Kevin Lawton wrote:

Timothy Daniels 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.