4-19
Figure 4-16.—Ac/dc differential voltmeter block diagram.
FUNCTIONAL BLOCK DIAGRAM
Figure 4-16 is a functional block diagram of a differential voltmeter. The circuitry is made up of a
reference supply, a resistive divider, a dc input divider, an ac converter, a null detector, and a meter. The
circuitry is interconnected by various switching arrangements when you perform the desired ac or dc
conventional or differential voltage measurements.
Placing the MODE switch in figure 4-16 to the AC position connects the instrument circuitry as a
conventional transistorized ac voltmeter. A full-scale input voltage at the input terminals of the instrument
results in a voltage being applied to the input of the null detector. The null detector drives the front panel
meter that indicates the value of the measured ac voltage.
Differential Null Mode
When the instrument is used as a dc differential voltmeter, the MODE and NULL SENS switches in
figure 4-16 are placed to their respective +/- dc and desired full-scale meter sensitivity positions. In this
mode of operation, the NULL SENS switch selects a suitable resistance value to determine the full-scale
sensitivity of the meter. The dc input voltage applied to the instrument is then compared with the null
detector, and any resulting difference is used to drive the meter. The meter terminals can be reversed
through the contacts of the MODE switch for +/- dc voltage measurements.
TRANSISTOR TESTERS
Laboratory transistor test sets are used in experimental work to test characteristics of transistors. For
maintenance and repair, however, checking all transistor parameters is not necessary. A check of two or
three performance characteristics is usually sufficient to determine whether a transistor needs to be
replaced.
