3.4 AC VOLTMETER

General Rectifier Type

• The meter scale is usually calibrated to give the rms value of an alternating sine wave input.

• Practical rectifier are non-linear devices particularly at low values of forward current as illustrated in Figure 3.3.

• Hence the meter scale is nonlinear.

In this part the meter has low sensitivity because of the high forward resistance of the diode ( I = V/R ). Also the diode resistance depends on the temperature.

Hence, shunt resistance, 𝑹𝒔𝒉 is added to improve the linearity.

• The meter movement is shunted by a resistor, Rsh in order to draw more current through the diode D1 and move the operating point into the linear portion of the characteristic curve.

• In the negative half cycle, diode D2 conducts and the current through the measuring circuit, which is in an opposite direction, bypasses the meter movement.

• To convert alternating current to unidirectional current flow, which produces positive deflection when passed through a PMMC, the diode rectifier is used. Several types of rectifiers are selected such as a copper oxide rectifier, a vacuum diode or semiconductor or “crystal diode”.

Vrms = Vp/2 (Half-wave)

Vavg = Vdc = 0.318Vp

Sine wave

For example, if the output voltage from a half wave rectifier is 10 Vrms so the voltmeter will provide an indication of approximately 4.5 Vdc. Therefore, we can see that the pointer that deflected full scale when 10Vdc signal was applied. When we apply a 10V rms sinusoidal as waveform, the pointer will deflect 4.5 V.

This means that the ac voltmeter is not as sensitive as dc voltmeter. In fact, an ac voltmeter using half wave rectifier is only approximately 45% as sensitive as a dc voltmeter.

Actually, the circuit would probably be designed for full-scale deflection with a 10V rms alternating current applied, which means the multiplier resistor would be only 45% of the value of the multiplier resistor for 10V dc voltmeter. Since we have seen that the equivalent dc voltage is equal to 45% of the rms value of the ac voltage

The additional D2 is reversed biased on the positive half cycle and has virtually no effect on the behavior of the circuit. In the negative half cycle, D2 is forward biased and provides an alternate path for reverse biased leakage current that would normally flow through the meter movement and D1. The purpose of the shunt resistor Rsh is to increase the current flow through D1 during the positive half cycle so that the diode is operating in a more linear portion of its characteristic curve. Although this shunt resistor improves the linearity of the meter on its low voltage ac ranges, it also further reduces the ac sensitivity.

Video on AC Voltmeter