DC voltmeter like PMMC meter works on DC supply but this meter can work on AC supply as if it is a AC voltmeter.
Figure 1.
What is required is an additional device to be connected between AC supply and PMMC meter is a Rectifier unit. If single diode is used as a rectifying unit for AC to DC conversion, then meter connected to output of rectifier serves as AC voltmeter with half wave type.
If for rectification 4-diode unit in the bridge form is used it will be a full wave type. Such rectifier instruments are suitable for measurement on communication circuits and also for low voltage light current work. Multiplier resistance (Rs) is used in series with the instrument for controlling the current to the safe value. Both such half wave and full wave rectifier units with the meter are explained with circuit arrangement below.
Half Wave Rectifier Circuit for AC Voltmeter
As shown in the Fig. 1 a single diode is connected to AC supply and PMMC instrument. Diode acting as rectifier converts AC into DC fed to the PMMC meter. Rs is the multiplier resistance connected in series. Current through the meter (Im) is given by the relation.
\[{{I}_{m}}=\frac{V}{{{R}_{m}}+{{R}_{s}}}\]
where Rm is the meter resistance, this produces full scale deflection.
Because of the inertia of moving parts PMMC indicates a deflection corresponding to the average value of current which is dependent upon the average value of the applied voltage. Sensitivity of this instrument is ac 0.45 times its sensitivity with dc.
Full Wave Rectifier Circuit for AC Voltmeter
Figure 2.
As said earlier, in this type for rectification (ac → to dc) a full wave bridge circuit is used using 4 diodes. See the connection of this bridge with AC and PMMC instrument in the Fig. 2. Across point A and B, ac voltage is connected to the bridge circuit, where as d.c. is available as output voltage from the bridge to which PMMC meter is connected. The current through the meter is given as,
\[{{I}_{m}}=\frac{V}{{{R}_{m}}+{{R}_{s}}}\]
This current causes full scale deflection of the meter. The sensitivity of full wave rectifier type of instrument with a sinusoidal ac as an input is 90% of that with dc voltage of the same magnitude. The sensitivity of the full wave rectifier type of instrument is two times that of the half wave rectifier type.
Note that the scale markings of the meters are made with using a multiplier factor of 1.11 in case of instrument using full wave rectifier and that factor is 2.22 in case of half wave type instrument.