How to Test a High Power Diode

Defective high power diodes are a common problem with microwave ovens.

Build the Test Jig

These diodes play a critical role by changing the high voltage alternating current (AC) supplied by the high voltage secondary winding of the power transformer into high voltage direct current (DC) that is needed by the magnetron tube that produces the electromagnetic energy that cooks your food. High power diodes subjected to thermal and electrical break down can cause your oven to fail to cook food although it appears to be working. A defective high power diode is found using a digital multimeter (DMM) or an analog volt-ohm-millimeter (VOM). A more accurate test can be made with a simple homemade test jig that allows you to measure the actual barrier voltage drop.

Step 1

Plug in the soldering iron and allow to heat up. Once hot, clean the tip by rubbing it on the wet soldering sponge.

Step 2

"Tin" the clean soldering iron tip by applying a coating of Rosin core solder. A properly tinned soldering tip will appear a bright silvery color and is essential for making proper solder connections.

Step 3

Strip ½ inch of insulation from the ends of the two lengths of hook-up wire.

Step 4

"Tin" the bare copper ends of the hook-up wire with the Rosin core solder by applying the tip of the soldering iron to the underside of the wire and the solder to the top. The heat will draw the molten solder completely around the wire and between the strands.

Step 5

Solder one of the lengths of hook-up wire to each battery connector lead. There will be no tension on these leads so you can simply lay the ends side by side and solder together. Wrap the soldered splices with electrical tape.

Step 6

Solder one lead of the resistor to an alligator clip and the other lead to one of the leads coming from the battery connector. Wrap that soldered splice with tape.

Testing a High Power Diode

Step 1

Identify the cathode and anode leads on the power diode. The cathode, or negative (-) lead, is usually identified by a solid line on the rectifier's body. The anode, the positive (+) lead, is identified by a solid arrowhead pointing against the cathode line.

Step 2

Attach the alligator clips to the diode leads, the negative battery lead to the cathode and the positive battery lead to the diode's anode lead.

Step 3

Snap the battery connector on the battery.

Step 4

Set the DMM for the DC Volts range and attach the positive probe to the diode's anode and the negative probe to the diode's cathode and note the voltage reading. A reading of 12 volts indicates an open diode and you need to replace it. A reading approximately 0.3 to 0.7 volts indicates a good power diode. Most silicon power diodes have a barrier voltage drop of approximately 0.5 volts.

Things You Will Need

  • Digital Multimeter (DMM) or an Analog Volt-Ohm-Millimeter (VOM)
  • 12-Volt battery
  • Battery connectors with leads
  • Two alligator test clips
  • Two 24-inch lengths of AWG 18 stranded copper hook-up wire
  • 240 Ohm, 1 Watt Carbon Resistor
  • 35-Watt soldering iron
  • Soldering sponge
  • Rosin core solder
  • Wire cutters
  • Wire strippers
  • Black plastic electric tape


  • An alternate means of testing a diode is to check its front to back resistance readings. Set the DMM to the Ohms range and connect the negative test lead to the cathode and the positive lead to the diode's anode, a reading of "0" or near "0" is normal. Reverse the test leads, now the meter should indicate infinite resistance. Low resistance both ways indicates a shorted diode. Infinite resistance both ways indicates an open diode. A "shorted" or "open" diode needs to be replaced.

About the Author

Based in Colorado Springs, Colo., Jerry Walch has been writing articles for the DIY market since 1974. His work has appeared in “Family Handyman” magazine, “Popular Science,” "Popular Mechanics," “Handy” and other publications. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College.