VAX 4000-500 PSU Overload?

[Paul Koning]

> On Dec 19, 2015, at 2:18 PM, Robert Jarratt <robert.jarratt at ntlworld.com> wrote:
> 
>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Dave
>> Wade
>> 
>> Seeing as it already seems to have over current sensing, I guess it has some
>> small resistors in there so perhaps it is possible to measure the current. I
>> notices one looks like this:-
>> 
>> http://uk.farnell.com/welwyn/oar1-r020fi/resistor-1-0r020/dp/1200359
>> 
> 
> 
> Ah! So that is what those things are! I definitely think this one is
> detecting something wrong and shutting down the 12V side. 

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.

	paul