Electrical indicating and test instruments

6.2.8 Electronic analogue voltmeters

Electronic voltmeters differ from all other forms of analogue voltmeters in being active rather than passive instruments. They have important advantages compared with other analogue instruments. Firstly, they have a high input impedance that avoids the circuit[1]loading problems associated with many applications of electromechanical instruments. Secondly, they have an amplification capability that enables them to measure small signal levels accurately.

The standard electronic voltmeter for d.c. measurements consists of a simple direct-coupled amplifier and a moving-coil meter, as shown in Figure 6.9(a). For measurement of very low-level voltages of a few microvolts, a more sophisticated circuit, known as a chopper amplifier, is used, as shown in Figure 6.9(b). In this, the d.c. input is chopped at a low frequency of around 250 Hz, passed through a blocking capacitor, amplified, passed through another blocking capacitor to remove drift, demodulated, filtered and applied to a moving-coil meter.

Three versions of electronic voltmeter exist for measuring a.c. signals. The average-responding type is essentially a direct-coupled d.c. electronic voltmeter with an additional rectifying stage at the input. The output is a measure of the average value of the measured voltage waveform. The second form, known as a peak-responding type, has a half-wave rectifier at the input followed by a capacitor. The final part of the circuit consists of an amplifier and moving-coil meter. The capacitor is charged to the peak value of the input signal, and therefore the amplified signal applied to the moving-coil meter gives a reading of the peak voltage in the input waveform. Finally, a third type is available, known as an r.m.s.-responding type, which gives an output reading in terms of the r.m.s. value of the input waveform. This type is essen[1]tially a thermocouple meter in which an amplification stage has been inserted at the input.

6.2.9 Calculation of meter outputs for non-standard waveforms

The examples below provide an exercise in calculating the output reading from various types of analogue voltmeter. These examples also serve as a useful reminder of the mode of operation of each type of meter and the form that the output takes.

Example 6.1

Calculate the reading that would be observed on a moving-coil ammeter when it is measuring the current in the circuit shown in Figure 6.10.

Solution

A moving-coil meter measures mean current.

Example 6.2

Calculate the reading that would be observed on a moving-iron ammeter when it is measuring the current in the circuit shown in Figure 6.10.

Solution

A moving-iron meter measures r.m.s. current.

 

Example 6.3

A dynamometer ammeter is connected in series with a 500 Ω resistor, a rectifying device and a 240 V r.m.s. alternating sinusoidal power supply. The rectifier behaves as a resistance of 200 Ω to current flowing in one direction and as a resistance of 2 k Ω to current in the opposite direction. Calculate the reading on the meter.

Solution