How to Test a Diode in a Circuit


Many small household appliances or circuit boards have devices called diodes placed on the circuit. A diode acts as a gateway to keep electrical current traveling in one direction. If a diode functions improperly, it will either allow complete flow of alternating current (AC) or no current at all. One of the main functions of a diode is to rectify AC into direct current (DC). A malfunctioning diode can ruin your appliances; you can test a diode's functionality with the resistance measurement function of a multimeter.


  • Plug the red lead of your multimeter into the socket marked "Voltage+."

  • Plug the black lead into the socket marked "COM."

  • Turn on the device you want to test.

  • Locate the diode you want to test. The diode is a small cylindrical piece with a band on one end. This band indicates the directional bias. One end will be the anode, or positive charge, and the other end will be the cathode, or negative charge.

  • Set the meter to measure resistance, indicated by the Greek letter omega and touch the leads to each end of the diode. You should get a low resistance measurement when the positive lead is touched to the anode and the negative to the cathode.

  • Switch the leads around. You should get a high reading, or "OL," indicating overload.

Diode Function

  • Connect the leads to the meter: red to "Volt+" and black to "COM."

  • Set the meter to the diode function. This is indicated with a cross with an arrow pointed toward the center.

  • Touch the leads to the ends of the diode. If you have the leads connected properly -- red lead to the anode, black to the cathode -- you should get a voltage measurement in millivolts.

  • Switch the position of the leads. You should get an undefined reading "OL" because the diode is not intended to allow voltage in that direction.

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