VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Sat Dec 19 08:15:34 CST 2015

> Bulbs have a lower than normal resistance when cold so it is possible that
> may be drawing more than 4A from the -12V line at the instant that you
> 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
> maximum.  In the latter case, the maximum load will be exceeded for a
> time at startup.  This is not a problem if the power supply is designed to
> with this (a power supply for motors for example) but computer power
> supplies will typically be designed to react quickly to overload
> Three or four 15 ohm resistors in parallel would probably make a better
> 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
> 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.



More information about the cctalk mailing list