Learning Event 6:

DETERMINE THE VALUE OF R FOR AC VOLTAGE MEASUREMENTS

1.

The measurement of AC voltage is a procedure very similar to the DC voltage

measurement procedure just discussed.

Again, the controls on the front panel of

the PSM-6 are the function and range switches. With the function switch set at ACV

and the range switch at any position, the circuit is basically the one shown in

Figure 1-22, with the values of R as shown in the accompanying table.

The ACV

ranges are also designed for a sensitivity of 1000 ohms/volts, and the total

resistance between points A and B should therefore be 450 ohms. The two rectifier

sections of CR101 rectify the incoming AC voltage, and the resulting pulsating DC

is read on the meter. Resistor R132 is a variable resistance, set at the factory,

to provide

compensations

for

variations

in

rectifier

characteristics

and

temperature correction.

Figure 1-22.

AC voltage measurement circuit

2.

The same basic precautions applying to AC voltage measurements apply to DC

measurements except that it is not necessary to observe polarities. One additional

precaution that should be observed, however, is one not necessarily related to

safety but one which results in proper use of the meter in AC voltage measurements.

The AC voltmeter section of the PSM-6 is designed to handle frequencies up to about

1000 hz. Above this frequency the voltage readings are subject to inaccuracies due

to inductive reactance.