I'm designing a box that has +48 and -48 power supplies, each bypassed
with 44,000 uF. I figure it will take forever to discharge when the
power is switched off, and I don't want to waste a heap of power on a
bleeder.
So I figured I could put an LED on the board to alert our test people
that the board is still hot, and have a DISCHARGE pushbutton. So what
I'm thinking is
Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
better than buying a switch rated for 1 amp at 100 volts DC.
I wonder if I could dump the energy into a mosfet, namely get rid of
the power resistors and add a smaller source resistor, to make the
power fet into a current sink. It's tempting, but then it might blow
up. There's about 100 joules to dump.
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
> I'm designing a box that has +48 and -48 power supplies, each bypassed
> with 44,000 uF. I figure it will take forever to discharge when the
> power is switched off, and I don't want to waste a heap of power on a
> bleeder.
> So I figured I could put an LED on the board to alert our test people
> that the board is still hot, and have a DISCHARGE pushbutton. So what
> I'm thinking is
> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> better than buying a switch rated for 1 amp at 100 volts DC.
> I wonder if I could dump the energy into a mosfet, namely get rid of
> the power resistors and add a smaller source resistor, to make the
> power fet into a current sink. It's tempting, but then it might blow
> up. There's about 100 joules to dump.
<robert.a.m...@gmail.com> wrote:
>On Apr 17, 2:30 pm, John Larkin <jlar...@highlandtechnology.com>
>wrote:
>> I'm designing a box that has +48 and -48 power supplies, each bypassed
>> with 44,000 uF. I figure it will take forever to discharge when the
>> power is switched off, and I don't want to waste a heap of power on a
>> bleeder.
>> So I figured I could put an LED on the board to alert our test people
>> that the board is still hot, and have a DISCHARGE pushbutton. So what
>> I'm thinking is
>> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>> better than buying a switch rated for 1 amp at 100 volts DC.
>> I wonder if I could dump the energy into a mosfet, namely get rid of
>> the power resistors and add a smaller source resistor, to make the
>> power fet into a current sink. It's tempting, but then it might blow
>> up. There's about 100 joules to dump.
>> Precision electronic instrumentation
>> Picosecond-resolution Digital Delay and Pulse generators
>> Custom laser controllers
>> Photonics and fiberoptic TTL data links
>> VME thermocouple, LVDT, synchro acquisition and simulation
>As a comparison, look at the size of your 5J and 10J MOV parts.
I think MOVs have some non-thermal long-term degradation mechanism. I
can certainly get a mosfet rated for 100, or even 300 watts, but lots
of mosfets die if you try to run up to their rtaed dissipation at
higher voltages. You can see that in the SOAR curves. And the heat
does have to go somewhere, and heat sinks are expensive.
>Is the MOSFET die about that size?
>What's the rule of thumb for abuse on a resistor? Isn't it more than
>20X rating? for a single event.
>that's what? 450 ohm for 10 seconds?
I tried some 6 watt wirewounds that we have in stock. They get warm at
50 watts*1 second, and pretty hot at 50 watts*3 seconds, which would
be 150 joules each, 300 for the pair. So the resistors look safe. It's
not like this will be used more than maybe 20 times ever.
>Hmmm..how about an incandescent 60w bulb?
That would be neat electrically, and fun to watch, but messy
mechanically. Where do I get a pcb-mount 60 watt lightbulb?
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
>I'm designing a box that has +48 and -48 power supplies, each bypassed
>with 44,000 uF. I figure it will take forever to discharge when the
>power is switched off, and I don't want to waste a heap of power on a
>bleeder.
>So I figured I could put an LED on the board to alert our test people
>that the board is still hot, and have a DISCHARGE pushbutton. So what
>I'm thinking is
>Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>better than buying a switch rated for 1 amp at 100 volts DC.
>I wonder if I could dump the energy into a mosfet, namely get rid of
>the power resistors and add a smaller source resistor, to make the
>power fet into a current sink. It's tempting, but then it might blow
>up. There's about 100 joules to dump.
Why not use a little PCB-mount power relay (powered from the
unfiltered supply)?
That way you don't need the LED or the pushbutton- it's always
discharged to a safe condition shortly after the power is turned off.
Or use a MOSFET and a bright blinkie-LED if you want solid state,
automation and confirmation, but the control gets a bit more complex.
Discharging from + to - might warrant some analysis to make sure there
are not any conditions under which Bad Things might happen of the
polarities of one of the supplies relative to ground flips.
>>I'm designing a box that has +48 and -48 power supplies, each bypassed
>>with 44,000 uF. I figure it will take forever to discharge when the
>>power is switched off, and I don't want to waste a heap of power on a
>>bleeder.
>>So I figured I could put an LED on the board to alert our test people
>>that the board is still hot, and have a DISCHARGE pushbutton. So what
>>I'm thinking is
>>Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>>better than buying a switch rated for 1 amp at 100 volts DC.
>>I wonder if I could dump the energy into a mosfet, namely get rid of
>>the power resistors and add a smaller source resistor, to make the
>>power fet into a current sink. It's tempting, but then it might blow
>>up. There's about 100 joules to dump.
>Why not use a little PCB-mount power relay (powered from the
>unfiltered supply)?
The supplies are commercial switchers, so I don't have an unfiltered
version. And if anything went wrong, something would catch fire. My
test people are smart enough to turn off power before they push the
DISCHARGE button. And I'd need a relay rated for 1 amp, 100 VDC, 48
volt coil.
>That way you don't need the LED or the pushbutton- it's always
>discharged to a safe condition shortly after the power is turned off.
>Or use a MOSFET and a bright blinkie-LED if you want solid state,
>automation and confirmation, but the control gets a bit more complex.
>Discharging from + to - might warrant some analysis to make sure there
>are not any conditions under which Bad Things might happen of the
>polarities of one of the supplies relative to ground flips.
Right. I think the diodes in the switchers will take care of that, but
I'll check.
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
> I'm designing a box that has +48 and -48 power supplies, each bypassed
> with 44,000 uF. I figure it will take forever to discharge when the
> power is switched off, and I don't want to waste a heap of power on a
> bleeder.
> So I figured I could put an LED on the board to alert our test people
> that the board is still hot, and have a DISCHARGE pushbutton. So what
> I'm thinking is
> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> better than buying a switch rated for 1 amp at 100 volts DC.
> I wonder if I could dump the energy into a mosfet, namely get rid of
> the power resistors and add a smaller source resistor, to make the
> power fet into a current sink. It's tempting, but then it might blow
> up. There's about 100 joules to dump.
> --
I assume you have antireversal diodes on the rails, right?
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
>> I'm designing a box that has +48 and -48 power supplies, each bypassed
>> with 44,000 uF. I figure it will take forever to discharge when the
>> power is switched off, and I don't want to waste a heap of power on a
>> bleeder.
>> So I figured I could put an LED on the board to alert our test people
>> that the board is still hot, and have a DISCHARGE pushbutton. So what
>> I'm thinking is
>> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>> better than buying a switch rated for 1 amp at 100 volts DC.
>> I wonder if I could dump the energy into a mosfet, namely get rid of
>> the power resistors and add a smaller source resistor, to make the
>> power fet into a current sink. It's tempting, but then it might blow
>> up. There's about 100 joules to dump.
>> --
>I assume you have antireversal diodes on the rails, right?
>Cheers
>Phil Hobbs
I figure the switching power supplies will take care of that. But I
suppose I should verify that.
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
>> On Tue, 17 Apr 2012 14:30:44 -0700, John Larkin
>> <jlar...@highlandtechnology.com> wrote:
>>> I'm designing a box that has +48 and -48 power supplies, each bypassed
>>> with 44,000 uF. I figure it will take forever to discharge when the
>>> power is switched off, and I don't want to waste a heap of power on a
>>> bleeder.
>>> So I figured I could put an LED on the board to alert our test people
>>> that the board is still hot, and have a DISCHARGE pushbutton. So what
>>> I'm thinking is
>>> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>>> better than buying a switch rated for 1 amp at 100 volts DC.
>>> I wonder if I could dump the energy into a mosfet, namely get rid of
>>> the power resistors and add a smaller source resistor, to make the
>>> power fet into a current sink. It's tempting, but then it might blow
>>> up. There's about 100 joules to dump.
>> Why not use a little PCB-mount power relay (powered from the
>> unfiltered supply)?
> The supplies are commercial switchers, so I don't have an unfiltered
> version. And if anything went wrong, something would catch fire.
Thems is incendiary words...pun intended.
My
> test people are smart enough to turn off power before they push the
> DISCHARGE button.
Think that's a false assumption. Who among us has never done a stupid
thing?
I'm paranoid when it comes to product liability.
If you adhere to standard safety practices, there are lots of places
to place blame.
Once you add a button that says, "press this to be safe", YOU become
the primary target. The injured party testifies that he pressed the discharge button before the explosion. Maybe your discharge circuit failed...maybe it didn't. Doesn't matter. You're screwed!
I think the legal ramifications of this decision far outweigh the
technical issues.
>> That way you don't need the LED or the pushbutton- it's always
>> discharged to a safe condition shortly after the power is turned off.
>> Or use a MOSFET and a bright blinkie-LED if you want solid state,
>> automation and confirmation, but the control gets a bit more complex.
>> Discharging from + to - might warrant some analysis to make sure there
>> are not any conditions under which Bad Things might happen of the
>> polarities of one of the supplies relative to ground flips.
> Right. I think the diodes in the switchers will take care of that, but
> I'll check.
> I'm designing a box that has +48 and -48 power supplies, each bypassed
> with 44,000 uF. I figure it will take forever to discharge when the
> power is switched off, and I don't want to waste a heap of power on a
> bleeder.
> So I figured I could put an LED on the board to alert our test people
> that the board is still hot, and have a DISCHARGE pushbutton. So what
> I'm thinking is
> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> better than buying a switch rated for 1 amp at 100 volts DC.
> I wonder if I could dump the energy into a mosfet, namely get rid of
> the power resistors and add a smaller source resistor, to make the
> power fet into a current sink. It's tempting, but then it might blow
> up. There's about 100 joules to dump.
> Precision electronic instrumentation
> Picosecond-resolution Digital Delay and Pulse generators
> Custom laser controllers
> Photonics and fiberoptic TTL data links
> VME thermocouple, LVDT, synchro acquisition and simulation
I'd keep the resistor. I have no equations to back this up.
I've got a little few watt heater. (120 C operating temp) I put both
the transistor and resistor on the thing I'm heating. Each disipates
about 1/2 the power at normal operating conditions. That seems
safest.
> I'm designing a box that has +48 and -48 power supplies, each bypassed
> with 44,000 uF. I figure it will take forever to discharge when the
> power is switched off, and I don't want to waste a heap of power on a
> bleeder.
> So I figured I could put an LED on the board to alert our test people
> that the board is still hot, and have a DISCHARGE pushbutton. So what
> I'm thinking is
> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> better than buying a switch rated for 1 amp at 100 volts DC.
> I wonder if I could dump the energy into a mosfet, namely get rid of
> the power resistors and add a smaller source resistor, to make the
> power fet into a current sink. It's tempting, but then it might blow
> up. There's about 100 joules to dump.
In article <jmkubn$u0...@dont-email.me>, mike <spam...@gmail.com> wrote:
>On 4/17/2012 3:28 PM, John Larkin wrote:
> My
>> test people are smart enough to turn off power before they push the
>> DISCHARGE button.
>Think that's a false assumption. Who among us has never done a stupid
>thing?
I agree, that's a poor assumption to make. Even if it's 99.99%
correct, if you do this operation 10 times a day, you'll blow up
something about once per three years if you have only a single machine.
I'm a software guy... and I don't trust software for safety
interlocks. If you've got a processor driving the gate of something
that can only sustain a 10% duty cycle without burning up, then it's a
*very* good idea to have a hardware safety to disconnect the drive
if the duty cycle exceeds this for more than a very short period of
time. All it takes is one software or processor glitch when the drive
is "on", the code jumps off into the weeds and never turns off the
drive, and you've got a fire on your hands.
I don't trust "wetware" any more than I trust software, for this sort
of safety. Human brains simply aren't reliable enough... and that's
just as true of smart people as unsmart people.
That's why electrical-worker safety regs require lockout and tagging
protocols (with one lock per worker!) on circuit breakers. It avoids
those embarrassing "Oh, I just did a thinko... and now I have to go
apologize to the grieving widow" moments.
In this situation, I'd stick a relay in the switched mains circuit,
and wire it up to disable the DISCHARGE button... make the discharge
circuit a no-op if the mains are still live.
-- Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
> I'm designing a box that has +48 and -48 power supplies, each bypassed
> with 44,000 uF. I figure it will take forever to discharge when the
> power is switched off, and I don't want to waste a heap of power on a
> bleeder.
> So I figured I could put an LED on the board to alert our test people
> that the board is still hot, and have a DISCHARGE pushbutton. So what
> I'm thinking is
> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> better than buying a switch rated for 1 amp at 100 volts DC.
> I wonder if I could dump the energy into a mosfet, namely get rid of
> the power resistors and add a smaller source resistor, to make the
> power fet into a current sink. It's tempting, but then it might blow
> up. There's about 100 joules to dump.
>On Apr 17, 5:30 pm, John Larkin <jlar...@highlandtechnology.com>
>wrote:
>> I'm designing a box that has +48 and -48 power supplies, each bypassed
>> with 44,000 uF. I figure it will take forever to discharge when the
>> power is switched off, and I don't want to waste a heap of power on a
>> bleeder.
>> So I figured I could put an LED on the board to alert our test people
>> that the board is still hot, and have a DISCHARGE pushbutton. So what
>> I'm thinking is
>> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>> better than buying a switch rated for 1 amp at 100 volts DC.
>> I wonder if I could dump the energy into a mosfet, namely get rid of
>> the power resistors and add a smaller source resistor, to make the
>> power fet into a current sink. It's tempting, but then it might blow
>> up. There's about 100 joules to dump.
> I'm designing a box that has +48 and -48 power supplies, each bypassed
> with 44,000 uF. I figure it will take forever to discharge when the
> power is switched off, and I don't want to waste a heap of power on a
> bleeder.
> So I figured I could put an LED on the board to alert our test people
> that the board is still hot, and have a DISCHARGE pushbutton. So what
> I'm thinking is
On a sunny day (Tue, 17 Apr 2012 14:30:44 -0700) it happened John Larkin
<jlar...@highlandtechnology.com> wrote in
<svmro7hdg45ta45u3ulqkk2scubto8s...@4ax.com>:
>I'm designing a box that has +48 and -48 power supplies, each bypassed
>with 44,000 uF. I figure it will take forever to discharge when the
>power is switched off, and I don't want to waste a heap of power on a
>bleeder.
>So I figured I could put an LED on the board to alert our test people
>that the board is still hot, and have a DISCHARGE pushbutton. So what
>I'm thinking is
>Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>better than buying a switch rated for 1 amp at 100 volts DC.
>I wonder if I could dump the energy into a mosfet, namely get rid of
>the power resistors and add a smaller source resistor, to make the
>power fet into a current sink. It's tempting, but then it might blow
>up. There's about 100 joules to dump.
tium experiment
That is exactly what I do in the tritium experiment with the power MOSFET
that has as only purpose heating the hot plate.
Small source resistor, CMOS opamp and feedback, Tim added a capacitor.
For a fixed input voltage the current is constant.
If you want power limiting during dischange add a 2$ PIC and also monitor
voltage across the MOSFET, and drive the MOSFET from filtered PWM [1].
Maybe you need a 3V button cell to keep that computah running to the last
volt (Vgs you need).
Oh what fun...
:-)
[1] you need power limiting as you cannot just current limit,
at high voltages you need less discharge current.
[2] you need to automate the triggring, either by a micro switch if
box is opened, ligh tdetector, mains presence detector, IR proximity,
or whatever.
[3] Giving training courses and exams, with a certificate when passed to your customers technicians at 1000$ an afternoon helps make money too.
[4] Hey I tho <censored becauce of politcal incorrectness>
On 2012-04-17, John Larkin <jlar...@highlandtechnology.com> wrote:
> I wonder if I could dump the energy into a mosfet, namely get rid of
> the power resistors and add a smaller source resistor, to make the
> power fet into a current sink. It's tempting, but then it might blow
> up. There's about 100 joules to dump.
can you use burn the energy in the ESR of the caps, perhps using an SCR.
-- ⚂⚃ 100% natural
--- Posted via news://freenews.netfront.net/ - Complaints to n...@netfront.net ---
>> I'm designing a box that has +48 and -48 power supplies, each bypassed
>> with 44,000 uF. I figure it will take forever to discharge when the
>> power is switched off, and I don't want to waste a heap of power on a
>> bleeder.
>> So I figured I could put an LED on the board to alert our test people
>> that the board is still hot, and have a DISCHARGE pushbutton. So what
>> I'm thinking is
>It requires a button to be pressed and held down by an operator - Murphy's >Law will apply.
There will be a bright orange LED on to remind the test tech that the
board is hot. They turn off power and push the button until the LED
goes off. I've done this before on other, bigger amps, and it worked
fine, no problems, but I used a chassis-mount power rocker switch and
a big aluminum-case bolt-down resistor.
Passive bleeders burn a lot of power and are slow and expensive.
>If one electro is not charged, faulty or has a broken connection, all hell >may break loose.
>At the very least, hefty reverse diodes across each electro are essential.
>... Phil
Across each cap? They are soldered to a PC board. If the connection to
the board is bad - unlikely - the connection to a diode will be bad,
too.
I checked my switching power supplies. Each, when off, seems to have
about a schottky drop in the reverse direction, so there's no worry
about reversing a supply through the discharge resistors.
> >On Apr 17, 5:30 pm, John Larkin <jlar...@highlandtechnology.com>
> >wrote:
> >> I'm designing a box that has +48 and -48 power supplies, each bypassed
> >> with 44,000 uF. I figure it will take forever to discharge when the
> >> power is switched off, and I don't want to waste a heap of power on a
> >> bleeder.
> >> So I figured I could put an LED on the board to alert our test people
> >> that the board is still hot, and have a DISCHARGE pushbutton. So what
> >> I'm thinking is
> >> Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> >> better than buying a switch rated for 1 amp at 100 volts DC.
> >> I wonder if I could dump the energy into a mosfet, namely get rid of
> >> the power resistors and add a smaller source resistor, to make the
> >> power fet into a current sink. It's tempting, but then it might blow
> >> up. There's about 100 joules to dump.
> On Apr 17, 5:30 pm, John Larkin <jlar...@highlandtechnology.com>
> wrote:
> > I'm designing a box that has +48 and -48 power supplies, each bypassed
> > with 44,000 uF. I figure it will take forever to discharge when the
> > power is switched off, and I don't want to waste a heap of power on a
> > bleeder.
> > So I figured I could put an LED on the board to alert our test people
> > that the board is still hot, and have a DISCHARGE pushbutton. So what
> > I'm thinking is
> > Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> > better than buying a switch rated for 1 amp at 100 volts DC.
> > I wonder if I could dump the energy into a mosfet, namely get rid of
> > the power resistors and add a smaller source resistor, to make the
> > power fet into a current sink. It's tempting, but then it might blow
> > up. There's about 100 joules to dump.
it on the limit but: it will barely do 500mA @ 100V, and the voltage
will quickly drop
abs. max Pd 60W, so that will also be just under the limit, and again
the voltage will
drop start to drop.
looks like anything over ~100ms is same as DC so the 2.5C/W tjc should
count
so with a block of alu big enough to eat the 150J without getting more
than a few deg
hotter even that little fet might just survive
On Tuesday, April 17, 2012 6:12:20 PM UTC-4, Spehro Pefhany wrote:
> On Tue, 17 Apr 2012 14:30:44 -0700, John Larkin
> <jlar...@highlandtechnology.com> wrote:
> >I'm designing a box that has +48 and -48 power supplies, each bypassed
> >with 44,000 uF. I figure it will take forever to discharge when the
> >power is switched off, and I don't want to waste a heap of power on a
> >bleeder.
> >So I figured I could put an LED on the board to alert our test people
> >that the board is still hot, and have a DISCHARGE pushbutton. So what
> >I'm thinking is
> >Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
> >better than buying a switch rated for 1 amp at 100 volts DC.
> >I wonder if I could dump the energy into a mosfet, namely get rid of
> >the power resistors and add a smaller source resistor, to make the
> >power fet into a current sink. It's tempting, but then it might blow
> >up. There's about 100 joules to dump.
> Why not use a little PCB-mount power relay (powered from the
> unfiltered supply)?
> That way you don't need the LED or the pushbutton- it's always
> discharged to a safe condition shortly after the power is turned off.
> Or use a MOSFET and a bright blinkie-LED if you want solid state,
> automation and confirmation, but the control gets a bit more complex.
> Discharging from + to - might warrant some analysis to make sure there
> are not any conditions under which Bad Things might happen of the
> polarities of one of the supplies relative to ground flips.
Wouldn't that just be a pair of reverse diodes across each capacitor to GND?
bloggs.fredbloggs.f...@gmail.com wrote:
>On Tuesday, April 17, 2012 6:12:20 PM UTC-4, Spehro Pefhany wrote:
>> On Tue, 17 Apr 2012 14:30:44 -0700, John Larkin
>> <jlar...@highlandtechnology.com> wrote:
>> >I'm designing a box that has +48 and -48 power supplies, each bypassed
>> >with 44,000 uF. I figure it will take forever to discharge when the
>> >power is switched off, and I don't want to waste a heap of power on a
>> >bleeder.
>> >So I figured I could put an LED on the board to alert our test people
>> >that the board is still hot, and have a DISCHARGE pushbutton. So what
>> >I'm thinking is
>> >Turns out that using a 30 cent mosfet and a 15 cent pushbutton is
>> >better than buying a switch rated for 1 amp at 100 volts DC.
>> >I wonder if I could dump the energy into a mosfet, namely get rid of
>> >the power resistors and add a smaller source resistor, to make the
>> >power fet into a current sink. It's tempting, but then it might blow
>> >up. There's about 100 joules to dump.
>> Why not use a little PCB-mount power relay (powered from the
>> unfiltered supply)?
>> That way you don't need the LED or the pushbutton- it's always
>> discharged to a safe condition shortly after the power is turned off.
>> Or use a MOSFET and a bright blinkie-LED if you want solid state,
>> automation and confirmation, but the control gets a bit more complex.
>> Discharging from + to - might warrant some analysis to make sure there
>> are not any conditions under which Bad Things might happen of the
>> polarities of one of the supplies relative to ground flips.
>Wouldn't that just be a pair of reverse diodes across each capacitor to GND?
Sure, maybe Schottky diodes, depending on what is there, should
protect just about anything.
On Wed, 18 Apr 2012 08:28:55 -0700 John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote in Message id:
<m0nto7199apsur57533lr0b60dvct3e...@4ax.com>:
>>> I'm designing a box that has +48 and -48 power supplies, each bypassed
>>> with 44,000 uF. I figure it will take forever to discharge when the
>>> power is switched off, and I don't want to waste a heap of power on a
>>> bleeder.
>>> So I figured I could put an LED on the board to alert our test people
>>> that the board is still hot, and have a DISCHARGE pushbutton. So what
>>> I'm thinking is
>>It requires a button to be pressed and held down by an operator - Murphy's >>Law will apply.
>There will be a bright orange LED on to remind the test tech that the
>board is hot. They turn off power and push the button until the LED
>goes off. I've done this before on other, bigger amps, and it worked
>fine, no problems, but I used a chassis-mount power rocker switch and
>a big aluminum-case bolt-down resistor.
>Passive bleeders burn a lot of power and are slow and expensive.
How about a passive bleeder that's switched into the circuit when the
power switch is turned off?
>>>> I'm designing a box that has +48 and -48 power supplies, each bypassed
>>>> with 44,000 uF. I figure it will take forever to discharge when the
>>>> power is switched off, and I don't want to waste a heap of power on a
>>>> bleeder.
>>>> So I figured I could put an LED on the board to alert our test people
>>>> that the board is still hot, and have a DISCHARGE pushbutton. So what
>>>> I'm thinking is
>>>It requires a button to be pressed and held down by an operator - Murphy's >>>Law will apply.
>>There will be a bright orange LED on to remind the test tech that the
>>board is hot. They turn off power and push the button until the LED
>>goes off. I've done this before on other, bigger amps, and it worked
>>fine, no problems, but I used a chassis-mount power rocker switch and
>>a big aluminum-case bolt-down resistor.
>>Passive bleeders burn a lot of power and are slow and expensive.
>How about a passive bleeder that's switched into the circuit when the
>power switch is turned off?
>Seems obvious, but maybe I'm missing something?
I'd have to tell when the power switch is off, or actually when
there's no AC power coming in. The switch is actually part of the AC
inlet/filter assembly, so I can't add a pole to it or anything like
that. So I'd have to sense 120-240 VAC and drive a discharge device.
This box will be CE/UL tested, so any primary circuits have to be
reviewed and hipot tested and all that. Hassle.
If anything went wrong with the logic, the resistors would smoke.
Unless I used huge power resistors.
A constant-power bleeder might be interesting, something with bipolar
transistors or depletion mosfets, but it would still need a heat sink.
100 joules into a 2-watt sink would still take 50 seconds to
discharge.
> How about a passive bleeder that's switched into the circuit when the
> power switch is turned off?
> Seems obvious, but maybe I'm missing something?
Like an interlock switch on a Plexiglas cover that switches on high
current bleeders? They worked well enough on high voltage, high current
power supplies in broadcast transmitters. I've also seen them on PC
board paneling routers, to keep idiots from losing their fingers.
-- You can't have a sense of humor, if you have no sense.
