough to allow anyone
to contradict you.
Sylvia.
Fredxx
I have seen two transistors cascaded in a Sony Trinitron, where the voltage
experienced between them was split using a capacitor across each which made
up the resonant circuit. Each was driven by a pulse transformer.
Don Klipstein
<SNIP to here>
>I have seen two transistors cascaded in a Sony Trinitron, where the voltage
>experienced between them was split using a capacitor across each which made
>up the resonant circuit. Each was driven by a pulse transformer.
Did they have problems with high cost or low beta of 1500V transistors?
--
- Don (d...@misty.com)
cbarn...@aol.com
Cascode is what I meant to say, hope that clears it up.
Sylvia Else
Well, it clarifies your intent.
I don't see that it addresses the problem, though. Referring to the
diagram in
http://en.wikipedia.org/wiki/Cascode
and paying due attention to the comment about biasing...
In the transition from the on state to the off state, the voltage on the
drain (Vout) that's connected to the transformer primary will rise from
near zero to whatever high voltage is produced by the primary. In the on
state, the gate of that transistor will also be not far from zero. In
the absence of some special handling of the gate voltage in the
transition to the off state, the drain-gate voltage will rise beyond the
tolerance of the transistor, which will fail.
Sylvia.
cbarn...@aol.com
As stated the upper device is a deflection transistor ( bjt ).
>
> and paying due attention to the comment about biasing...
You would normaly bias it from the low voltage supply via a suitabe
resistor to give enough on state base current.
>
> In the transition from the on state to the off state, the voltage on the
> drain (Vout) that's connected to the transformer primary will rise from
> near zero to whatever high voltage is produced by the primary.
Yes these transistors are rated at a least 1500 volts, this is exactly
what the op wants.
In the on
> state, the gate of that transistor will also be not far from zero. In
> the absence of some special handling of the gate voltage in the
> transition to the off state, the drain-gate voltage will rise beyond the
> tolerance of the transistor, which will fail.
I dont see what your talking about here!
Fredxx
"Don Klipstein" <d...@manx.misty.com> wrote in message
news:slrniq9s...@manx.misty.com...
I have no idea, but it was in the early days of solid state TVs. I would
guess they went for the most cost effective method to cope with the flyback
voltage.
josephkk
wrote:
Glad to find someone was awake. The term for HV V(ce) splitting usage is
totem pole. Also used (borrowed) to describe TTL outputs by similarity.
Doing totem pole with MOS transistors includes some interesting design
trade-offs.