By Chris Deziel Updated Mar 24, 2022
A diode is a semiconductor device that permits current flow in only one direction. Commonly referred to as a rectifier, it converts AC into pulsating DC. Diodes are integral to many household appliances, such as microwave ovens, where a microwave diode works with a capacitor to boost the transformer voltage that powers the magnetron.
In schematic diagrams, a diode is shown as a triangle pointing toward a line; the triangle’s apex indicates the direction of forward current. The side pointed toward the triangle is the cathode (negative terminal), while the opposite side is the anode (positive terminal). Correct polarity is essential—installing a diode backward will render it ineffective.
If the current through a diode exceeds its rating, the device can short out, losing its reverse‑blocking capability. Age or deterioration can also open the internal circuit, preventing current flow in either direction. In either case, the diode must be replaced. A multimeter can diagnose such faults.
Use your multimeter’s diode test mode if available; otherwise, set it to measure resistance and test the diode in both forward and reverse directions.
On a multimeter with a dedicated diode test setting, the dial often displays a triangle‑on‑line symbol. Selecting this mode applies a small voltage across the meter leads. Connect the black (negative) lead to the diode’s cathode and the red (positive) lead to the anode. A healthy diode will show a forward‑bias voltage drop of roughly 0.5 V to 0.8 V. A reading near 0 V indicates a defective diode. Reversing the leads should give an open‑loop (OL) or infinite reading; a similar forward voltage in reverse indicates a shorted diode.
When a diode‑test mode is unavailable, use the multimeter’s resistance (Ω) setting. First, disconnect the circuit and fully discharge all capacitors—especially the high‑voltage capacitor in microwave ovens—to avoid shock hazards. Remove the diode from the board. Set the meter to resistance, place the black lead on the cathode and the red lead on the anode. The forward‑biased diode should register a resistance between 1 kΩ and 10 MΩ. Switching the leads to reverse bias should yield an infinite or OL reading. Identical readings in both directions mean the diode is bad.
Safety reminder: Always unplug the device and discharge capacitors before testing. High‑voltage circuits can deliver lethal shocks.
