Cap reformation question

Dave Dunfield dave04a at
Wed Dec 28 09:07:36 CST 2005

>    I understand the principle of using a light as a current limiter, but 
> I have one question - say I start with a 7.5W bulb, how long should I 
> run the PSU at that level, and should I then step up through 20W, 40W, 
> etc?  If so, I have bulbs up to 200W; where should I stop?

Part 1 answer:

You should stop when the line voltage to the equipment reaches a normal
value - with an unloaded linear PS, this should be LONG before you get to
a 200w bulb.

You should perform the procedure with the power-supply under no load.
(ie: remove all the cards). If it is truly under no load, then you won't need
to move "up" in lamp sizes, because as the caps charge, the current drawn
by the supply will fall off toward zero, and the voltage dropped across the
lamp will drop toward zero. As the current drops, the lamp filiment will cool
causing it's resistance to drop as well. If there is no load at all, then the line
voltage to the system would reach "normal" even with a low wattage lamp in

In practice this may not happen with a really low wattage bulb due to some
fixed current draw by such things as the chassis fans and a bleed resistor
on the supply (if present) - You can disconnect these things and still bring
up up to normal voltage through a low-wattage bulb. I prefer to do this,
because if a component fails (like a cap shorting), the lower wattage bulb
you have in series, the lower the current that will flow in the "short" circuit
- I like to let long out of service linear power supplies "idle" thought a 25 or
40w bulb for a while before I go to higher power/load tests.

Part 2 answer.

Assuming you have the power supply under no load, even with a small
bulb, the input voltage will climb toward line level rather quickly. It is best
to reform caps at a slower rate.

Best thing to use is a variac, so you can actually control the voltage being
applied to the caps, and bring it up in steps, allowing the caps to form for
a good long time at each step. If you don't have a variac, you can fairly easily
divide line voltage down to 1/3 and later 1/2 by using 3 and then 2 same
wattage bubs in series, and connecting the equipment across the bulb closest
to the neutral side of the line - again is is assuming that the equipment will
eventually charge to virtually no power draw allowing the line voltage to drop
evenly across the bulbs. Note that using different wattage bulbs to control the
division does not usually work as well as you would like, as the higher watt
bulb will stay "cold", dropping most of the voltage across the lower watt bulb.

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools:
com             Collector of vintage computing equipment:

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