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Testing diodes: As was discussed earlier, the diode is a semiconductor device, which conducts direct current in one direction only. In other words, the diode exhibits a very low resistance when it’s forward-biased and an extremely high resistance when it’s reverse-biased. Earlier it was noted that an ohmmeter applies a known voltage from an internal source (batteries) to the measured resistor. Theoretically, this voltage can reach 1.5 or 3 V. The diode requires a voltage of 0.7V to become forward-biased. Therefore, if the positive test lead of the ohmmeter is connected to the anode and the negative test lead of the ohmmeter is connected to the cathode, the diode becomes forward-biased. In this case, the ohmmeter reads a very low resistance. If the test leads are reversed with respect to the anode and the cathode, the diode becomes reverse-biased. Then, the ohmmeter reads a very high resistance. Therefore, an ordinary ohmmeter can be used to test a diode. Most DMMs have a diode-test function. It’s marked on the select switch with a small diode symbol. When the DMM is set to the diode-test mode, it provides a sufficient internal voltage to test the diode in both directions. ++++1 illustrates the testing procedure of a diode. The positive test lead of the DMM (in red color) is connected to the anode, and the negative test lead of the DMM (in black color) is connected to the cathode. If the diode is in a good working order, the multimeter should display a value in the range between 0.5 and 0.9 V (typically 0.7 V). Then the test leads of the DMM are reversed with respect to the anode and the cathode. As the diode in this case appears as an open circuit to the multimeter, practically all of the internal DMM voltage will appear across the diode. The value on the display depends on the meter's internal voltage source and it’s typically in the range between 2.5 and 3.5 V. A defective diode appears either as an open circuit or as a closed circuit in both directions. The first case is more common and it’s mainly caused by an internal damage of the PN junction due to overheating. Such a diode exhibits a very high resistance when it’s both forward-biased and reverse-biased. On the other hand, the multimeter reads 0 V in both directions, if the diode is shorted. Sometimes a failed diode may not exhibit a complete short circuit (0 V) but may appear as a resistive diode, in which case the meter reads the same resistance in both directions ( For example, 1.5 V). ++++1 Properly functioning diode. ++++2 Defective diodes. As was mentioned earlier, if a special diode-test function is not provided in a particular multimeter, the diode still can be checked, by measuring its resistance in both directions. The selector switch is set to Ohms. When the diode is forward-biased, the meter reads from a few hundred to a few thousand ohms. The actual resistance of the diode normally does not exceed 100 ohm, but the internal voltage of many meters is relatively low in the Ohms range and it’s not sufficient to forward-bias the PN junction of the diode completely. For this reason, the displayed value is higher. When the diode is reverse-biased, the meter usually displays some type of out-of-range indication, such as 'OL', because the resistance of the diode in this case is too high and cannot be measured from the meter. The actual values of the measured resistances are unimportant. What is important, though, is to make sure that there is a major difference in the readings, when the diode is forward-biased and when it’s reverse-biased. In fact, this is all that is important to note, for this indicates that the diode is working properly. Testing SCRs: The SCR is a diode with an additional gate terminal. The SCR can be brought into conduction only if it’s forward-biased and if it’s triggered from a pulse applied to the gate. Thus, the SCR can be checked in a similar manner to the conventional diode, which is by employing a DMM with a diode-check function, or with an ordinary ohmmeter. The positive (red) test lead of the meter is connected to the anode of the SCR and the negative (black) test lead is applied to the cathode. The instrument should show an infinite high resistance. A jumper can be used to trigger the SCR. Without disconnecting the meter, use the jumper to short-circuit the gate terminal of the SCR with the positive lead of the meter. The SCR should exhibit a great decrease of resistance. ++++3 Testing the SCR. When the jumper is disconnected, the device may continue to conduct or may turn off. This depends on the properties of both the SCR and the meter. If the holding current of the SCR is small, the ohmmeter could be capable of supplying enough current to keep it turned on. However, if the holding current of the SCR is high, the device will turn off upon the disconnection of the jumper. Some high-power SCRs may have an internal resistor connected between the cathode and the gate. This resistor prevents the SCR from triggering due to the small interference surges. A maintenance technician, who is not aware of the existence of this resistor, may mistakenly diagnose such an SCR as being leaky between the cathode and the gate. The resistor's value can be measured with an ohmmeter during the test. Testing TRIACs: The TRIAC actually consists of two SCRs connected in parallel and in opposite directions; therefore, the procedure for testing a TRIAC is essentially the same as the testing of an SCR. The positive test lead of the meter is connected to MT2 and the negative test lead is applied to MT1. When the gate is open, the ohmmeter should indicate an infinite resistance. Then, similar to the SCR testing procedure, a jumper is used to touch the gate terminal to MT2 (a positive triggering pulse is applied to the gate). The TRIAC should exhibit a great decrease in resistance. This indicates that one of the SCRs in the pair functions properly. Then the test leads of the ohmmeter are reversed with respect to the anode and the cathode. Again, if the gate is open, the ohmmeter should exhibit an out-of-range resistance. Using the jumper, the gate terminal is briefly touched to MT2 (a negative triggering pulse is applied to the gate). The resistance of the TRIAC greatly decreases, which indicates the proper functioning of the second SCR in the pair. ++++4 Testing the TRIAC. Testing BJTs: The BJTs are devices consisting of three layers of semi conductive material and can be either of pnp or npn type. Therefore, each transistor can be represented as a combination of two diodes. The equivalent base of pnp-type transistors appears as connected to the cathodes of both the diodes. If the transistors are of the npn type, the equivalent base appears as connected to the anodes of both the diodes. The two remaining terminals of the diodes represent the emitter and the collector. Both the PN junctions of the transistor are tested separately as two independent diodes. If both of them show no defects, the transistor is working properly. ++++5 A transistor, represented as two diodes. The diode-test function of a DMM can be also used to test the transistors. Let us assume that a pnp-type transistor has to be tested. The negative test lead (black) of the multimeter is applied to the base of the transistor. The positive test lead (red) is applied first to the emitter and then to the collector. In this arrangement, both the junctions will be forward-biased when tested. The DMM should read a low resistance in both cases. Then the red test lead is applied to the base of the transistor instead of the black one. The procedure is repeated. Both the PN junctions are now reverse-biased, when tested. The multimeter reads high resistance in both cases. The procedure for testing the npn transistors is identical. The difference is that the DMM will now read a high resistance, when the black lead is applied to the base, and a low resistance, when the red lead is connected to it. If a multimeter without a diode-test mode is used, the transistor can be tested with the OHMs function. The test operations are similar to the OHMs function diode checking, described in the previous section. It’s important to emphasize again, that the reading of a few hundred to a few thousand ohms for the forward-bias condition does not necessarily indicate a faulty transistor. It’s rather a sign that the internal power supply of the meter is not sufficient to completely forward-bias the PN junction. The out-of-range indication for reverse-biasing of the same transistor clearly shows that the device is functioning properly. The important consideration here is the difference between the two readings and not their actual value. The transistor is faulty if both the PN junctions exhibit approximately the same resistance in both directions. In a similar way to diodes, the PN junctions of the defective transistors exhibit either a very high resistance in both directions (an internal open-circuit), or a zero resistance in both directions (an internal short-circuit). Sometimes the faulty PN junction exhibits a small resistance, which is equal in both directions. For example, the meter readings in both directions are 1.2 V instead of the correct 0.7 V and the 2.9 V readings, respectively. In this case, the transistor is defective and has to be discarded. Most DMMs are capable of measuring the current gain of the transistor bDC. The three transistor terminals are placed in special slots, marked E, B, and C, respectively. Then, a known value of IB is applied to the transistor and the respective IC is measured. As you know, the ratio IC/IB is equal to bDC. Though this is a convenient and quick method to check the transistor, one should be aware that some DMMs measure the value of bDC with a low accuracy. The specifications of the DMM have to be checked before relying on the measured value of the current gain. Some testers have the useful feature of an in-circuit bDC check. Here there is no need to disconnect the suspected transistor from the rest of the circuit and it can be tested directly on the PCB. Troubleshooting biased BJTs: Sometimes the transistor itself may not be faulty, but due to faults in the external circuitry, it may not operate correctly. For example, a cold junction on the transistor base terminal effectively isolates the base from the rest of the circuit. Therefore, the bias voltage on the transistor is 0 V, which will drive it into a cutoff. When checking such a transistor from the component side of the PCB, it will appear to be functioning correctly. Yet, the signal is not present at the output. To better understand how to troubleshoot a biased BJT, consider the amplifier stage example. It’s built on the transistor 2N3946. According to the data sheets, bDC for this transistor is in the range of 50-150. Therefore, we can assume that bDC for the specified transistor is 100. The bias voltages are chosen VBB = 3 V and V = 9 V. Performing some simple calculations, we can determine that: The voltages and the component values are specified. All the measured voltages are with respect to the ground. If the circuit operates correctly, the following voltages should be measured: +0.7 V in point A, +4.9 V in point B, and 0V in point C. ++++6 Troubleshooting a single amplifier stage. First, the transistor has to be checked. If the transistor is not defective, the PCB has to be inspected visually for mechanical defects, burned components, and badly soldered joints. Finally, the voltages on the transistor terminals have to be measured. Three typical abnormal conditions may occur due to faults in the external circuitry. Measuring the voltages on the transistor terminals can help to more effectively detect these faults. If the voltage at point B is only several mV instead of the normal +0.7 mV, then this is an indication that the base of the transistor is open. The soldered joints at the base of the transistor and at RB have to be checked. The value of the RB has to be measured. Any external circuitry, leading to the base of the transistor has to be inspected for badly soldered joints and for components that are out of tolerance. If the meter reads a few mV on the collector terminal (point B) it’s an indication that the collector is not connected to the rest of the circuitry. At the same time, the voltage on the base terminal should be around 0.7 V, as the base-emitter PN junction is forward-biased. The soldered joints on the collector and the collector resistor to the PCB have to be inspected. The value of RC has to be measured. Any component, connected to the collector resistor, has to be checked. If there is an open ground connection, the symptoms are as follows: +3 V at the base terminal and +9 V at the collector terminal, as there is no collector and no emitter currents. The voltage measured at the emitter is +2.5 V or more. This occurs because the internal resistance of the measuring voltmeter provides a forward current path. It flows from VBB, through RB, the base-emitter junction and through the measuring voltmeter to the ground. Thus, the voltmeter registers the voltage drop across the PN junction. The soldered joint on the emitter has to be checked. All external circuitry connected to the emitter also has to be checked and tested. ++++7 Typical abnormal conditions in a biased BJT: (a) Open base; (b) Open collector; (c) Open emitter. Testing FETs: FETs are more difficult to test than BJTs. Before testing a FET, it must be ascertained if the transistor is a JFET- or a MOSFET-type. Thereafter, it has to be clarified whether it’s a p-channel or an n-channel device. JFETs can be tested with an ordinary ohmmeter. ++++ 8 ... an equivalent circuit of a JFET. It appears to the ohmmeter as two diodes connected in series between the drain and the source. The polarity of the diodes is inverted. The gate terminal is taken from the midpoint between them. In the case of an n-channel type, the gate is connected to the anodes of both the diodes. If the transistor is a p-channel type, the gate is connected to the cathodes of both the diodes. The insulation layer of SiO2 appears to the ohmmeter as a resistor connected between the drain and the source in parallel to both the diodes. Therefore, the JFET transistors can be checked using an ohmmeter, by testing the PN junctions between the gate and the drain on one side and the gate and the source on the other. If the JFET is in good working order, both PN junctions should behave as ordinary diodes, exhibiting a high resistance in one direction and a low resistance in the other. Then the resistance between the drain and the source is measured. The meter should indicate some amount of resistance, which depends on the JFET properties. ++++8 A JFET, represented with two diodes and a resistor. In a best-case scenario, testing MOSFETs with an ohmmeter is a very difficult task. This is so because a very thin layer of metal oxide insulation separates the gate junction and the channel. This property of the MOSFET ensures extremely high input impedance of the device, but makes it vulnerable to permanent damage, even when minimal static voltages are built up at the transistor terminals. In fact, a MOSFET can be easily damaged even when it’s lightly touched with a finger. For this reason, MOSFETs come in packages that provide an electrical connection between all terminals, which prevents the static voltages from building up. MOSFETs can be tested very carefully with a low-voltage ohmmeter, set to the highest possible range. D-MOSFETs that are in a good working order, exhibit some continuity between the source and the drain. However, there should be no resistance, between the gate and drain and the gate and source terminals. E-MOSFETs that are working properly show no continuity between any of the terminals. Troubleshooting-biased JFETs: It’s not a recommended practice to unsolder a FET transistor in order to test it. After the visual inspection for damaged components or badly soldered joints, the voltages on the drain and the source have to be measured with respect to the ground. A typical faulty symptom is the drain voltage, which is nearly equal to the power supply voltage. This condition occurs when the drain current is zero and therefore there is no voltage drop across RD. The following faults may be the cause:
++++9 Typical abnormal conditions in a biased JFET. Another typical faulty symptom is a drain voltage that is much less than the normal value. This condition occurs when the drain current is at a far higher level than normal, in which case there is a high voltage drop across RD. The following faults may cause this to happen:
Some faults are very difficult to troubleshoot. One such example is an internally opened gate in a zero-biased D-MOSFET. After the fault occurs, the gate to the source voltage remains the same (0 V). For this reason, the drain current does not change its value and the bias appears to be normal. In general, troubleshooting FETs is a much more difficult task and requires more skills and experience than troubleshooting BJTs. Troubleshooting op-amps: Op-amps are complex and sophisticated devices and are subject to several internal failures while in operation. However, the operational amplifier as such cannot be tested. Should there be an internal problem, it’s not possible to troubleshoot and fix it. Therefore, if the op-amp fails, the only option is to replace it. Usually there are only a few external components in the op-amp circuits. A typical circuit consists of an input resistor, a feedback resistor, and a potentiometer for an offset voltage compensation. If the circuit malfunctions, the external components have to be checked first. There could be dry joints, or the components may be burnt, or out of tolerance. If this is not so then, the contacts on the op-amp itself have to be checked. It’s possible that some of them are faulty. Finally, if everything else appears to be in good working order, but the circuit is still non-operational, it has to be assumed that the amplifier itself is faulty. In this case, the op-amp is simply replaced as one would replace a resistor, a transistor, or any other component. Some typical faults in op-amp circuits are given below:
Summary: Most DMMs provide special functions for testing diodes and BJTs. However, if such functions are unavailable, most electronic components can be tested with an ordinary ohmmeter. If a diode is in a good working order, the ohmmeter readings should change from high to low (and the vice versa) every time the test leads are reversed with respect to the anode and the cathode. The SCR is tested in a similar way. The difference is that in addition a jumper is used between the gate and the anode to trigger the SCR. When the SCR is triggered, its resistance drops significantly from high to low. TRIACs are tested as SCRs, but in both directions (i.e., the test leads of the meter are reversed and the procedure is repeated). BJTs are treated as two diodes, connected in series. Each equivalent diode is tested independently. FETs are more difficult to test. Special care must be taken not to damage the device due to the static charge build up. JFET transistors can be represented as two diodes connected in series, with an additional resistor connected in parallel across them. Both equivalent diodes are tested independently. The value of the resistor is also measured. To find out faults in biased transistor circuits, initially, the approximate voltages on each transistor terminal are calculated. Then the voltages are measured. Any deviations from the calculated values are analyzed logically, which essentially leads to finding and fixing the problem. Op-amps cannot be tested, as the other devices. All external components and the soldered joints have to be checked and if the circuit still does not operate properly, the op-amp has to be replaced. Quiz: 1. A diode, tested with a DMM, exhibits 1.1 V in one direction and an out-of range indication ('OL') in the other. Is the diode faulty? 2. A diode, tested with a DMM, exhibits the same value of 1.1 V in both directions. Is the diode faulty? 3. Does it make any difference if the jumper is connected to MT1 or MT2 during the test of a TRIAC? 4. Can you describe how to identify the type of an unknown transistor (pnp or npn) and its terminals (E, B, C), using just an ordinary ohmmeter? 5. The power supply of a one-stage BJT amplifier is 12 V. What voltage would you expect to measure at the collector? 6. A single-stage, small-signal amplifier is built using a JFET. The voltage measured at the drain is 0 V. What could be the cause of the fault? 7. A single-stage, small-signal amplifier is built using a BJT. Assume that the input signal is a sinusoid with a peak value of 0.5 V and the power supply is 12 V. There is a dry joint on the emitter. What is the form of the output voltage? |
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