Test a diode

Simply put to test:a diode check continuity and then reverse the leads. You should have extremely high readings in one direction, low readings in the other. If you get low readings in both directions, the diode is shorted. If high readings in both directions, the diode is open.

Digital multi-meters 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 diode mode when testing diodes:
  • A good forward-based diode displays a voltage drop ranging from 0.5 to 0.8 volts for the most commonly used silicon diodes. Some germanium diodes have a voltage drop ranging from 0.2 to 0.3 V.
  • The multi-meter displays OL when a good diode is reverse-biased. The OL reading indicates the diode is functioning as an open switch.
  • A bad (opened) diode does not allow current to flow in either direction. A multi-meter will display OL in both directions when the diode is opened.
  • A shorted diode has the same voltage drop reading (approximately 0.4 V) in both directions.
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.

A multi-meter’s Diode Test mode produces a small voltage between test leads. The multi-meter 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 multi-meter 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.

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

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