In this Tutorial we are going to see how to Troubleshoot- Diodes , BJTs, TRIACs, SCRs etc. Diodes have the major applications in preventing reverse voltage , rectification (AC – DC) , freewheeling etc. Diodes like zener also helps us in regulating the voltage. Transistors are always finding its own way in amplification and switching applications. Its depends upon the purpose where we use NPN / PNP transistors. SCR ( Thyristors ) are mainly used in controlling motors whereas TRIACs find its way in dimmer / speed control of AC devices like Light Bulbs , Induction motor respectively. Below given are the simple techniques to test the components using a simple multimeter.
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 is forward-biased and an extremely high resistance, when it is reverse-biased. An ohmmeter applies a known voltage from an internal source (batteries) to the measured resistor. Theoretically, this voltage can reach 1.5 V or 3 V. The diode requires a voltage of 0.7 V 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. Thus an ordinary ohmmeter can be used to test a diode.
Most digital multimeters (DMMs) have a diode test function. It is marked on the select switch with a small diode symbol. When the DMM is set to diode test mode, it provides a sufficient internal voltage to test the diode in both directions. 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 good working order, the multimeter should display a value in the range between 0.5 V 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 is typically in the range between 2.5 V 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 is mainly caused by internal damage of the pn-junction due to overheating. Such a diode exhibits a very high resistance when it is 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).
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 thousands ohms. The actual resistance of the diode normally does not exceed 100 Ω, but the internal voltage of many meters is relatively low in the OHMs range and it is 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 great difference in the readings, when the diode is forward-biased and when it is reverse-biased. In fact, that is all you need to know. This indicates that the diode is working properly.
The SCR is a diode, with an additional gate terminal. The SCR can be brought into conduction only if it is forward-biased and if it is triggered from a pulse applied to the gate. Thus, the SCR can be checked in a similar manner to the conventional diode, 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.
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 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 small interference surges. A maintenance technician, who is not aware of the existence of this resistor, may mistakenly diagnose such SCR as being leaky between the cathode and the gate. The resistor’s value can be measured with an ohmmeter during the test.
Since the TRIAC actually consists of two SCRs connected in parallel and in opposite directions, the procedure for testing a TRIAC is essentially the same as testing 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, similarly 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.
Bipolar Junction Transistors (BJTs) are devices, consisting of three layers of semiconductive material and can be either pnp or npn type. Therefore, each transistor can be represented as a combination of two diodes, connected together as shown in figure. The equivalent base of pnp type transistors appears as connected to the cathodes of both diodes. If transistors are npn type, the equivalent base appears as connected to the anodes of both diodes. The two remaining terminals of the diodes represent the emitter and the collector. Both pn-junctions of the transistor are tested separately as two independent diodes. If both of them show no defects, the transistor is working properly.
The diode-test function of a digital multimeter can be also used to test 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 junctions will be forward-biased when tested. The DMM should read 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 pn-junctions are now reverse-biased, when tested. The multimeter reads high resistance in both cases. The procedure for testing 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 is important to emphasize again, that the reading of a few hundred to a few thousand ohms for forward the bias condition does not necessarily indicate a faulty transistor. It is rather a sign that the internal power supply of the meter is not sufficient to forward bias completely the pn- junction. The out-of-range indication for reverse-biasing 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 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 should be discarded.
Most digital multimeters are capable of measuring the current gain of the transistor βDC. 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 βDC. Though this is a convenient and quick method to check the transistor, one should be aware that some DMMs measure the value of βDC 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 βDC 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.
What if the transistor is biased ? then here is the solution !
Sometimes the transistor itself may not be faulty, but due to faults in the external circuitry, it may not operate properly. 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 cutoff. When checking such transistor from the component side of the PCB, it will appear to be functioning correctly. And yet, the signal is not present at the output. To better understand how to troubleshoot a biased BJT, let us consider a simple amplifier stage as shown in Figure 8.6. It is built on the transistor 2N3946. According to the data sheets, βDC for this transistor is in the range of 50 to 150. Therefore, we can assume that βDC for the specified transistor is 100. The bias voltages are chosen VBB = 3 V and VCC = 9 V. Performing some simple calculations, we can determine that:
Consider the base terminal as “A” , Collector terminal as “B” , Emitter terminal as “C” . 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 μV instead of the normal +0.7 mV, 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 bad soldered joints and for components that are out of tolerance.