[GreenKeys] Milliamp meter shunting

[John Nagle]

> From: On Behalf Of Kenneth Gartland
> Subject: [GreenKeys] Milliamp meter shunting...
 >
> I have a couple of old Simpson ma meters rated
> at 100 ma...
> I went to use them to test a known 40ma cct.
> the other day, and both pegged at over the scale limit...
>
> Do these older meters require some type of "Shunt"
> across the terminals to get an accurate reading?
 >
> Ken
> VE3-HMQ

    Meter movements are voltage devices.  Current
meters are voltage meters with a resistor in
parallel with the meter movement.  Sometimes
the resistor is internal; sometimes it's
external.

    Figuring out the shunt resistor value
for a meter movement is straightforward.
You need a voltage meter such as a DVM
and an adjustable voltage source that
can deliver a few volts DC. Hook the
DVM and your meter in parallel, and
apply a small test voltage. Turn up the
voltage until you reach the 100mA reading
on the meter. Call that 100ma value I1.
Record the voltage on the DVM.  Call that V1.

    Then use the DVM to measure the resistance
of the meter.  Call that R1.

    Given R1, V1, and I1, compute
the shunt resistor value R2.

    From E = I*R, we have

    V1 = I1 * Rtot
or
    Rtot = V1 / I1

where Rtot is the total resistance of the
meter and the shunt resistor R2 in parallel.
As  a rough approximation, Rtot can be used as
a shunt resistor value. If Rtot is much smaller than
R2, because the meter has a high resistance, that's
enough. I not, you have to do the parallel resistor
calculation.

    So, from the parallel resistor formula

    Rtot = 1/(1/R1 + 1/R2)

We know Rtot and R1, and need to solve for R2

    1/Rtot = 1/R1 + 1/R2

    1/R2 = 1/Rtot - 1/R1
    R2 = 1/(1/Rtot - 1/R1)

or

    R2 = 1/(1/(V1/I1) - 1/R1)

or

    R2 = 1/(I1/V1 - 1/R1)

R2 is thus the resistance for the shunt resistor.

Wattage computation is left as an exercise.

			John Nagle