1. Chelmsford Amateur Radio Society Advanced Course (10) Measurements Part-1 - Meters

2. Types of Movement & Constructional Details
Meter movements for DC and AC measurements can be moving-coil, moving-iron, hot-wire and thermocouple. The latter are good for measuring RF amps.
For DC and low frequency AC, we will only consider the moving coil type.
For AC a diode rectifier is needed in the meter circuit
The maximum movement possible is called Full Scale Deflection, FSD S N

3. Ammeter
The basic meter movement is typically 1mA for FSD and has internal resistance of 100 ohms.
Its range has to be extended to the required range by a Shunt Resistor, which be calculated by OHMS LAW.
Meter
Internal ‘r’
Shunt R
Example for 1 Amp FSD scale:-
Calculate the voltage drop across the meter from V = I.R
We get V = 1*10-3 x 100 = 0.1 Volts.
If 1mA flows through the meter then amps must flow through shunt.
There is 0.1 volts across the shunt.
As R = V / I, then the shunt is 0.1 / = Ohms.

4. Voltmeter
Meter
Internal r
Multiplier Resistor
To measure voltages consider the basic meter movement with typical 1mA FSD and internal resistance of 100 Ohms. This is extended to the required voltage range by a series resistor. Called a Multiplier Resistor.
The volts drop across the meter is 0.1 Volts so to have a full scale of 1V it is required to drop ( ) Volts across the series resistor
Apply OHMS LAW: R = V / I, with V = = 0.9 Volts and I = 1 mA
Therefore the multiplier resistor is V / I = 0.9 / .001 = 900 Ohms.
- and the meter is a total of 1000 Ohms input resistance

5. Ohmmeter
Red
Meter
Internal r
Calibration R +
Black
To set for ZERO OHMS the two probe leads red and black are joined and the calibration R adjusted for FSD.
Measuring a resistance introduces another series resistor into the circuit which reduces the current through the meter.
Thus the scale can be calibrated in Ohms.

6. Meter Loading
470K 1V
Internal
Resistance &
Multiplier
Potential Divider opposite should have a voltage at the midpoint of 0.5 Volts
Total resistance of the meter is 1000 Ohms; 100 Ohms internal resistance plus 900 Ohms multiplier resistance.
The lower 470k is shunted by 1000 Ohms so the bottom pair becomes; 470k x 1000 Ohms / (470 k ) Ohms =
The volts measured will be / x 1 = volts. Oops!
Care must be taking when measuring voltages across large resistors.
Use a high impedance digital voltmeter or a very high ohms per volt analogue voltmeter. NB: DVMs may be >10MOhm impedance