The differential voltmeter can be used as a conventional TRANSISTORIZED ELECTRONIC VOLTMETER (TVM) and a DIFFERENTIAL NULL VOLTMETER. It can also be used to measure variations of a voltage near some known value (NULL DETECTOR), high resistance values (MEGGOMETER), and for dBm measurements.

METER DESIGN CHARACTERISTICS

The differential voltmeter is a solid-state instrument that provides the capability of making dc voltage measurements from +/- 10 microvolts to +/- 1,100 volts. Ac voltages from 0.001 to 1,100 volts can be measured over a frequency range from 5 hertz to 100 kilohertz. Both of these measurements can be made without concern of loading the circuit. The differential voltmeter has four voltage readout dials that vary the resistance of the divider assembly as described above.

The differential voltmeter uses a built-in NULL DETECTOR to measure an unknown voltage. The meter circuitry compares the unknown voltage to a known, adjustable reference voltage supplied by the meter. The reference voltage is provided by a high-voltage dc power supply and decade resistor divider assembly strings that are set by voltage readout dials. In this way, the output from the high-voltage power supply can be precisely divided into increments as small as 10 microvolts. The readout dials are used to adjust the meter pointer to 0 and the unknown voltage is then read from the voltage dials.

A primary feature of the differential voltmeter is that it does not draw current from the unknown source for dc measurements when the measurement is obtained. Therefore, the determination of the unknown dc potential is independent of its source.

FRONT PANEL CONTROLS

The front panel of a typical differential voltmeter is shown in figure 4-15. With a few differences, the controls and terminals are similar to those used on other differential voltmeters. The NULL SENSE switch selects the conventional TVM mode of operation and the various full-scale null detector sensitivity ranges when the instrument is operated in the differential mode of operation. The RANGE switch allows selection of the desired input voltage range, positions the readout dial decimal point, and selects the various ranges of the NULL SENSE switch. The readout dials provide a digital readout of the measured voltage when the instrument is in the differential mode.

Figure 4-15. - Controls, terminals, and indicators.

MODES OF OPERATION

There are two primary modes of operation: the conventional transistorized voltmeter mode and the differential null mode. These modes are described in the next paragraphs.

Conventional Transistorized Voltmeter (TVM) Mode

When the instrument is used as a conventional transistorized dc voltmeter, the circuitry is connected as shown in figure 4-16. The null detector drives the front panel meter and provides a full-scale meter deflection for any full-scale input. Positive or negative voltage measurements are made by reversing the meter terminals through the contacts of the MODE switch.

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.

Two of the most important parameters used for transistor testing are the transistor CURRENT GAIN (BETA) and the COLLECTOR LEAKAGE or REVERSE CURRENT (ICO). Two other tests that can be accomplished include the electrode resistance and diode measurements. You may want to review NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, for a review of transistors before continuing this section.

The Semiconductor Test Set AN/USM-206A (figure 4-17) is a rugged, field-type tester designed to test transistors and semiconductor diodes. The set will measure the beta of a transistor, the resistance appearing at the electrodes, and the reverse current of a transistor or semiconductor diode. It will also measure a shorted or open condition of a diode, the forward transconductance of a field-effect transistor, and the condition of its own batteries.

Figure 4-17. - Semiconductor test set.

To assure that accurate and useful information is gained from the transistor tester, you should make the following preliminary checks of the tester before testing any transistors:

1. With the POLARITY switch in the OFF position, the meter pointer should indicate exactly zero. (When required, rotate the meter ZERO ADJUST KNOB on the front of the meter to fulfill this requirement.) To prevent battery drain, be sure to leave the POLARITY switch in the OFF position when measurements are not actually being made.

2. Always check the condition of the test set batteries. To make this check, disconnect the test set power cord, place the polarity switch in the PNP position, and place the function switch first to BAT. 1 and then to BAT. 2. In both BAT positions, the meter pointer should move so as to indicate within the red BAT box.

BETA MEASUREMENTS

If the transistor is to be tested out of the circuit, plug it into the test jack located on the right-hand side below the meter. If the transistor is to be tested in the circuit, at least 300 ohms must exist between E-B (emitter to base), C-B (collector to base), and C-E (collector to emitter) for accurate measurement. Initial setting of the test set controls is performed as follows:

• 1. Set the function switch to BETA.
• 2. Set the POLARITY switch to PNP or NPN (depending on the type of transistor under test).
• 3. Set the RANGE switch to X10.
• 4. Adjust METER ZERO for zero meter indication (transistor disconnected).
• 5. The POLARITY switch should remain OFF while the transistor is connected to or disconnected from the test set; it should then be set to PNP or NPN, as in step 2 above.

If the beta reading is less than 10, perform the following steps:

• 1. Reset the RANGE switch to X1 and reset the meter to zero.
• 2. After connecting the yellow test lead to the emitter, the green test lead to the base, and the blue test lead to the collector, plug the test probe (not shown) into the jack located at the lower right-hand corner of the test set.
• 3. When testing grounded equipment, unplug the 115-volt line cord and use battery operation. A beta reading is attained by multiplying the meter reading times the RANGE switch setting. Refer to the transistor characteristics book provided with the tester to determine if the reading is normal for the type of transistor under test.