VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Sat Dec 19 15:05:50 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Paul
> Koning
> Sent: 19 December 2015 19:26
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> Something to watch out for with high current sense resistors: they often
> have four connections rather than two.  This is a "Kelvin connection": two
> connections are for the current to be sensed, and the other two pick up
> resulting voltage difference.  The idea is that you get the voltage drop
> the resistor itself, as opposed to the voltage drop across the resistor
> the solder joints.  That makes a difference if you have a sense resistor
of just
> a few milliohms.
> If you have a high current supply but it doesn't use Kelvin connected
> sense, you're vulnerable to false shutdown if the solder joints are less
> perfect.  If so, switching to the Kelvin hookup, if you can see how, is a
> answer; failing that, make sure the joints are in excellent condition.

I can't tell if this is a Kelvin connected one or not, although I suspect it
probably is. You can see a picture with two here:
These are from a second board in the PSU, but they look identical to the one
on the problem board. It would appear to have 4 connections, but doesn't
look anything like the only Kelvin connected one I could find.

I tried testing it with my ESR/Low Ohms Meter, but the nominal value appears
to be 0.003R, which is below the range of my meter, the meter shows a value
between 0.00 and 0.02, but mostly 0.01. This is with the resistor in
circuit. It would be quite difficult to remove it. But I suspect that it may
be more the sense circuitry around the resistor than the resistor itself,
since the value seems to be OK. If that is the case then I may be in
trouble, the sense circuitry seems to be on a little riser board with loads
of surface mount components, including an LM339 comparator.



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