VAX 4000-500 PSU Overload?

Peter Coghlan cctalk at
Sat Dec 19 05:50:40 CST 2015

> Actually, I may have just narrowed it down a bit. I connected a 12V DC bulb
> to the -12 output, in parallel to a 15R resistor, and the PSU shuts down
> immediately. The 12V bulb has a 21W rating. If my calculations are correct,
> that means the bulb draws 1.75A. The 15R resistor would draw 0.8A. So a
> total of 2.5A, which is well under the spec of 4A, but causes the PSU to
> shutdown. Using the 5W filament of the bulb does not cause the PSU to shut
> down.
> Is there a flaw in my calculations, or am I really not overloading the -12V
> supply? Just to recap I had a 15R resistor and a 12V DC 21W bulb in
> parallel, on the -12V output.

Bulbs have a lower than normal resistance when cold so it is possible that
you may be drawing more than 4A from the -12V line at the instant that you
apply power.

Bulbs are great for supplying a minimum load for a power supply that won't
regulate without it but they are not so great for supplying a load near the
maximum.  In the latter case, the maximum load will be exceeded for a short
time at startup.  This is not a problem if the power supply is designed to
cope with this (a power supply for motors for example) but computer power
supplies will typically be designed to react quickly to overload conditions.

Three or four 15 ohm resistors in parallel would probably make a better
load for the -12V line.

It seems hard to imagine what could normally draw something approaching 4A
on the -12V line in a VAX 4000 though.  If it was possible to get an ammeter
in series with that line maybe you could get an idea of what the normal draw
is on that line, if the power supply stays running long enough.  This would
give a better idea of what sort of dummy load is needed to simulate it.

Peter Coghlan.

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