How Do I Test Diodes With a Multimeter?

By Nathan E. Baker

Test diodes with the multimeter's diode test setting. If the multimeter lacks a diode test setting, use the ohmmeter function or the continuity test function. Prepare the diode prior to testing for the most accurate result.

Tips

Semiconductor components like diodes may fail in a variety of ways, including failure from thermal breakdown or failure at specific frequencies. Simple test procedures can not find these types of defects. When in doubt about a diode, replace it with a known-good component.

Warnings

These test procedures do not work with Zener diodes.

Prepare the Diode for Testing

Make sure both diode terminals are clean and free of corrosion or contamination. If the diode is part of an electrical circuit, remove it from the circuit. If you cannot remove the diode from its circuit, disconnect one terminal of the diode from the circuit to electrically isolate the component for testing.

Diodes let current flow in one direction, from anode to cathode. Manufacturers identify the cathode terminal of the diode with a contrasting color band. The anode is the opposite terminal. Current moving forward, from anode to cathode, encounters a small forward resistance. Current flowing in the opposite direction encounters high resistance. Test procedures depend upon measuring or comparing forward resistance and reverse resistance values.

Multimeter With Diode Test Option

Configure the multimeter to test diodes. Identify the polarity of the test probes or leads. The red lead is positive for most multimeters, but some differ. Refer to the multimeter's manufacturer instructions.

Tips

Some multimeter leads end in a sharp point, or a probe, and others end in an alligator clip. Both leads work equally well for testing diodes.

Attach or touch the positive lead to the anode terminal.
Attach or touch the negative lead to the cathode terminal.
Read the multimeter. The display indicates the voltage drop between the probes. A good diode creates a voltage drop from 0.5 to 0.6 volts. A good germanium diode creates a voltage drop from 0.25 to 0.3 volts. No voltage drop or an excessive voltage drop indicates the diode may be defective.
Reverse the test leads.
Read the multimeter. A good diode displays an open circuit. A measurement other than open circuit may indicate the diode is defective.

If the test indicates good results in both directions then the diode is good. Other results may indicate the diode is defective.

Ohmmeter or Continuity Setting

Configure the multimeter for resistor or continuity testing.

Warnings

Use the following method only if the multimeter lacks a diode test setting.

Tips

Test probe or lead polarity does not matter when testing a diode in the following manner.

Attach or touch one test lead to one diode terminal.
Attach or touch the other test lead to the opposite diode terminal.
Read the resistance value on the multimeter, or note the continuity test result.
Reverse the test leads.
Read the resistance value or note the continuity test result.
Compare the measurements.

An ohmmeter will measure very high resistance or an open circuit in one direction, and very low resistance in the opposite direction. A typical measurement for a working diode, for example, is a ratio of 10-to-1, with reverse resistance being 10 times or more greater than forward resistance. The exact measurement depends on the type of diode you are testing, its voltage rating and manufacturer.

A good diode displays continuity in one direction only. A defective diode displays continuity in both directions or open circuit in both directions.

References

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