If you are trying to heat food in the microwave, but it keeps coming out just as cold as when it went in, one of the most common causes is a faulty diode. Other causes include a defective magnetron, a faulty high-voltage capacitor, or a defective door switch. To work out if the diode is to blame, the diode should be tested with a multimeter. Other clues that the diode needs replacing include an electrical burning smell, the diode splitting in two, or a burned crack on the diode.
Microwave diodes can be easy to replace, but make sure to follow the safety warning and discharge the high-voltage capacitor.
Safety Warning
Due to the very high voltage and high current that microwave ovens require, the risk of electrocution is high when repairing the appliance. Microwave diodes are usually located close to the high-voltage capacitor. The high-voltage capacitor can store a lethal amount of electricity, even after the microwave has been disconnected from the power for several months. To safely access electrical components in the microwave, the capacitor must be discharged.
The capacitor can be discharged by touching both the positive and negative terminals of the capacitor with a metal screwdriver blade. You must insulate yourself against electrocution, and the terminals must be touched at the same time. You could use either a screwdriver with a rubber handle or needle-nose pliers with rubber handles. If you are unsure, you should leave it to a trained professional.
What Does a Diode Do?
The microwave’s diode converts alternating current (AC) to direct current (DC), which doubles the voltage and powers the magnetron that heats the food. Without the diode, the magnetron would not receive enough voltage to do its job.
Testing the Diode with a Multimeter
While a faulty diode will likely have visible signs that it needs to be replaced, to be sure, you should test it with a multimeter.
Before accessing the microwave to remove the diode for testing, make sure the microwave is disconnected from the power source. Before removing the diode from the microwave, it is recommended that the high-voltage capacitor be discharged (see safety warning above).
When testing a microwave diode with a multimeter, you will need a multimeter that is powered by a 9-volt battery and with a setting of Rx10,000. Alternatively, you can use a multimeter in conjunction with a 9-volt battery to test the microwave diode.
A healthy diode will only show continuity – a continuous electrical path – in one direction. Therefore, you should test for continuity in one direction and then the other direction. If there is continuity in both directions, the diode has shorted and needs to be replaced. If there is no continuity, the diode is open and needs to be replaced.
To test a diode with a multimeter:
- Set the multimeter to Rx10,000 or higher.
- Calibrate the meter leads.
- Touch the black multimeter lead to one end of the diode and the red multimeter lead to the other.
- Note the multimeter reading.
- Swap the multimeter leads to test for continuity in the opposite direction.
- Note the multimeter reading.
If the multimeter showed continuity in both directions or not at all, the diode needs to be replaced.
To test a diode with a 9-volt battery:
- Set the multimeter to DC voltage.
- Hold the black multimeter lead against one end of the diode.
- Touch the opposite end of the diode to the battery’s negative terminal.
- Touch the red multimeter lead to the battery’s positive terminal.
- Note the multimeter reading.
- Test the diode for continuity in the opposite direction by holding the black multimeter lead against the opposite end of the diode. The other end of the diode should touch the battery’s negative terminal and the red multimeter lead, the positive terminal, like the first test. You can also reverse the battery, holding the same end of the diode against the positive terminal on the battery.
- Note the multimeter reading.
A healthy diode will show a drop in voltage by a few volts when pressed against one end of the diode and next to no change when pressed against the other end of the diode.
Accessing the Diode
Accessing the microwave will depend upon the microwave that you have and whether it is part of a kitchen cabinet or is free-standing. On some microwaves, the control panel may need to be removed; on others, unscrewing the rear panel may give you access. Refer to the microwave’s manual if you are unsure which panel to remove.
To remove and replace the diode:
- Disconnect the microwave from the power source.
- Remove the turntable plate and support.
- Unscrew and/or unclip the relevant panel(s).
- Discharge the capacitor (note the safety warning above). Some diodes and capacitors may be behind another panel that will need to be removed.
- If you need to disconnect any wires or remove any parts, like the waveguide, take a picture to remember how to reassemble the microwave. Using needle-nose pliers may assist with the disconnecting of wires.
- The diode is usually secured with a screw, with the other end connected to the capacitor. Remove the screw securing the diode.
- Use needle-nose pliers to remove the other end of the diode from the capacitor. Note which end of the diode connects to the capacitor. If the new diode is installed the wrong way, it likely will not work.
- With the old diode removed, install the new diode, making sure to get the right polarity.
- Reassemble the microwave, making sure to reconnect any wires and/or put back any parts you may have removed.
Your microwave should now be ready for use. If the microwave still has a heating issue, the diode may need to be turned around or the magnetron or capacitor replaced.
