Date   : Tue, 14 Feb 2006 22:17:01 -0000
From   : "Mike" <profpep@...>
Subject: Re: Pop goes the PSU

Further to my other comments, it might be worth mentioning, in
connection with very old switcmodes, that it may be best to run them up
on a dummy load ( a couple of high wattage resistors), before risking
them going wrong whilts attached to the valuable old electronics. For
the BBC pwer unit, 4.7 Ohms at 10 Watts (so it will under run), on the 5
Volt rails and 12/15 ohms, 20 Watts on the 12Volt is good. I do this
with old units, and then look at the rail voltages and the noise. The
BBC power supply has an over voltage 'crowbar' circuit built in, so that
keeps the risk down. 

If you want to play very safe, run the switchmode upwith a 60 or 100
watt lightbult in SERIES with the live lead. If the power supply is
internally shorted, the bulb will light, if OK it will stay out or glow
dimly. This is an old TV engineers trick: the cold resistance of the
bulb is of the order of 40 Ohms, the hot resistance is about 500 Ohms.
It can stop a blown switching transistor damaging the rest of the
circuit, or a crowbar blowing the front end rather than the fuse.

I'd like to stress, however, that direct mains switchmode power supplies
are inherently dangerous things, there are lethal voltages preset, and
they should be handled with the greatest of care.

||\/||ike