Electronics Principles V11

Clive W. Humphris

ANALOGUE MULTI-METER: Measuring Voltage. (Voltmeter)  

To enable an analogue meter movement to measure voltages greater than its designed capability, a resistor needs to be inserted in series to limit the current flowing through the meter coil, to that which produces a full scale needle deflection (FSD).

A typical circuit diagram is shown in the lower half of the display which has four voltage ranges. 1Volt, 10V, 100V and 1kVolt. In practice these would be switched positions from a single input.

Each of the series resistor values is calculated using the formula shown. Here you can experiment with not only different voltage ranges, but also various values for the meter movement.

Suppose you are measuring 50V and using our values for FSD and Ra. You would select the 100V input. The total circuit resistance would be 999k + 1k = 1Mohms. Therefore the current would be 50 ÷ 1000,000 = 50µA = half full scale deflection. Note. The total meter circuit resistance (Rm + Ra) always equals input voltage range ÷ the current required to give a full scale needle deflection on the meter.

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