19 Dec
2015
19 Dec
'15
6:15 a.m.
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.
Unfortunately I don't have enough of these to hand, and the local shop
(Maplin) does not have enough in stock.
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.
I checked the resistance of the backplane across the -12V inputs and it is
very high, so it seems unlikely that it could be that. But getting something
in series to measure load is going to be really awkward, I will see if I can
do it somehow. What would really help would be a way to bench test all the
outputs at high-ish loads to see if it is the PSU or not, but I don't know
of a practical way to do this with the high currents required.
Regards
Rob