> 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
