52 mm diameter.
The voltmeter is connected to the plus and minus (ground) polarity for voltage display. A turning magnet ratio measuring movement is used. The dial is graduated according to the movement characteristics. The voltage range to be ignored, below 8 V or 18 V, is electronically suppressed by a Z diode. The limitation of the dial to a range of 8 - 16 V or 18 - 32 V (instead of 0 - 16 V or 0 - 32 V for the same pointer deflection) gives a better resolution of the reading.
The turning magnet ratio measuring movement comprises three stationary coils wound 90
against each other, and a rotating permanent magnet disk with an axle and a pointer in these coils.
The three coils constitute two measuring circuit branches, coil III constituting branch 1; branch 2 consists of coil II with opposite sense of winding and coil I wound vertical to coil II.
No current passes the Z diode between voltage 0 and Z voltage, the current distribution in all three coils is unchanged, and thus the resulting magnetic field remains unchanged.
A partial current passes the Z diode when the voltage rises above the minimum value indicated on the dial. Now the currents in both circuit branches are not equal any more. The strength of the magnetic field in measuring circuit 1 containing coil III increases with the measured voltage, whereas it remains constant in measuring ciruit 2 with coilsi and II. The turning magnet carrying the pointer follows the direction of the field resulting from measuring circuits 1 and 2, thereby indicating measured voltage.
A magnetic shield prevents effects of external magnetic fields.
The voltmeter measuring range is adapted to various on-board voltages by selection of adequate dropping resistor and Z diode values.