VAX 4000-500 PSU Overload?

Jon Elson elson at
Sat Dec 19 12:59:29 CST 2015

On 12/19/2015 04:56 AM, Robert Jarratt wrote:
> 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.
Cold light bulbs draw WAY more current than when the 
filament is hot.  Not sure if a 21 W 12V bulb could draw 20 
A cold, but it might.


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