How to test diodes using a digital multimeter

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Steps for using a multimeter in the Diode Test mode.
 

How to test diodes

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:

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.

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.

Diode test analysis

A multimeter set to the Resistance mode (Ω) can be used as an additional diode test or, as mentioned previously, if a multimeter does not include the Diode Test mode.

Diode Test Analysis

A diode is forward-biased when the positive (red) test lead is on the anode and the negative (black) test lead is on the cathode.

A diode is reverse-biased when the positive (red) test lead is on the cathode and the negative (black) test lead is on the anode.

Diode Test with Resistance Mode

The resistance mode 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 to Resistance mode (Ω). It may share a space on the dial with another function.
  3. Connect the test leads to the diode after it has been removed from the circuit. Record the measurement displayed.
  4. Reverse the test leads. Record the measurement displayed.
  5. For best results when using the Resistance mode to test diodes, compare the readings taken with a known good diode.

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.

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