# Moving Coil Meter + Shunt

Discussion in 'Homework Help' started by scol, Aug 2, 2011.

1. ### scol Thread Starter New Member

Aug 2, 2011
2
0
Hi,
I'm doing some course lab work and need to write up a report.
This is because of my laziness throughout the year and I missed a lab anyway. I'm a little hazy on the subject of moving coil meters and shunts/multipliers - I've been learning electronics for years and have a good understanding I just wanted to ask for some help to clear it up as this just had be a bit confused.
I have a MCM with an F.S.D. of 100μA and resistance of 1KΩ.
I have to A, modify the following circuits and calculate the component values to allow for voltages of 0 - 1V and 0 - 5V so I can read the voltages across V1, V2, V3, V1 + V2, V2 + V3, V1 + V2 + V3. The first is using the multiplier, i.e. resister in series with the MCM.
I worked out 8050Ω for the resistor to go in before the MCM (0 - 1V).
Then 48050Ω for 0 - 5V.
[Series]
>-- 5V PSU --- R1(470Ω) --- R2(150Ω) --- R3(330Ω) -->

The second circuit involves using a shunt which is the one I got really stuck with I have pages of formulas and calculations but can't get a consistent answer. Remeber this time it's parallel.
[Parallel]
>-- 5V PSU --- R1(470Ω) --- R2(150Ω) --- R3(330Ω) -->

Any help would be great or even just simplified explanations as I'm unsure on what to do?
Thanks

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Shunts are placed across MCM's to allow the user to measure higher currents than the meter's full scale deflection allows. They are not used for voltage measurements.

Suppose you want to transform your 100uA MCM to a 1A full scale current meter.

At 100uA and 1kΩ the meter requires 100mV to give full scale indication.

So your parallel shunt carries close enough to 1A (less only 100uA) which will produce 100mV and will therefore require a resistance of 100 milli-ohm, since Rshunt=100mV/1A=100 mΩ.

One used to buy meter shunts denoted as say 100A 50mV, 50A 50mV etc - meaning these would respectively produce 50mV with 100A [R=500uΩ] and 50mV with 50A [R=1mΩ] of current flowing. These would be readily compatible for range extension with a MCM of 50mV FSD.

For setting a smaller full scale current reading you may need to be more careful with the calculations. Say you want to expand the MCM range from 100uA to 1mA. In that case the shunt carries 900uA (1mA-100uA) for 100mV full scale. So the shunt resistance needs to be Rshunt=100mV/900uA=111.11Ω

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