measuring meter shunt resistance

[Jim Strohm]

>Gents,
>Is there a simple way to accurately measure the low resistance of a current
>meter shunt? I am trying to get my plate current and grid current meters
>calibrated and I need to verify the values of the shunt R but the Ip shunt
>is less than .3 ohms and the Ig shunt is less than 1.6 ohms. My Fluke 77BN
>is of no real help at these low values.

Lane,

If you have a known low-value precision resistance and a stable low-voltage
source, you can put the unknown resistance (here, your shunt resistor) in
series with the known resistance, put the voltage across the string, and
measure
the voltage in the middle.

Naturally, you'll need a stiff enough voltage source to not sag when it's
connected to the resistance string, and a powerful enough string to
handle the current.  If you're looking to measure 0.3 ohms, though,
you should be able to use 10 volts from your bench supply and a 30 to 50
ohm 1-watt resistor from your junk box.

The Fluke will give a close enough ohm reading on that high a resistance
to be fairly accurate (resistive heating taken into consideration), and
the volt reading should allow you to calculate a 100-to-1 divider with
close enough precision.

Naturally, this assumes your 3 1/2-digit Fluke 77 is still somewhere close to
being in cal....

Jim N6OTQ

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