Re: Inside power supplies: Where are the shocks?

[Rubywand <rubywand@swbell.net>]

stephen e buggie writes ...
> 
>        WHERE ARE THE ELECTRIC SHOCKS INSIDE COMPUTER POWER SUPPLIES?
>        ============================================================
> 
> As regular readers already know, I modify IBM power supplies to be used as
> external units on Apple IIgs and IIe.   Have made about 175 units since
> April 1996.
> 
> Virtually every unit has a sticker with ominous warnings --- DO NOT OPEN
> CASE --- DANGEROUS SHOCKS ARE INSIDE!
> 
> I've hacked inside MANY power supplies and have not yet felt a jolt!
> Where are these shocks lurking?  Of course, the power is OFF when I do the
> hacking.  I stay away from the large heat-sinks.  There are big capacitors
> inside, but how long do these hold the power?
> 
> I do various tests during assembly of Bugg-Power units, and I am usually
> inside within 20-30 seconds after the power has been on with some tests.
>
 ....


     You are most likely to find potentially dangerous voltages lurking
near the AC cord 'end' of the unit. For sure, if the power supply is
still plugged in to the AC line, there are going to be dangerous AC
voltages near the ON/OFF switch and on unswitched connections to the AC
power cord. Naturally, you should unplug the AC power cord of the power
supply before opening the power supply case.

     The capacitors inside a switching power supply will tend to
discharge once the power supply is turned OFF. How much of a charge
remains and where after a few minutes depends upon the particular
design.

     The most dangerous charges are likely to remain on capacitors with
high voltage ratings-- like 200VDC or so-- marked on the capacitor. They
have high voltage ratings because they need them. There are often 2-4
such capacitors and they are typically located near the AC power cord.
Some of these capacitors may retain a high enough voltage to deliver a
shock for a hour or more after the power supply is turned OFF and
unplugged.

     You are unlikely to find dangerous voltages on the output side of a
typical PC power supply. Since the power supply should not be powered ON
without a decent load on the outputs-- at least an Amp or so on the +5V
line-- the outputs usually have a chance to discharge fairly quickly
once the power supply is turned OFF. Besides, the highest PC supply
output voltages are normally +12V and -12V. Any charge remaining on the
16VDC, etc. rated output capacitors can cause a spark when grounded but
is unlikely to produce much of a shock.

     A good way to discharge the higher voltage PC power supply
capacitors is via a 50-100 Ohm 10 Watt power resistor. Touch or clip one
end of the resistor to one end of the capacitor and touch the other end
of the resistor to the other end of the capacitor several times. To
discharge the lower voltage output caps, you can connect one end of the
resistor to the Ground lead and touch the other end to the other output
leads. 

     Even after discharging the caps, it is best to act as though a
charge may linger somewhere (like on a smaller cap with a high V
rating).  If you are going to be working on and touching circuit board
connections, discharge big capacitors and use a Volt meter to check
around for stray voltages.



Rubywand