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

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

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 0.999 amps must flow through shunt. There is 0.1 volts across the shunt. As R = V / I, then the shunt is 0.1 / 0.999 = 0.1001 Ohms.

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 (1 - 0.1) Volts across the series resistor Apply OHMS LAW: R = V / I, with V = 1 - 0.1 = 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

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.

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 + 1000) Ohms = 997.79 The volts measured will be 997.79 / 470997.79 x 1 = 0.002 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