Bob
While I agree with the general principle that a power supply connected to
a motherboard will tend to discharge its capacitors when unplugged, there are
a few caveats:
1. The power supply may shut down before all the power is drained from
ALL the capacitors.
2. Large capacitors that hold high voltages (in this case over 500 Volts)
for long periods of time tend to develop a "set" - this is one
aspect of the "electret effect". I have seen a capacitor that was
shorted (via a 150 Ohm resistor) to reduce its potential to
essentially zero, (this was observed on a meter connected
across the capacitor) which was then removed from the circuit,
and left standing overnight in a locked room. The next morning,
it had "recovered" over 60% of its original voltage.
3. "Good" power supply designs will have a small, high resistance
resistor across the large, high-voltage capacitors to "bleed"
the voltage down to safe levels within a minute or two. This is
NO guarantee of safety, as not all power supplies have a bleeder
resistor, and I once saw one that originally had a resistor, but
it burned out (OPEN).
Your only assurance of safety is to use a BIG, low-value resistor
(say 25 Watt, 100-150 Ohm), securely grounded, with an insulated
handle, to ground EVERY terminal of EVERY significant capacitor
just before working on the power supply. Depending on the capacitance,
it may take several seconds to reduce the voltage to a safe level.
One time constant ( = Resistance x Capacitance ) will reduce the
voltage by approx. 63%. [Example: 100 Ohm x 0.01 Farad (10,000 microFarad)
= 1 second. 1,000 Volts drops to 370 Volts after 1 second, to about 137
Volts after 2 seconds.] If the resistor gets hot, it should have been
bigger. You definitely DON'T want it to fail!
A lower resistance would be quicker, but runs the risk of causing arc
scars or damaging other parts.
Boyd Ramsay
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