19 Dec
2015
19 Dec
'15
1:05 p.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] 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
the
resulting voltage difference. The idea is that you
get the voltage drop
across
the resistor itself, as opposed to the voltage drop
across the resistor
PLUS
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
current
sense, you're vulnerable to false shutdown if the
solder joints are less
than
perfect. If so, switching to the Kelvin hookup, if
you can see how, is a
good
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: http://1drv.ms/1micVN7.
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.
Regards
Rob