Finding Faults on Transistors
Testing BJTs and JFETs
Once you have identified the transistor pins, or at least the base, if the fault is not already obvious you can
use the method in Fig. 3.1.27b to identify a fault on any bipolar transistor that is not co
nnected in a circuit.
Use a multi meter switched to a range suitable for testing about transistor junctions, as discussed on the
Meters for transistor testing
page. Follow the n
umbered sequence of tests below 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. If you are not sure, read part 2 of this section
"Meters for transistor testing"
1. Test the resistance between collector and emitter.
n 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
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, in our test we
are simply testing a 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 resistanc
e reading of less than 1k in both cases.
5. Now connect the negative lead of your meter to the base and the positive lead to another pin as shown
at 5 in the diagram above.
6. Lastly connect the positive probe to the other untested pin as shown at 6 in the
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 o
n 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 is by substituting a known good device, making sure
that the handling precaut
ions mentioned earlier are observed. JFETs however can, with care, be tested
with a multi
meter in much the same way as bipolar transistors.
Fig 3.1.28 JFE
T Junction Model (A single PN junction and a
Fig 3.1.28 shows a junction model for testing a JFET. With this device we can consider that we are testing
a single PN junction attached to a channel that is basically a resistor. The resistan
ce of the channel
between source and drain will be very high (several Megohms) but may vary considerably if we have the
positive meter lead connected to the drain and then even touch the very high impedance gate with our
fingers. This can put enough voltag
e on the gate to operate the transistor! The actual results you get we
vary depending on such things as the type of meter used, the resistance of your skin and even the
humidity of the room.
We can normally test the PN junction by connecting the meter betw
een 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
tion (negative to gate).
Testing transistors in circuit
Although you can sometimes use the above methods for testing transistors still in a SWITCHED OFF
circuit, provided that any resistors in the circuit around the transistor have high values and don´t ha
much effect on the actual transistor resistances you are measuring, the above methods assume you are
going to test the transistor, having first unsoldered it and removed it from the circuit. Of course this is only
one way to test a transistor, and usual
ly 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 if an already suspect transistor is faulty.
1. More than 0.7V difference between base and emitter voltages indicates an open circuit b
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 conducing heavily
4. A HIGHER than expected collector voltage generally means that the transistor is not conducting (turned
Note: Indications 2,3 and 4 can also be caused by other circuit conditions.
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
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