Learn how to use an ohmmeter to test a bipolar junction transistor (BJT), identify its leads, and evaluate its forward- and reverse-biased junctions.
Bipolar junction transistor (BJT) testing is a procedure that is performed periodically when working with semiconductor devices. Lead identification, gain, open leads, shorted conditions, and leakage are some of the tests that can be performed on a transistor. Curve tracers, gain testers, and sound-producing instruments can be used for these tests. These instruments, as a general rule, are not always available. An ohmmeter, however, can be used to perform many of these tests. An ohmmeter is one of the functions of a volt-ohm milliammeter (VOM) or digital volt-ohm-meter (DVOM). In this article, you will learn how to use an ohmmeter to test a bipolar junction transistor and identify its leads.
Junction Testing using Ohmmeter
A BJT transistor has two P–N junctions in its construction. Both of these junctions can be tested with an ohmmeter. Figure 1 shows the junctions of PNP and NPN transistors. Each junction responds as a diode when it is tested. Forward biasing occurs when the material polarity of the junction matches the voltage polarity of the ohmmeter. When the ohmmeter polarity is reversed, the same junction becomes reverse biased. Recall that forward biasing of a diode indicates a low resistance reading, while reverse biasing indicates an extremely high resistance reading. Both junctions of a transistor will respond to this test in the same way if the transistor is good.
A faulty P–N junction does not show a difference in its forward and reverse resistance connections. Low resistance in both directions indicates that a junction is shorted. Excessive current generally causes this type of failure in a transistor. Infinite or exceedingly high resistance in both directions indicate an open condition. An open junction is normally caused by a broken internal connection. This may be the result of a current overload or excessive shock. Open and shorted transistor failures, as a general rule, occur suddenly and cannot be corrected.
A rather quick test of both junctions can be made simultaneously with the ohmmeter. Connect one ohmmeter probe to the emitter and the other to the collector. Ohmmeter probe polarity in this case is not important. This connection should cause an extremely high resistance reading. Then reverse the two ohmmeter probes. High resistance should also occur in this direction. A good silicon transistor will show an infinite resistance in either direction. Any measurable resistance in either direction indicates leakage. Germanium transistors will show some leakage in this test. Unless the leakage is excessive, it can be tolerated in a germanium transistor. An ideal BJT transistor would have no indication of emitter−collector leakage.
Figure 1. Junction polarity of PNP and NPN transistors.
Lead Identification
An ohmmeter can also be used to identify the leads of a BJT transistor. The polarity of the ohmmeter voltage source should be checked for this test to be meaningful. Straight-polarity ohmmeters have the black or common probe negative and the red probe positive. Reverse polarity ohmmeters would be connected in the opposite direction for these tests. The polarity of an ohmmeter can be tested with a separate DC voltmeter if it is unknown. The ohmmeter used in this explanation has straight polarity (black-negative and red-positive).
Base Identification
To identify transistor leads, inspect the lead location of the device under test. Pick out the center lead of the transistor. Assume that it is the base lead. Connect the negative probe of the ohmmeter to it as shown in Figure 2. Then, alternately touch the positive ohmmeter probe to the two outside transistor leads. If a low resistance indication occurs for each lead, the center lead is actually the base. The test also indicates that the transistor is the PNP type. If the resistance is high between the center lead and the two outside leads, reverse the meter polarity. The positive ohmmeter probe should now be connected to the assumed base. Once again, alternately switch the negative ohmmeter probe between the two outside transistor leads. If a low resistance reading is obtained, the transistor is an NPN type.
Figure 2. Ohmmeter BJT transistor testing.
If the center lead does not produce low resistance in either of the two conditions, it is not the base. Then select one of the outside leads as the assumed base. Try the same procedure again with the newly assumed base. If this does not produce results, try the other outside lead. One of the three leads must respond as the base if the transistor is good. If a response cannot be obtained, the transistor must be open or shorted.
Gain Test and Collector and Emitter Identification
Thus far, we have identified the base lead and determined the polarity of the transistor to be NPN or PNP. It is now possible to test the gain and to identify the remaining two leads. A 100,000-Ω resistor and an ohmmeter are needed for this part of the test. The resistor is used to provide base current from the ohmmeter to the transistor. If a power transistor is tested, use the R meter range and a 1000-Ω base resistor.
The process of troubleshooting transistor gain should be more meaningful to you if you know how this test works. Figure 3 shows how PNP and NPN transistors respond to the ohmmeter test. The energy source of the ohmmeter is used to supply the bias voltage to each of the transistor elements.
For the PNP transistor of Figure 3(a), the positive ohmmeter probe is connected to the emitter and the negative lead to the collector. When a resistor is connected between the collector and the base, it causes base current. With the emitter and base forward biased and the collector reverse biased, the transistor becomes low resistant. The ohmmeter responds to this condition by showing a change in resistance. The resistance of the transistor decreases in value. A low resistance reading on the ohmmeter indicates the transistor has gain and correct lead selection.
The ohmmeter test of an NPN transistor is shown in Figure 3(b). For this type of transistor, the emitter is connected to the negative ohmmeter probe and the collector to the positive ohmmeter probe. When a resistor is connected between the collector−base, it causes the base current to flow. With the emitter−base forward biased and base−collector reverse biased, the transistor becomes low resistant. A low resistance reading on the ohmmeter indicates the transistor has gain and correct lead selection.
The procedure for checking an NPN transistor is shown in Figure 4. Note that the ohmmeter is connected to the two outside leads. The center lead has been previously identified as the base. If another lead were found to be the base, it would be used in place of the center lead. One end of the resistor is connected to the base with the other lead open. In this case, it is assumed that the negative ohmmeter probe is connected to the emitter and the positive lead to the collector. If the assumed leads are correct, the emitter will be forward biased, and the collector is reverse biased. Touching the open end of the base resistor to the positive ohmmeter probe will cause a base current flow. If it does, the ohmmeter will indicate a low resistance. In effect, the emitter−base junction is forward biased, and the base−collector junction is reverse biased. If no base current flows, the assumed emitter−collector leads are reversed. Simply reverse the ohmmeter’s probes and again touch the resistor lead to the positive ohmmeter probe. If the transistor is good, and the assumed leads are correct, the ohmmeter will show a low resistance. If it does not, the transistor has a low gain. In some transistors, low gain is permissible. As a rule, low gain is an indication of some type of transistor problem. The ohmmeter test of transistor gain is only a close approximation of the status of the device.
Figure 3. BJT Transistor gain test. (a) PNP test circuit. (b) NPN test circuit.
Figure 4. NPN gain test diagram
Figure 5 shows the procedure for testing a PNP transistor. In this case, the ohmmeter is connected to the two outside leads. We have identified the center lead as the base. The base resistor is now connected to this lead. For a PNP transistor, the positive ohmmeter lead goes to the emitter and the negative lead to the collector. If the assumed leads are correct, touching the open end of the base resistor to the negative lead will cause the base current to flow. If it does, the ohmmeter will show a low resistance reading. This indicates that the emitter−base is forward biased, and the base−collector reverse biased as in the circuit. If no current flows, the assumed emitter−collector leads are reversed. Reverse the two ohmmeter probes, and again touch the base resistor to the negative ohmmeter probe. A good transistor with correct lead identification will indicate low resistance. A transistor with low gain will not cause much of a change in resistance.
Figure 5. PNP gain test.
BJT Transistor Testing Key Takeaways
- BJT Transistor testing, lead identification, and polarity of the material from which it is constructed can be determined with an ohmmeter.
- The two junctions of a transistor are tested as diodes.
- Lead and material polarity identification of a transistor is achieved by using the ohmmeter’s voltage source to bias the transistor into operation.
- When identifying the lead and material polarity of a transistor, a base lead is assumed; one ohmmeter lead is attached to this lead and the other lead is switched between the two remaining leads.
Review Questions
1. Each P–N junction of a good transistor will respond as a(n) _____ when tested with an ohmmeter.
2. A(n) _____ P–N junction will not show a difference in its forward and reverse resistance connections.
3. When an ohmmeter is connected between the emitter−collector leads of a good bipolar transistor, it will show (high, low) resistance in either polarity.
4. A good bipolar transistor is connected so that the positive ohmmeter lead goes to the base and the negative lead is switched alternately between the other two leads. If this test shows low resistance between each of the alternate leads, the device is (NPN, PNP).
5. If a bipolar transistor shows low resistance between any two leads in either direction of ohmmeter polarity, it indicates that the device is _____.
6. Infinite or exceedingly high resistance in both directions across either P–N junction of a bipolar transistor indicates that the junction is _____.
Answers
- Diode
- Faulty
- Low
- NPN
- Shorted
- Open