Voltage Checks

Voltage measurements provide a means of checking circuit conditions in a transistorized circuit just as they do in checking conditions in a tube circuit. The voltages, however, are much lower than in a tube circuit. The bias voltage between the base and emitter, for instance, is usually 0.05 to 0.20 volts. When making checks, observe polarity.

Resistance Checks

Transistors have little tendency to burn or change value because of low voltage in their circuits. They can, however, be permanently damaged by high-voltage conditions that occur when the collector voltage is increased. They can also be permanently damaged when the ambient temperature increases and causes excessive collector current flow. Transistors are easily damaged by high current; therefore, resistance measurements must not be taken with an ohmmeter that provides a maximum current output in excess of 1 milliampere. If you are not sure that the range of ohmmeter you want to use is below the 1 milliampere level, connect the ohmmeter to a milliammeter and check it. See figure 2-19 for a method of measuring the current from an ohmmeter.

Figure 2-19. - Measuring current passed by an ohmmeter.

Resistance measurements usually are not made in transistorized circuits, except when you are checking for open windings in transformers and coils. When a resistance check is required, the transistors are usually removed from the circuit. Resistance checks cannot test all the characteristics of transistors, especially transistors designed for high frequencies or fast switching. The ohmmeter is capable of making simple transistor tests, such as open and short tests.

Refer to NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, for a review of transistor and semiconductor terms and theory.