Capacitors in PSU are dangerous?

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kony wrote:

> >This mustn't be relied on. From my own personal experience (with a
> >monitor): the PSU failed and the monitor died. The cause of the failure
> >was an open-circuit high-value resistor (10 megohms?). This allowed a
> >large capacitor to charge with no discharge path other than leakage. The
> >effect was utterly dead-looking equipment holding a large charge even
> >when switched off.
>
> One difference would be that an ATX power supply is going to continue
> supplying 5VSB, that being another drain.

The +5vsb typically uses a small bias transformer and is unrelated to
the +300vdc buss. To confirm, monitor the +5vsb while you unplug the
AC cord or switch OFF the rear panel switch. The +5vsb goes away instantly
(or as soon as the +5vsb caps discharge.)

 

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On Thu, 15 Apr 2004 18:38:30 +0100, Tim Auton
<tim.auton@uton.[groupSexWithoutTheY]> wrote:


>You feel free to advise people to do whatever you like, but some
>people prefer to advise people of undetermined ability to err on the
>side of caution.
>
>Do I wait? No. But then I know what I'm doing.

It's beyond overkill to advise waiting 24 hours. Even if you didn't know
what you were doing you should've known that there's another very obvious
way an ATX power supply drains besides the bleeder resistors.

 

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Piotr Makley wrote:

> VWWall <vwall@DEADearthlink.net> wrote:
>
>
>>Regal wrote:
>>
>>>I read on some website that the capacitors in a PC's PSU can
>>>hold a charge for long after they have been switched off and
>>>that the charge could be fatal.
>>>
>>>Is this really so? Surely that is exaggerating?
>>
>>The capacitors in the input circuit of a PC PSU are charged to
>>~320 V DC. This can, indeed, be lethal, but they are shunted
>>by bleeder resistors which will reduce the voltage to a safe
>>value in a minute or two. By the time you get the unit out of
>>the computer case and remove it's cover, the voltage is
>>probably safe. To be sure, wait five minutes after unplugging
>>the PSU before touching anything inside.
>>
>>A PC monitor uses high voltage, (up to 25,000 V), on the CRT.
>>The tube glass envelope is used as a capacitor, and can hold a
>>charge for some time. Because the energy content is quite
>>low, contact with this very high voltage is usually not
>>deadly, but may result in serious injury from muscle reaction.
>> Don't remove the housing from a CRT monitor unless you
>>understand how to safely discharge this voltage!
>
>
>
> Thinkingof power supplies ...
>
> if a faster processor needing extra power was installed (say, it
> needs an extra 30 W) then would that noticeably reduce the life of
> the power supply?

The main enemy of any electronics is heat. 30W additional output
means ~10 W additional heat produced in the PSU. Some marginal
units can't support even their label wattage. It all depends on
how close to the limit you're pushing the supply. Also the PSU
fan takes its input air from within the case. This air will be hotter
due to the 30 additional CPU watts, and without good case ventilation
will raise the internal temperature of the PS still more.

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.

 

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Hello

"Regal" <stua_NOTTHISBITsmith@yahoo.co.uk> wrote in message
news:Xns94CCAAEC4846D628D1@208.42.66.156...
> I read on some website that the capacitors in a PC's PSU can hold a
> charge for long after they have been switched off and that the
> charge could be fatal.
>
> Is this really so? Surely that is exaggerating?

I hate to throw another issue into this thread but....everyone seems to have
an opinion so here is another. Voltage will burn you or your equipment,
furniture etc. amps will kill. Voltage can knock you off a platform or in
certain situations kill. Pacemaker comes to mind and certain heart
conditions. Why do I say this OSHA electrical safety course among many
others. I haven't looked at PSU for their specs but I do believe that common
sense will keep you safe be it voltage current/amps or what ever. Yes I know
that amps are related to watts and voltage but that is dependent on
resistance. Here is link to some terms and guidance. To many issues to
address in a limited time but the hazards are more than what will kill you.
Fire, Shock and death being the 3 foremost. The hazards are real the warning
labels are overkill CYA most of the time and common sense is what is needed.
I applaud someone just asking a question to find out. By the way I am
Superintendent of Safety for an Air Force base so I have some knowledge, no
expert but have researched more injuries than I have suffered by several
orders of magnitude. LOL

Something to think about:
Good judgment come from experience, experience well that comes from bad
judgment.
If you don't like that quote here is an original.
Stupid should be painful its up to you if its Motrin or morphine. Ask the
question!

http://www.utsa.edu/compliance/enviro/Section%2004/Sec04-01.html

 

[Guest]

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Kevin Lawton wrote:

> Timothy Daniels <TDaniels@NoSpamDot.com> wrote:
> | "Regal" wrote:
> || I read on some website that the capacitors in a PC's PSU can hold a
> || charge for long after they have been switched off and that the
> || charge could be fatal.
> ||
> || Is this really so? Surely that is exaggerating?
> |
> | A well-designed power supply has bleed resistors across the
> | big capacitors (usually the filtering electrolytics) and the charge
> | should be essentially dissipated within seconds, certainly a minute,
> | of shutdown. As for a "fatal" charge, what's the maximum voltage
> | used in a PC - 12 volts for the fans? Bottom line - have you ever
> | seen a "Danger! High Voltage" warning on a PC case?
>
> You do sometimes see that warning on the PSU !
> A switched-mode power supply works by 'pumping up' capacitors to a high
> voltage with sudden bursts of energy and then regulating the output down to
> the required level, switching the current off and on as needed. Draw more
> current and it uses bigger bursts of energy in each 'pump'. That's why it is
> so efficient in size terms. If you were using a linear power supply to
> supply the currents used inside a modern PC it would be far bigger and
> dissipate a lot more heat.

Close, but no cigar! There is no "pumping" of the input capacitors. They
are either charged to the peak line voltage from a 240 V line input, or they
are charged with a voltage doubler rectifier circuit from a 120 V line input.
The approximately 340 V DC derived is then switched at a rate of about 50,000
cycles and then transformed down to the desired output voltages. The time
the "switch" is on determines the output voltages, and PWM (pulse width
modulation) is used to regulate these voltages. The main reduction in
size is due to the small core needed for the output transformer. In addition,
the input transformer is eliminated completely.

> Under normal circumstances, the bleed resistors should do their job - but
> you wouldn't open up the PSU under normal circumstances would you ? Under
> fault conditions - what if a bleed resistor has failed ?

The most frequent reason I have opened a PSU is to replace the fan. Even
with a failed bleeder there is enough leakage current to discharge the
capacitors in less than the "many hours" often quoted. If you're really
worried, let the supply remain un-plugged overnight. Then be very careful
not to cut yourself on the sharp metal edges in most PSUs!

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.

 

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kony wrote:

> On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem <a$-b$1@ms3.org.uk>
> wrote:
>
>
>>VWWall wrote:
>>
>>>The capacitors in the input circuit of a PC PSU are charged to
>>>~320 V DC. This can, indeed, be lethal, but they are shunted by
>>>bleeder resistors which will reduce the voltage to a safe value
>>>in a minute or two. By the time you get the unit out of the computer
>>>case and remove it's cover, the voltage is probably safe. To be sure,
>>>wait five minutes after unplugging the PSU before touching anything inside.
>>
>>This mustn't be relied on. From my own personal experience (with a
>>monitor): the PSU failed and the monitor died. The cause of the failure
>>was an open-circuit high-value resistor (10 megohms?). This allowed a
>>large capacitor to charge with no discharge path other than leakage. The
>>effect was utterly dead-looking equipment holding a large charge even
>>when switched off.
>
>
> One difference would be that an ATX power supply is going to continue
> supplying 5VSB, that being another drain.

Also the main switching supply will continue to operate until the capacitors
are down to <~ 250 V, still more drain. With the excption of those PSUs
that have heat sinks at an input voltage potential, it's very hard to
get even your finger tips on the full voltage. A little care is still
advised.

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.

 

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On Thu, 15 Apr 2004 14:12:48 -0700, ric <nospam@home.com> wrote:

>kony wrote:
>
>> >This mustn't be relied on. From my own personal experience (with a
>> >monitor): the PSU failed and the monitor died. The cause of the failure
>> >was an open-circuit high-value resistor (10 megohms?). This allowed a
>> >large capacitor to charge with no discharge path other than leakage. The
>> >effect was utterly dead-looking equipment holding a large charge even
>> >when switched off.
>>
>> One difference would be that an ATX power supply is going to continue
>> supplying 5VSB, that being another drain.
>
>The +5vsb typically uses a small bias transformer and is unrelated to
>the +300vdc buss. To confirm, monitor the +5vsb while you unplug the
>AC cord or switch OFF the rear panel switch. The +5vsb goes away instantly
>(or as soon as the +5vsb caps discharge.)

It hasn't been unrelated in the units I've traced. They looked pretty
standard... right after rectified there was the voltage doubler, the
large caps everyone is concerned about, with the bleeder resistors across
them, and the power leading to the 5VSB transformer was directly
connected, parallel to the bleeder resistors. Perhaps I haven't looked
closely enough, at enough different PS 5VSB circuits, but I've not seen
anything to suggest any unit deviated from this.

 

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VWWall wrote:

> Also the main switching supply will continue to operate until the capacitors
> are down to <~ 250 V, still more drain.

Not ATX PSUs. The switching circuitry is inhibited as soon as the
PS_ON signal goes high. If the PSU is under light load, the 300v
buss could still be well over 300vdc.

 

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True.

--
DaveW



"Regal" <stua_NOTTHISBITsmith@yahoo.co.uk> wrote in message
news:Xns94CCAAEC4846D628D1@208.42.66.156...
> I read on some website that the capacitors in a PC's PSU can hold a
> charge for long after they have been switched off and that the
> charge could be fatal.
>
> Is this really so? Surely that is exaggerating?

 

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kony wrote:

> >The +5vsb typically uses a small bias transformer and is unrelated to
> >the +300vdc buss. To confirm, monitor the +5vsb while you unplug the
> >AC cord or switch OFF the rear panel switch. The +5vsb goes away instantly
> >(or as soon as the +5vsb caps discharge.)
>
> It hasn't been unrelated in the units I've traced. They looked pretty
> standard... right after rectified there was the voltage doubler, the
> large caps everyone is concerned about, with the bleeder resistors across
> them, and the power leading to the 5VSB transformer was directly
> connected, parallel to the bleeder resistors.

The primary of the +5vsb bias transformer connected to a 300VDC source?
In parallel with the bleed resistors? Poor transformer. Must have HEAVY
GAUGE windings, or a short life span.

 

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kony <spam@spam.com> wrote:
>On Thu, 15 Apr 2004 18:38:30 +0100, Tim Auton
><tim.auton@uton.[groupSexWithoutTheY]> wrote:
>
>>You feel free to advise people to do whatever you like, but some
>>people prefer to advise people of undetermined ability to err on the
>>side of caution.
>>
>>Do I wait? No. But then I know what I'm doing.
>
>It's beyond overkill to advise waiting 24 hours. Even if you didn't know
>what you were doing you should've known that there's another very obvious
>way an ATX power supply drains besides the bleeder resistors.

I'm assuming the worst case - ie component failure, where the only
discharge of the caps is self-discharge. In that case though, 24 hours
may not be enough. Hmmm, I think we need some experimental data.


Tim
--
Love is a travelator.

 

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>Some switched mode power supplies start by rectifying the mains voltage to
>350+volts dc and storing this on a capacitor.That is enough to kill you in
>certain circumstances.All capacitors from main powered equipment should be
>treated with respect.
>Ken Reynolds

I'd tend to agree, esp. with switched mode supplies. Even powerful audio amps
with linear or switching supplies can be potentially dangerous. Old CRT's are
dangerous, possibly newer ones as well.

I work on all this stuff occasionally, but I take precautions. I try to use
one hand only, which avoids a circuit across the heart.

There should be bleeder resistors that will leak off the charge eventually.
You can also make a shorting rod and discharge them before repairs.

I'm just a hobbyist / low voltage tech, not a high-voltage tech, but I can't
recall anyone dying from servicing a home computer. There have certainly been
a few deaths among hams servicing 2kw homebrew amps. Generally among older
persons, but it's enough juice to kill younger guys too.

Obviously i am talking about _unplugged_ gear, I hope this goes without saying.

Michael

 

[overlord]

Archived from groups: alt.comp.hardware,alt.comp.hardware.pc-homebuilt,uk.comp.homebuilt,alt.comp.hardware.homebuilt (More info?)

Under the right<?> circumstances yes.
Your UK voltage is like 230volts? That's the rms value, like an average
effective value. The peak value would be 325 volts then.
Also, the capacitors take a finite amount of time to charge up in a circuit.
They also take a finite amount of time to discharge. They can be shorted
in such a way to discharge more quickly, kicking out many times their inputted
voltage in one great electron orgasm.

You have a camera?
You push the button half way and see the little blinking light; it's charging a capacitor.
The light goes steady; the capacitor is charged.
You push the button the rest of the way in and it discharges in a smaller time constant
to flash the flash bulb. Those itty bitty batteries in the camera can't flash it on their output
voltage on their own. The discharge is many times the voltage that the battery slowly put
into the capacitor. If those little 1.5volt (or whatever) batteries can charge that capacitor
to discharge and flash that flashbulb, think what a nice flash you can get from discharging
the capacitor that you charged with the 230volt line current, properly or improperly
handled....

You probably won't light up like a flash bulb.... but you'll think you did.


On Thu, 15 Apr 2004 16:48:08 +0100, Regal <stua_NOTTHISBITsmith@yahoo.co.uk> wrote:

>I read on some website that the capacitors in a PC's PSU can hold a
>charge for long after they have been switched off and that the
>charge could be fatal.
>
>Is this really so? Surely that is exaggerating?

~~~~~~
Bait for spammers:
root@localhost
postmaster@localhost
admin@localhost
abuse@localhost
postmaster@[127.0.0.1]
uce@ftc.gov
~~~~~~
Remove "spamless" to email me.

 

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Overlord wrote:

> Under the right<?> circumstances yes.
> Your UK voltage is like 230volts? That's the rms value, like an average
> effective value. The peak value would be 325 volts then.
> Also, the capacitors take a finite amount of time to charge up in a circuit.
> They also take a finite amount of time to discharge. They can be shorted
> in such a way to discharge more quickly, kicking out many times their inputted
> voltage in one great electron orgasm.

Many times their charging *current*? Yes.
Many times their charging *voltage*? No.

> You have a camera?
> You push the button half way and see the little blinking light; it's charging a capacitor.
> The light goes steady; the capacitor is charged.
> You push the button the rest of the way in and it discharges in a smaller time constant
> to flash the flash bulb. Those itty bitty batteries in the camera can't flash it on their output
> voltage on their own. The discharge is many times the voltage that the battery slowly put
> into the capacitor. If those little 1.5volt (or whatever) batteries can charge that capacitor
> to discharge and flash that flashbulb, think what a nice flash you can get from discharging
> the capacitor that you charged with the 230volt line current, properly or improperly
> handled....

Nice story, but technically flawed. A capacitor will only charge up to
the peak voltage applied across it. I think you will find that cameras
use voltage multipliers to achieve their flash voltage, like a small
fly back circuit or something. [I've never taken one apart, so I don't
know exactly.]

 

[Guest]

Archived from groups: alt.comp.hardware.homebuilt (More info?)

>The capacitors in the input circuit of a PC PSU are charged to
>~320 V DC.

Most home DMM's measure to 2k volts. So you can easily test the circuit before
putting your hands in there.

Obviously that may not work on CRT's and TV's.

I picked up a TV repair manual for 5 bucks used. Had plenty of tips on working
with high-voltage circuits. Should be plenty of stuff available at the local
library. A little reading may save your life.

Michael

 

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>The main enemy of any electronics is heat. 30W additional output
>means ~10 W additional heat produced in the PSU.

Are you sure? I'd believe that ratio in a linear supply, but in a switching
supply?

Michael

 

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jealous xmp wrote:

> >The main enemy of any electronics is heat. 30W additional output
> >means ~10 W additional heat produced in the PSU.
>
> Are you sure? I'd believe that ratio in a linear supply, but in a switching
> supply?

Being that switching PSUs used in PCs are typically 70-75% efficient,
yeah - that sounds right. 40w more input, 30w more output, 10w more heat.
That would be 75% efficiency.

Now, how well that heat is dealt with....*that* is the big question.

 

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VWWall <vwall@DEADearthlink.net> wrote:

> Close, but no cigar! There is no "pumping" of the input
> capacitors. They are either charged to the peak line voltage
> from a 240 V line input, or they are charged with a voltage
> doubler rectifier circuit from a 120 V line input. The
> approximately 340 V DC derived is then switched at a rate of
> about 50,000 cycles and then transformed down to the desired
> output voltages. The time the "switch" is on determines the
> output voltages, and PWM (pulse width modulation) is used to
> regulate these voltages. The main reduction in size is due to
> the small core needed for the output transformer. In
> addition, the input transformer is eliminated completely.

As a related aside ....

I think those room light dimmers (for normal incandescent bulbs)
work in a similar way to the way you describe for a switched mode
PSU. In other words chopping up the output voltage into very small
slices.

Would such a device set to "dim" register less on the household
electric meter than when set to "bright"?

 

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Piotr Makley wrote:

> VWWall <vwall@DEADearthlink.net> wrote:
>
>
>>Close, but no cigar! There is no "pumping" of the input
>>capacitors. They are either charged to the peak line voltage
>>from a 240 V line input, or they are charged with a voltage
>>doubler rectifier circuit from a 120 V line input. The
>>approximately 340 V DC derived is then switched at a rate of
>>about 50,000 cycles and then transformed down to the desired
>>output voltages. The time the "switch" is on determines the
>>output voltages, and PWM (pulse width modulation) is used to
>>regulate these voltages. The main reduction in size is due to
>>the small core needed for the output transformer. In
>>addition, the input transformer is eliminated completely.
>
>
> As a related aside ....
>
> I think those room light dimmers (for normal incandescent bulbs)
> work in a similar way to the way you describe for a switched mode
> PSU. In other words chopping up the output voltage into very small
> slices.

They are similar except they work directly with the AC voltage. When
set to "dim" they let less than the full half of the sinewave AC in each
direction through to the lamp. The thermal lag of the lamp filament filters
out the short "pulses" and the lamp produces less light. The PDM switch
in a PSU works with DC voltage of both polarities and is switched at
a high (~50 KHz) frequency. This produces a waveform the output transformer
can handle. At the high frequency the core can be much smaller than a
transformer working at line (50/60 Hz) frequency. The efficiency is also
much higher since the transistor switches are either on or off and don't
dissipate much power internally in either state.
>
> Would such a device set to "dim" register less on the household
> electric meter than when set to "bright"?

Yes. The electric meter is an integrating device. Less total energy flow
when the lamps are on dim equals less energy registered.

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.

 

[Guest]

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>Close, but no cigar! There is no "pumping" of the input capacitors. They
>are either charged to the peak line voltage from a 240 V line input, or they
>are charged with a voltage doubler rectifier circuit from a 120 V line input.

I could see how the voltages could get high. It's 120 volts, but that's RMS.
Each alternating dip is 170 volts peak. And 170*2 would be 340 volts (doubling
circuit).

A doubling or quadrupling circuit could be viewed as "pumping" IMHO.

FWIW, anything over 14 volts is generally referred to as dangerous. I've built
small supplies and I personally wouldn't put my hands across a +/- 30 volts.

Even the low current 90 volts of a phone circuit (it was ringing) gave me a
jolt once.

Michael

 

[Guest]

Archived from groups: alt.comp.hardware.homebuilt (More info?)

>The maximum voltage inside a PSU is the mains of course!

I haven't looked at the schematics of PC PSU lately, but it would depend on the
circuit. There may be a doubling circuit as someone noted, for use in US as
well as Europe's higher rails.

You can't say definitively say that the PSU is at mains voltage peak. 170 to
340 volts is my best guess. I'd have to see the specific circuit. Don't
forget all voltages are 50 percent higher potentially due to the mains being
rated in RMS, not peak or peak-to-peak.

In the end, the voltage perhaps isn't the only factor. The capacitors would
vary from supply to supply and determine the upper bound of energy stored by
the cap.

Q=CV

The couloumbs of charge is just capacitance times voltage. I believe the
energy would be 1/2CV^2. So the energy is proportional to capacitance, but the
square of the voltage.

In the end, that energy and charge determine the current and time across the
body during it's discharge phase.

Michael

 

[Guest]

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>They are similar except they work directly with the AC voltage. When
>set to "dim" they let less than the full half of the sinewave AC in each
>direction through to the lamp. The thermal lag of the lamp filament filters
>out the short "pulses" and the lamp produces less light.

Some of the old dimmers were just a thyristor like a triac in the circuitry.
The "gate" voltage would determine when it "tripped." Thus the output is a
chopped up waveform, which lowers the average energy emitted by the element in
the bulb.

I believe this is why the dimmers and related stuff sometimes cause a visible
flashing.

Michael

 

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Chris Stolworthy wrote:
> "Regal" <stua_NOTTHISBITsmith@yahoo.co.uk> wrote in message
>
>> I read on some website that the capacitors in a PC's PSU can
>> hold a charge for long after they have been switched off and
>> that the charge could be fatal.
>>
>> Is this really so? Surely that is exaggerating?
>
> No they are serious, I had the Unfortunate experience not too
> long ago of puncturing one on accident. Nasty little shock,
> let me tell ya. Some nice electrical burns as well.

We used to take a 200V 0.1 uF capacitor and stick the leads into a
110 V a.c. socket. Everyone knows you can't charge a cap from an
a.c. supply, right? Then hand it to someone to hold. Carefully.

--
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

 

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On Fri, 16 Apr 2004 00:11:09 GMT, kurt_SPAMLESS@hotmail.com (Overlord)
wrote:

<snip>

>They also take a finite amount of time to discharge. They can be shorted
>in such a way to discharge more quickly, kicking out many times their inputted
>voltage in one great electron orgasm.

A rapid discharge will do nothing to raise the voltage, it's the current
you're thinking of.... at most the voltage would be the peak input voltage
(per the input from, position in the circuit, not necessarily the mains AC
voltage).

 

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ric <nospam@home.com> wrote in message news:<407EFAD0.A2AF5241@home.com>...
> kony wrote:

> The +5vsb typically uses a small bias transformer and is
> unrelated to the +300vdc buss. To confirm, monitor the
> +5vsb while you unplug the AC cord or switch OFF the
> rear panel switch. The +5vsb goes away instantly (or as
> soon as the +5vsb caps discharge.)

I checked some +5Vstandby circuits because I wanted to know why the
one in my Raidmax/Powmax had a much smaller transformer than the
others, and I found that they didn't use separate capacitors but were
powered from the same big ones that ran the main PSU. Most even ran
the +5Vstandby transformer at exactly the same frequency, although
Enermax used 2x and Raidmax/Powmax used 4x. The one exception may have
been a super-cheap 250W (PC Power & Cooling shows it as an example of
a bad PSU) where the +5Vstandby circuit was held on a completely
separate circuit board, but I don't really remember.