How to Test Diodes with a Digital Multimeter

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How to Test Diodes with a Digital Multimeter

Digital multimeters, Fundamentals

Digital multimeters can test diodes using one of two methods:

  1. Diode Test mode: almost always the best approach.
  2. Resistance mode: typically used only if a multimeter is not equipped with a Diode Test mode.
Note: In some cases it may be necessary to remove one end of the diode from the circuit in order to test the diode.
Things to know about the Resistance mode when testing diodes:
  • Does not always indicate whether a diode is good or bad.
  • Should not be taken when a diode is connected in a circuit since it can produce a false reading.
  • CAN be used to verify a diode is bad in a specific application after a Diode Test indicates a diode is bad.

A diode is best tested by measuring the voltage drop across the diode when it is forward-biased. A forward-biased diode acts as a closed switch, permitting current to flow.

Steps for using a multimeter in the diode test mode

A multimeter’s Diode Test mode produces a small voltage between test leads. The multimeter then displays the voltage drop when the test leads are connected across a diode when forward-biased. The Diode Test procedure is conducted as follows:

  1. Make certain a) all power to the circuit is OFF and b) no voltage exists at the diode. Voltage may be present in the circuit due to charged capacitors. If so, the capacitors need to be discharged. Set the multimeter to measure ac or dc voltage as required.
  2. Turn the dial (rotary switch) to Diode Test mode. It may share a space on the dial with another function.
  3. Connect the test leads to the diode. Record the measurement displayed.
  4. Reverse the test leads. Record the measurement displayed.

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