Testing a Diode with a Multimeter

- method to test a diode or transistor with an analogue multimeter..

Unlike digital multimeters, analogue multimeters do not have a specific positions to provide a diode or transistor test.

Nevertheless it is still possible to test a diode using an analogue multimeter, and it is also possible to undertake a simple transistor test using an analogue multimeter.

It is quite easy to perform a simple go / no-go transistor or diode test using an analogue multimeter.

Analogue multimeter diode test

This form of diode test using an analogue multimeter is able to detect whether a diode is operational or not. However it cannot provide any parameters about the performance of the diode. This is not normally a problem because the performance of the diodes is normally tested or guaranteed by the manufacturer.

Most failures of diodes are catastrophic, rendering the diode completely non-functional. This simple analogue multimeter test is able to detect whether the basic diode function of the component is intact.

The test basically relies on using the meter to test for the resistance of the diode in both directions. In the reverse direction, the diode should not conduct and will indicate a high resistance, whereas in the forward direction, it will conduct and indicate a lower resistance.

When testing a diode with a multimeter in this way, it should be remembered that the terminal used as positive terminal for measuring voltage, etc is actually negative with respect to negative terminal. In other words the polarity on these terminals appears to be the opposite of what might be expected. This is because of the way the analogue multimeter works.

As a result the readings for testing a diode with a multimeter in this way are as given below in the diagram.

Testing a diode with a multimeter

Note: the meter markings are as they appear on the meter, i.e. the Negative terminal may be marked COM or where the negative voltage would be connected when measuring a voltage.

The readings taken are only indicative. The reading of the diode in the reverse direction should indicate virtually infinite ohms and show no deflection. Some germanium diodes may show visible current in the reverse direction, but this is often normal.

The level of deflection in the forward direction of the diode will vary considerably dependent upon the range and meter in question. Often it will deflect about half way across the scale.

Testing a transistor with an analogue multimeter

It is quite easy to extend the diode testing using a multimeter to provide a simple transistor test.

The key to the test is to understand the make-up of a bipolar transistor.

From the point of view of this test a transistor can be considered as a pair of diodes wired back to back.

Testing a transistor with a multimeter

The diagram shows an NPN transistor, but the diode direction can be reversed for a PNP transistor.

To perform the test, simply test for the diode functionality between the base and collector and base and emitter.

Testing a transistor with a multimeter

A final test should be made between the collector and emitter. This should show high resistance in both directions. Sometimes when a large current has been carried by the transistor it is possible for the collector emitter path to punch through the thin emitter region. Often there will still be a diode present between base-emitter and base-collector, but the collector and emitter will show a very low resistance.

While digital multimeters often have specific diode and transistor test functions, it is still possible to undertake a good test of a diode or transistor with an analogue multimeter.

By Ian Poole

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