A-C Meter Readings

Author: N.H. Crowhurst

If an ordinary D'Arsonval moving-coil voltmeter is connected to an alternating voltage, one half-wave will try to make it read in the normal way, while the other half-wave will try to make it read backwards, as if it had been connected the wrong way around. As the pointer will not have time to move back and forth so rapidly, it will stand still, or vibrate slightly, without moving far from the zero marking. Because of this, moving-coil meters must be used with rectifiers to get a reading.

Half-wave - the simplest rectifier meter

The rectifier is a metal-contact device that acts in the same fashion as a diode - it lets current flow in one direction, but not the other. In the symbol for the rectifier, the arrow points in the direction of current flow (opposite to electron flow). As a result of the action of the rectifiers, when this kind of meter is connected to an alternating voltage, one half-wave is bypassed through one rectifier, while the other half goes through the meter. Of course, the meter pointer will not have time to follow the fluctuations, so it will average out the current that flows through it.

Average voltage measurement

During one half-wave, no current flows through the meter, while during the other half-wave, it follows half a sine wave in form. The average of half a sine wave is 0.637 of peak value. However, during half the time, no current goes through the meter; therefore, the average over the whole time will be half of 0.637 or 0.31S5. (If an ordinary meter movement is used in this circuit to measure an alternating voltage, a 1-volt peak - 2-volt peak-to-peak - voltage will only give a reading of 0.3185 volt.)

In an arrangement that uses four rectifiers, the meter gets both halves of the wave, and it will read 0.637 of the peak voltage if a regular d-c meter is used.

Full-wave rectifier meter - current paths on alternate half-waves

Thus all rectifier-type meters indicate a value that is some kind of average, but the average value is not directly related to practical circuit behavior. In audio work, we must know either the maximum voltages or currents or the power (E X I) associated with them. If only the level changes (impedance or resistance is constant), the power is proportional to voltage or current squared, at any instant. Average power is determined by the average (or mean) of the squares.

Relationship between voltage or current and power in sine wave

The power-equivalent reading of any a-c waveform could be read on a meter where both the field and the current in the moving coil come from the circuit being measured. This is called a dynamometer movement. Torque at every instant is proportional to power. So, the reading will be the average power represented by the current or voltage. If it is marked with square-root readings, or the steady current or voltage that would give the same reading, this meter will read true rms of any waveform.

Different kinds of meters give different readings according to the waveform