How to Test a Transistor & a Diode with a Multimeter

It is very quick and easy to learn how to test a transistor and a diode with a analogue multimeter - normally this is good enough for most applications.

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While many digital multimeters these days have a specific capability for testing diodes and sometimes transistors, not all do, especially the older analogue multimeters that are still in widespread use.

However it is still quite easy to perform a simple go / no-go test using the simplest of equipment.

This form of testing is able to detect whether transistor or a diode is operational, and although it cannot provide details of the parameters.

The lack of parameter testing is seldom a problem because these components will have been tested at manufacture and it is comparatively rare for the performance to fall to a point where they do not operate in a circuit.

Most failures are catastrophic, rendering the component completely inoperable. These simple multimeter tests are able to detect these problems very quickly and easily.

Diodes of most types can be tested in this way - power rectifier diodes, signal diodes, zener / voltage reference diodes, varactor diodes and many more forms of diode can all be tested.

How to test a diode with a multimeter

The basic diode test is very simple to perform. Just two tests are needed with the multimeter to ensure that the diode works satisfactorily.

The diode test relies on the fact that the diode will only conduct in one direction and not the other. This means that its resistance will be different in one direction to that of the other.

By measuring the resistance in both directions, it is possible to ascertain whether the diode is working, and also which connections are the anode and cathode.

As the actual resistance in the forward direction is dependent upon the voltage, it is not possible to give exact values for the expected forward resistance as the voltage from different meters will be different - it will even be different between different ranges on the meter.

... the band on the diode package represents the cathode....

Diode circuit representation & orientation

The method of testing a diode with an analogue meter is quite straightforward.

Step by step instructions:

Set the meter to its ohms range - any range should do, but the middle ohms range if several are available is probably best.
Connect the cathode terminal of the diode to the terminal marked positive on the multimeter, and the anode to the negative or common terminal.
Set the meter to read ohms, and a "lowish" reading should be obtained.
Reverse the connections.
This time a high resistance reading should be obtained.

Notes:

In step 3 above the actual reading will depend upon a number of factors. The main thing is that the meter deflects, possibly to half way or more. The variation depends on many items including the battery in the meter, and the range used. The main point to note is that the meter deflects significantly.
When checked in the reverse direction, silicon diodes are unlikely to show any meter deflection whatsoever. Germanium ones that have a much higher level of reverse leakage current may easily show a small deflection if the meter is set to a high ohms range.

This simple analogue multimeter test of a diode is very useful because it gives a very quick indication of whether the diode is basically operational. It cannot, however, test more complicated parameters such as the reverse breakdown, etc.

Nevertheless it is an essential test for maintenance and repair. Although it is possible for the diode characteristics to change, this very rarely happens and it is far ore likely that a complete breakdown of the diode occurs, and this would be immediately visible using this test.

Accordingly this type of test is exceedingly useful in a number of areas of electronics test and repair.

How to test a transistor with a multimeter

The diode test using an analogue multimeter can be extended to give a simple and straightforward confidence check for bipolar transistors. Again the test using a multimeter only provides a confidence check that the bipolar transistor has not blown, but it is still very useful.

As with the diode, the most likely failures result in the destruction of the transistor, rather than a slight degradation in the performance.

The test relies on the fact that a bipolar transistor can be considered to comprise of two back to back diodes, and by performing the diode test between the base and collector and the base and emitter of the transistor using an analogue multimeter, most of the basic integrity of the transistor can be ascertained.

Transistor equivalent circuit with diodes for multimeter test.

One further test is required. The transistor should present a high resistance between the collector and emitter with the base left open circuit as there are two back to back diodes.

However it is possible for the collector emitter path to be blown and a conduction path created between the collector and the emitter whilst still having a diode function to the base. This also needs to be tested.

It should be noted that a bipolar transistor cannot be functionally replicated using two separate diodes because the operation of the transistor depends upon the base which is the junction of the two diodes, being one physical layer, and also very thin.

Step by step instructions:

The instructions are given primarily for an NPN transistor as these are the most common types in use. The variations are shown for PNP varieties - these are indicated in brackets (.. .. ..):

Set the meter to its ohms range - any range should do, but the middle ohms range if several are available is probably best.
Connect the base terminal of the transistor to the terminal marked positive (usually coloured red) on the multimeter
Connect the terminal marked negative or common (usually coloured black) to the collector and measure the resistance. It should read open circuit (there should be a deflection for a PNP transistor).
With the terminal marked positive still connected to the base, repeat the measurement with the positive terminal connected to the emitter. The reading should again read open circuit (the multimeter should deflect for a PNP transistor).
Now reverse the connection to the base of the transistor, this time connecting the negative or common (black) terminal of the analogue test meter to the base of the transistor.
Connect the terminal marked positive, first to the collector and measure the resistance. Then take it to the emitter. In both cases the meter should deflect (indicate open circuit for a PNP transistor).
It is next necessary to connect the meter negative or common to the collector and meter positive to the emitter. Check that the meter reads open circuit. (The meter should read open circuit for both NPN and PNP types.
Now reverse the connections so that the meter negative or common is connected to the emitter and meter positive to the collector. Check again that the meter reads open circuit.
If the transistor passes all the tests then it is basically functional and all the junctions are intact.

Notes:

The final checks from collector to emitter ensure that the base has not been "blown through". It is sometimes possible that there is still a diode present between collector and base and the emitter and the base, but the collector and emitter are shorted together.
As with the germanium diode, the reverse readings for germanium transistors will not be as good as for silicon transistors. A small level of current is allowable as this results from the presence of minority carriers in the germanium.

Analogue multimeter overview

While most of the multimeters on sale today are digital ones, there are nevertheless many analogue meters still in use. While they may not be the latest in technology, they are still ideal for many uses, and can be used easily for measurements such as the ones above.

Although the tests detailed above are aimed at analogue meters, similar ones can be undertaken with digital multimeters , DMMs.

Often digital multimeters may incorporate a specific bipolar transistor test function, and this is very convenient to use. The overall test performance by the specific bipolar transistor test function is often very similar to that mentioned here, although some digital multimeters may give a value for the current gain.

Using the simple test for diodes and transistors is very useful in many service and repair scenarios. Being able to have a good idea of whether a diode or transistor is functional is very useful. As transistor testers are not widely sold, being able to use any multimeter to provide this capability is particularly useful. It is even more convenient because the test is very easy to perform.