VAX 4000-500 PSU Overload?
robert.jarratt at ntlworld.com
Sat Dec 19 04:56:28 CST 2015
> -----Original Message-----
> From: Robert Jarratt [mailto:robert.jarratt at ntlworld.com]
> Sent: 19 December 2015 09:56
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Cc: 'cctech at classiccmp.org'
> Subject: RE: VAX 4000-500 PSU Overload?
> A bench test of the PSU with a small dummy load saw the PSU appear to
> work fine. It might help to load it more to see if it still operates
> under load. The specs for the PSU are:
> 22 amperes at +12.1 Vdc
> 60 amperes at +5 Vdc
> 7 amperes at +3.4 Vdc
> 4 amperes at -12.1 Vdc
> Has anyone got any suggestions for how to create a higher load in a fairly
> simple manner? It would take a lot of resistors or vehicle light bulbs to
> some of those loads, and something heftier than a breadboard to connect
> them together.
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
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.
Looks like I have an avenue to explore.
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