Navy Electricity and Electronics Training Series (NEETS), Module 21, 2-21 through 2-30

Module 21 - Test Methods and Practices

Pages i , 1-1, 1-11, 1-21, 2-1, 2-11, 2-21, 2-31, 2-41, 3-1, 3-11, 3-21, 3-31, 4-1, 4-11, 5-1, 5-11, 5-21, 5-31, AI-1 to AI-3, Index

different magnitudes and frequencies. Some diodes may be damaged by the excessive current produced by some range settings of a standard multimeter. Therefore, you should use a digital multimeter when performing this measurement.

Q-14. As a rule of thumb, what is an acceptable ratio of back-to-forward resistance for a diode?

Many naval electronic equipments use silicon-controlled rectifiers (SCRs) for the control of power. Like other solid-state components, SCRs are subject to failure. You can test most SCRs with a standard ohmmeter, but you must understand just how the SCR functions.

As shown in figure 2-12, the SCR is a three-element, solid-state device in which the forward resistance can be controlled. The three active elements shown in the figure are the anode, cathode, and gate. Although they may differ in outward appearance, all SCRs operate in the same way. The SCR acts like a very high-resistance rectifier in both forward and reverse directions without requiring a gate signal. However, when the correct gate signal is applied, the SCR conducts only in the forward direction, the same as any conventional rectifier. To test an SCR, you connect an ohmmeter between the anode and cathode, as shown in figure 2-12. Start the test at R x 10,000 and reduce the value gradually. The SCR under test should show a very high resistance, regardless of the ohmmeter polarity. The anode, which is connected to the positive lead of the ohmmeter, must now be shorted to the gate. This will cause the SCR to conduct; as a result, a low-resistance reading will be indicated on the ohmmeter. Removing the anode- to-gate short will not stop the SCR from conducting; but removing either of the ohmmeter leads will cause the SCR to stop conducting - the resistance reading will then return to its previous high value. Some SCRs will not operate when you connect an ohmmeter. This is because the ohmmeter does not supply enough current. However, most of the SCRs in Navy equipment can be tested by the ohmmeter method. If an SCR is sensitive, the R x 1 scale may supply too much current to the device and damage it. Therefore, try testing it on the higher resistance scales.

Q-15. When testing an SCR with an ohmmeter, the SCR will conduct if what two elements are shorted together?

Triac is General Electric's trade name for a silicon, gate-controlled, full-wave, ac switch, as shown in figure 2-13. The device is designed to switch from a blocking state to a conducting state for either polarity of applied voltages and with either positive or negative gate triggering. Like a conventional SCR, the Triac is an excellent solid-state device for controlling current flow. You can make the Triac conduct by using the same method used for an SCR, but the Triac has the advantage of being able to conduct equally well in either the forward or reverse direction.

To test the Triac with an ohmmeter (R x 1 scale), you connect the ohmmeter's negative lead to anode 1 and the positive lead to anode 2, as shown in figure 2-13. The ohmmeter should indicate a very high resistance. Short the gate to anode 2; then remove it. The resistance reading should drop to a low value and remain low until either of the ohmmeter leads is disconnected from the Triac. This completes the first test.

The second test involves reversing the ohmmeter leads between anodes 1 and 2 so that the positive lead is connected to anode 1 and the negative lead is connected to anode 2. Again, short the gate to anode 2; then remove it. The resistance reading should again drop to a low value and remain low until either of the ohmmeter leads is disconnected.

Q-16. When a Triac is properly gated, what is/are the direction(s) of current flow between anodes 1 and 2?

The unijunction transistor (UJT), shown in figure 2-14, is a solid-state, three-terminal semiconductor that exhibits stable open-circuit, negative-resistance characteristics. These characteristics enable the UJT

to serve as an excellent oscillator. Testing a UJT is a relatively easy task if you view the UJT as being a diode connected to the junction of two resistors, as shown in figure 2-15. With an ohmmeter, measure the resistance between base 1 and base 2; then reverse the ohmmeter leads and take another reading. Readings should show the same high resistance regardless of meter lead polarity. Connect the negative lead of the ohmmeter to the emitter of the UJT. Using the positive lead, measure the resistance from the emitter to base 1 and then from the emitter to base 2. Both readings should indicate high resistances that are approximately equal to each other. Disconnect the negative lead from the emitter and connect the positive lead to it. Using the negative lead, measure the resistance from the emitter to base 1 and then from the emitter to base 2. Both readings should indicate low resistances approximately equal to each other.

The junction field-effect transistor (JFET) has circuit applications similar to those of a vacuum tube. The JFET has a voltage-responsive characteristic with a high input impedance. Two types of JFETs that you should become familiar with are the junction p-channel and the junction n-channel types, as shown in figure 2-16. Their equivalent circuits are shown in figures 2-17 and 2-18, respectively. The only difference in your testing of these two types of JFETs involves the polarity of the meter leads.

Using an ohmmeter set to the R x 100 scale, measure the resistance between the drain and the source; then reverse the ohmmeter leads and take another reading. Both readings should be equal (in the 100- to 10,000-ohm range), regardless of the meter lead polarity. Connect the positive meter lead to the gate. Using the negative lead, measure the resistance between the gate and the drain; then measure the resistance between the gate and the source. Both readings should indicate a low resistance and be approximately the same. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate to the drain; then measure the resistance between the gate and the source. Both readings should show infinity.

Using an ohmmeter set to the R x 100 scale, measure the resistance between the drain and the source; then reverse the ohmmeter leads and take another reading. Both readings should be the same (100 to 10,000 ohms), regardless of meter lead polarity. Next, connect the positive meter lead to the gate. Using the negative lead, measure the resistance between the gate and the drain; then measure it between the gate and the source. Both readings should show infinity. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and the drain; then measure it between the gate and the source. Both readings should indicate a low resistance and be approximately equal.

Another type of semiconductor you should become familiar with is the metal oxide semiconductor field-effect transistor (MOSFET), as shown in figures 2-19 and 2-20. You must be extremely careful when working with MOSFETs because of their high degree of sensitivity to static voltages. As previously mentioned in this chapter, the soldering iron should be grounded. a metal plate should be placed on the workbench and grounded to the ship's hull through a 250-kilohm to 1-megohm resistor. You should also wear a bracelet with an attached ground strap and ground yourself to the ship's hull through a 250-kilohm to 1-megohm resistor. You should not allow a MOSFET to come into contact with your clothing, plastics, or cellophane-type materials. a vacuum plunger (solder sucker) must not be used because of the high electrostatic charges it can generate. Solder removal by wicking is recommended. It is also good practice to wrap MOSFETs in metal foil when they are out of a circuit. To ensure MOSFET safety under test, use a portable volt-ohm-milliammeter (VOM) to make MOSFET resistance measurements. a VTVM must never be used in testing MOSFETs. You must be aware that while you are testing a MOSFET, you are grounded to the ship's hull or station's ground. use of a VTVM would cause a definite safety hazard because of the 115-volt, 60-hertz power input. When the resistance measurements are complete and the MOSFET is properly stored, unground both the plate on the workbench and yourself. You will understand MOSFET testing better if you visualize it as equivalent to a circuit using diodes and resistors, as shown in figures 2-21 and 2-22.

Using an ohmmeter set to the R x 100 scale, measure the resistance between the MOSFET drain and the source; then reverse the ohmmeter leads and take another reading. The readings should be equal, regardless of meter lead polarity. Connect the positive lead of the ohmmeter to the gate. Using the negative lead, measure the resistance between the gate and the drain and between the gate and the source. Both readings should show infinity. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and the drain; then measure it between the gate and the source. Both readings should show infinity. Disconnect the negative lead from the gate and connect it to the substrate. Using the positive lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both of these readings should indicate infinity. Disconnect the negative lead from the substrate and connect the positive lead to the substrate. Using the negative lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both readings should indicate a low resistance (about 1,000 ohms).

Using an ohmmeter set to the R x 100 scale, measure the resistance between the drain and the source; then reverse the leads and take another reading between the drain and the source. Both readings should show infinity, regardless of meter lead polarity. Connect the positive lead of the ohmmeter to the gate. Using the negative lead, measure the resistance between the gate and the drain and then between the gate and the source. Both readings should indicate infinity. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and the drain and then between the gate and the source. Both readings should indicate infinity. Disconnect the negative lead from the gate and connect it to the substrate. Using the positive lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both readings should indicate infinity. Disconnect the negative lead from the substrate and connect the positive lead to the substrate. Using the negative lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both readings should indicate a low resistance (about 1,000 ohms).

Integrated circuits (ICs) constitute an area of microelectronics in which many conventional electronic components are combined into high-density modules. Integrated circuits are made up of active and passive components, such as transistors, diodes, resistors, and capacitors. Because of their reduced size, use of integrated circuits can simplify otherwise complex systems by reducing the number of separate components and interconnections. Their use can also reduce power consumption, reduce the overall size of the equipment, and significantly lower the overall cost of the equipment concerned. Many types of integrated circuits are ESDS devices and should be handled accordingly.

Your IC testing approach needs to be somewhat different from that used in testing vacuum tubes and transistors. The physical construction of ICs is the prime reason for this different approach. The most frequently used ICs are manufactured with either 14 or 16 pins, all of which may be soldered directly into the circuit. It can be quite a job for you 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 all of them.

Although there are a few IC testers on the market, their applications are limited. Just as transistors must be removed from the circuit to be tested, some ICs must also be removed to permit testing. When ICs are used in conjunction with external components, the external 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 operating characteristics of the component.

Any linear (analog) IC is sensitive to its supply voltage. This is especially the case among ICs that use bias and control voltages in addition to a supply voltage. If you suspect a linear IC of being defective, all voltages coming to the IC must be checked against the manufacturer's circuit diagram of the equipment for any special notes on voltages. The manufacturer's handbook will also give you recommended voltages for any particular IC.

When troubleshooting ICs (either digital or linear), you cannot be concerned with what is going on inside the IC. You cannot take measurements or conduct repairs inside the IC. You should, therefore, consider the IC as a black box that performs a certain function. You can check the IC, however, to see that it can perform its design functions. After you check static voltages and external components associated with the IC, you can check it for dynamic operation. If it is intended to function as an amplifier, then you can measure and evaluate its input and output. If it is to function as a logic gate or combination of gates, it is relatively easy for you to determine what inputs are required to achieve a desired high or low output. Examples of different types of ICs are provided in figure 2-23.

Digital ICs are relatively easy for you to troubleshoot and test because of the limited numbers of input/output combinations involved. When using positive logic, the logic state of the inputs and outputs of a digital IC can only be represented as either a high (also referred to as a 1 state) or as a low (also referred to as a 0 state). In most digital circuitry, a high is a steady 5-vdc level, and a low is a 0-vdc level. You can readily determine the logic state of an IC by using high-input-impedance measuring devices, such as an oscilloscope. Because of the increased use of ICs in recent years, numerous pieces of test equipment have been designed specifically for testing ICs. They are described in the following paragraphs.

Logic clips, as shown in figure 2-24, are spring-loaded devices that are designed to clip onto a dual- in-line package IC while the IC is mounted in its circuit. It is a simple device that usually has 16 light emitting diodes (LEDs) mounted at the top of the clips. The LEDs correspond to the individual pins of the IC, and any lit LED represents a high logic state. An unlit LED represents a low logic state. Logic clips require no external power connections, and they are small and lightweight. Their ability to simultaneously monitor the input and output of an IC is very helpful when you are troubleshooting a logic circuit.

The logic comparator, as shown in figure 2-25, is designed to detect faulty, in-circuit-DIP ICs by comparing them with ICs that are known to be good (reference ICs). The reference IC is mounted on a small printed-circuit board and inserted into the logic comparator. You then attach the logic comparator to the IC under test by a test lead, which is connected to a spring-loaded device similar in appearance to a logic clip. The logic comparator is designed to detect differences in logic states of the reference IC and the IC being tested. If any difference in logic states does exist on any pin, an LED corresponding to the pin in question will be lit on the logic comparator. The logic comparator is powered by the IC under test.

Logic probes, as shown in figure 2-26, are extremely simple and useful devices that are designed to

help you detect the logic state of an IC. Logic probes can show you immediately whether a specific point in the circuit is low, high, open, or pulsing. a high is indicated when the light at the end of the probe is lit and a low is indicated when the light is extinguished. Some probes have a feature that detects and displays high-speed transient pulses as small as 5 nanoseconds wide. 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 for you to locate the fault. They can also display that single, short-duration pulse that is so hard to catch on an oscilloscope. The ideal logic probe will have the following characteristics: