At 02:50 AM 7/30/01 +1000, you wrote:

>How and why did you go about this??

Kelvin connections are just 4 -wire resistors, so the end
resistance doesn't matter. Imagine you took a bar of low
tempco metal and drilled 4 holes into it, like this:

     <--------- R ------------->
  A  B                         C  D
 ----------------------------------
| o  o                         o  o |
 ----------------------------------

On the outer two (A & D) put some hefty bolts in there to connect to
copper bus bars, on the inner two you stake a couple of pins in there
and solder to the bar and to wires. Want to trim the resistor?
Get your file and thin the middle down a bit. You use B & C as
the output. Typically 50, 60 or 100mV would be the full-scale
output voltage. Notice that if there is a bit of extra resistance
in connection A, say, it won't affect the measurement, there are
effectively 3 resistors, and only the middle one matters for
the measurement. Ra-b and Rc-d can vary a bit. Rb-c is the
critical one, but there are no high-current connections to it,
very little current flows through wires to B & C.

You don't usually make these, of course, there are companies that
specialize in just this sort of thing.

>And also would you recommend a 1mohm sense system? This seems
>insane to me.

I don't!, but mostly because it's floating on top of a +/-100V
signal. This means a very high CMRR is required, and it is
AC, so you may have to worry about a pF here and there.
Were it to be grounded or close to grounded and DC,
no problem, mon! I just wanted to point out (perhaps for
other applications) that it's not impossible.

> I often do medium/high power DC motor
>work and appreciate any info you can share here...

Probably useful for that kind of work, particularly
for low voltage. A 2 HP 24V motor would be ~63 A, so
0.001 Ohm would be reasonable (4W dissipation, 63mV
signal). Low side sensing is easier but you can high-
side sense it and level shift it with a current source.

Best regards,
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