Silicon Controlled Rectifier Testing

Although the transistor tester is not specifically designed to test SCRs, it will test many types. The SCR specification that determines whether or not it can be tested is the gate trigger voltage or current.

Two tests are available for SCR testing: switching action (gain) and leakage. To test for SCR switching action:

1. Connect the three leads of the tester to the SCR as if it were a transistor.

2. Perform the transistor gain test.

3. An SCR that is being triggered by the tester and switching properly will show one position as a good PNP transistor and one as a good NPN transistor.

4. If no good readings are obtained, use a comparison test by testing a replacement SCR of the same type. If the replacement SCR tests properly, use the results of Step 3 as the. test results.

5. If the replacement SCR does not test, the trigger characteristics are beyond the test signal levels supplied by the tester. Proceed with the SCR leakage test.

Testing for SCR leakage:

1. Connect the RED and GREEN leads of the tester to the anode and cathode leads of the SCR under test.

2. Perform the diode test.

3. If the SCR is shorted, both positions of the test will show high leakage.

4. If the SCR is not leaky, NEITHER position will show high leakage.

5. An open SCR will not be detected unless it is one that can be tested using the transistor gain test.

TESTING INTEGRATED CIRCUITS

Digital ICs are relatively easy to troubleshoot and test because of the limited numbers of input/output combinations involved. Any particular IC can be tested by simply comparing it to a known good one. A device that can be of great value in troubleshooting integrated circuits is the logic probe shown in figure 4-24.

Use of a suitable logic probe can greatly simplify the troubleshooting of logic levels through integrated circuitry. It can show you immediately whether a specific point in the circuit is low, high, open, or pulsing. Some probes have a feature that detects and displays high-speed transient pulses as small as 10 nanoseconds wide.

Figure 4-24.\Typical logic probe.

These probes are usually connected directly to the power supply of the device being tested, although a few also have internal batteries. Since most IC failures show up as a point in the circuit stuck either at a high or low level, these probes provide a quick, inexpensive way of locating the fault. They can also display that single short-duration pulse that is hard to catch on an oscilloscope.

Testing ICs uses an approach somewhat different from that used in testing transistors. The physical construction of ICs is the prime reason for this different approach. The most frequently used ICs are manufactured with either fourteen or sixteen pins, all of which are soldered directly into the circuit. It can be quite a job to unsolder all of these pins, even with the special tools designed for this purpose. After unsoldering all of the pins, you then have the tedious job of cleaning and straightening them. Although there are a few IC testers on the market, their applications are highly limited. Just as the transistor must be removed from the circuit for checking on a tester, the IC must be removed to permit testing. When ICs are used in conjunction with external components, these components should first be checked for proper operation. This is particularly important in linear applications, where a change in the feedback of a circuit can adversely affect the component's entire operating characteristics. Any linear (analog) IC is sensitive to its supply voltage. This is especially the case among those that use bias and control voltages in addition to a supply voltage. If a linear IC is suspected of being defective, it is important to check all voltages coming to the IC against the circuit diagram of the equipment manufacturer for any special notes on voltages. The manufacturer's handbook will also give recommended voltages for any particular IC. When troubleshooting ICs (either digital or linear), a technician cannot be concerned with what is going on inside the IC. He cannot take measurements or conduct repairs inside the IC. He can therefore consider the IC as a "black box" that performs a certain function. The IC can be checked, however, to see that it can perform its design characteristics. After checking static voltages and external components associated with the IC, check it for dynamic operation. If the IC is intended to function as an amplifier, then measure and evaluate its input and output. If it is to function as a logic gate or combination of gates, it is relatively easy to determine what inputs are required to achieve a desired high or low output.