How to Test a Microwave Diode Also Known As A rectifier
How to Test a Microwave Oven Diode
Before you can begin the process of troubleshooting a microwave oven diode, you must first learn what a microwave oven diode is and what it does. A microwave oven diode is a semiconductor. A semiconductor is an electrical device that allows current to flow in only one direction. Imagine for a second that you had a gate for a fence that only opened in. Everyone who entered that area could only go in through that gate, they could not go back out. They would have to find another way out. A microwave oven diode is kind of like that gate that only opens in one direction, it only lets electricity flow in one direction. That's why it is called a semiconductor. It only conducts electricity in one direction.
On a microwave oven, a high-voltage diode is used to create a pulsating direct current circuit. It does this when connected to a 120 volt AC circuit; because it only lets half of the voltage get through. Alternating current is a type of electricity that moves in both directions, whereas, direct current moves in one direction.
One of the components on the high-voltage side of a microwave oven is called a capacitor. A capacitor has the ability to store electricity even when the system is unplugged or off. In the case of a microwave oven capacitor, it can hold a very high voltage. The voltage stored in a capacitor of a microwave can be around 2,000 volts and will definitely get your attention if you do not respect it. Before you service anything inside a microwave, the microwave oven should be disconnected from the electrical supply, and the high voltage capacitor should be discharged. There are two sides to a capacitor, that is to say there are separate terminals on the top. One side will always have a high charge, once again even if the microwave is unplugged. To neutralize or discharge a capacitor on a microwave, you need to assist the charge that is built up on the plates on one side of the capacitor, to move it over to the other side. This is done using a screwdriver with a good insulated handle if the wires and connectors going into the capacitor are not shielded or insulated. If they are you will have to use either needle nose pliers, or snap ring pliers with tips on the ends to make a connection between both sides of the capacitor. Either way, needle nose pliers or snap ring pliers, make sure that they have insulated handles.
Once the capacitor has been discharged, you can now remove the diode from the circuit and check it. A lot of digital multimeters today have a function to check diodes. When it comes to microwave diodes, this function is essentially worthless. And that's because a microwave diode can be good, but the meter will read it as defective. The reason for this is that the testing system in the meter relies on that low voltage battery inside the multimeter. This is not enough current to check a microwave diode. The best way to check a microwave diode, is to hook it to a digital multimeter and connect it to 120 volts AC, as shown in the picture above.
Notice that one line of the test cord is attached to the diode. On the other end of the diode is one of the leads that goes into the digital multimeter. The other lead that comes out of the digital multimeter is connected to the other side of the line on the test cord. Notice that the meter is reading 60.9 volts. This is because the diode is only permitting one side of the line from passing through. When you read around 60 volts on the meter, that means you are reading one half of the cycle from the test cord, confirming that this is a good diode. If the diode were shorted, it would act as a conductor instead of a semiconductor and you would be reading 120 volts on the meter. If the diode were open, then you would read nothing on the meter.