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Finding Faults on Transistors

Fig. 6.1 Testing BJTs and JFETs.

Identifying the pins on bipolar transistors

Once the transistor pins have been identified, or at least the base, if the fault is not already obvious you can use the method in Fig. 6.1 to identify a fault on any bipolar transistor that is not connected in a circuit.

Use a multimeter switched to a range suitable for testing transistor junctions, as discussed on the Meters for transistor testing page. Follow the numbered sequence of tests in Fig. 6.1 to find out if the transistor is good or faulty.

Before you begin these tests, make sure you know which of your meter leads is positive and which is negative. Remember that analogue meters may have the polarity of their red and black meter leads reversed when measuring resistance.

Test for short circuit between collector and emitter.

1. Test the resistance between collector and emitter.

2.Then reverse the positive and negative meter connections and test again.

If the meter reads zero or a few ohms in tests 1 and 2, there is a short circuit between collector and emitter and the transistor is faulty. If both readings are infinity, continue with test 3.

Test forward resistance of base−emitter and base−collector junctions.

3. Now connect the positive meter lead to the base, and test the resistance of both junctions by connecting the negative meter probe to one of the other two pins. It doesn't really matter whether this is the collector or the emitter, this test is the same for any junction.

4. Now leave the positive lead on the base, and move the negative lead to the other untested (collector or emitter) pin, and measure the resistance of this junction.

For tests 3 and 4 you should get a typical forward resistance reading of around 500 to 1K ohms in both cases. A reading of zero ohms indicates a short circuit and a faulty transistor. In this case, as a double check, continue with tests 5 and 6.

Test reverse resistance of base−emitter and base−collector junctions.

5. Now connect the negative lead of your meter to the base and the positive lead to another pin as shown at 5 in Fig. 6.1 above.

6. Finally connect the positive probe to the other untested pin as shown at 6 in Fig. 6.1 above.

In tests 5 and 6 both junctions should read infinity. If all of these six tests are ok you have a good transistor. If one or more of the tests has failed, so has the transistor!

Fault identification on FETs

The results of resistance tests on FETs are generally not as easy to interpret as in bipolar transistors. Because of the high impedances involved, the results will be more variable and practice is needed to gain confidence in the results obtained. In addition, the handling requirements for IGFETS with regard to electrostatic voltages, mean that testing these devices out of the circuit is very likely to cause more damage than good! The only effective test for IGFETs (where voltage tests on the transistor in a "live" circuit suggest a faulty transistor), is by substituting a known good device, making sure that the handling precautions mentioned earlier are observed. JFETs however can, with care, be tested with a multi-meter in much the same way as bipolar transistors.

Identifying the pins on bipolar transistors

Fig 6.2 JFET Junction Model (A single PN junction and a Resistive Channel)

Fig 6.2 shows a junction model for testing a JFET. For testing purposes, the JFET can be considered to be a single PN junction, attached to a channel that is basically a resistor. The resistance of the channel between source and drain will be very high (several Megohms), but may vary considerably if the positive meter lead is connected to the drain and the negative lead to the source, then the very high impedance gate is touched with a finger. This can create enough static voltage on the gate to operate the transistor! The actual results you get will vary, depending on such things as the type of meter used, the resistance of your skin and even the humidity of the room.

The PN junction can be tested by connecting the meter between gate and source, first one way and then reversing the polarity. The result should be a low reading of about 1k ohms in the forward bias direction (positive to gate in the case of a N channel device), and infinity (open circuit) in the reverse bias direction (negative to gate).

Testing transistors in circuit.

Although the above methods can sometimes be used for testing transistors still connected in a SWITCHED OFF circuit, this is only possible where any other components in the circuit around the transistor have high values of resistance, and therefore have little effect on the actual transistor resistances measured.

Most resistance tests on semiconductors assume that the transistor, diode etc. has first been unsoldered and removed from the circuit. However this is only one way to test a transistor, and is usually used to confirm earlier tests done with the circuit in "working" (though faulty) condition. These tests involve measuring the voltages on the suspect transistor with the circuit switched on and are part of a full fault finding process. There are however some simple voltage indications that can indicate with a good degree of certainty, whether a suspect transistor is faulty.

1. More than 0.7V difference between base and emitter voltages indicates an open circuit b-e junction.

2. The same voltage on two or more terminals MAY indicate one or more short circuit junctions.

3. A LOWER than expected collector voltage generally means that the transistor is conducting heavily (turned on).

4. A HIGHER than expected collector voltage generally means that the transistor is not conducting (turned off).

Note: Whilst conditions 2,3 and 4 can indicate a faulty transistor, they can also be caused by other circuit conditions. For this reason further voltage tests on other transistors (mainly transistors or supply lines feeding the suspect transistor) should be carried out, before deciding the location of the fault.

Warning: You should never work on "live" circuits unless you know AND USE safe working practices. Many circuits that derive power from the mains (line) supply (and some that don´t) contain LETHAL voltages as well as other hazards. Live circuits must only be worked on by fully trained personnel. Before attempting any work on live circuits using any information provided on this web site, please read the important DISCLAIMER.

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