A microwave oven high voltage capacitor works the same as every other capacitor. Meaning that it stores electrons for half of an electrical cycle and releases them back into the circuit on the other half of the cycle. Let me explain using an example.
Imagine that you had a roll of aluminum foil as wide as a plastic coffee container is tall. Pull about 3 feet of foil off that roll and lay it on a table. Now pull another sheet off, also 3 feet long and lay it next to the first one on the table. Take your trusty soldering iron and solder a 6 inch piece of insulated wire to the very top of both pieces of foil.
Now we have two pieces of aluminum foil as wide as a coffee can is tall with a small section of wire soldered to the very top of the foil.
The next thing we’re going to need is a roll of paper as wide as our coffee container is tall. That’s because we are going to pull off a 3 foot section of paper and place it on top of the first section of aluminum foil. Next we will take the other section of foil and place it on top of the paper. So now we have a sandwich of aluminum foil with wires sticking out of the top, and a piece of paper in the middle.
Next we roll the foil and the paper into a log. So now we have a roll consisting of two pieces of aluminum foil with a piece of paper in between with two wire sticking out the top. The paper will insulate the two pieces of aluminum foil from coming in contact with each other. Now let’s grab our imaginary plastic coffee can and stuff our log of foil and paper inside. If we solder a couple of spade terminals to the other ends of the wires attached to the foil and push them through the lid of the coffee can, we now have a capacitor. So if we put our extremely large imaginary capacitor in the high voltage circuit of a microwave oven, the foil would act as a momentary storage device for electrons in that circuit doubling the available power 60 times per second.
Even though the capacitor is in series with the circuit there is no electrical connection between the two pieces of foil. Some microwave oven capacitors use a bleed resistor to neutralize a capacitor when the circuit is not in use. This resistor is electrically irrelevant when the capacitor circuit is active. By the way never assume a resistor has neutralized a capacitor when it is not active in the circuit because you could be in for the shock of your life.
Photo of a Capacitor analyzer used to check a microwave oven capacitor
With the microwave oven unplugged, always use a screwdriver, snap ring pliers or long nose pliers to short a capacitor from one side to the other before touching the terminals.
The only way to really test a microwave oven capacitor is the same way you would test any other capacitor. In the old days when we used analog multimeters we could watch for a deflection of the meter when it was set to ohms. Today however we use electronic capacitor analyzers with a digital display. This very inexpensive tool is essential if you service microwave ovens. Capacitor analyzers have a digital display with wires that have alligator clips on them. Once you discharge a capacitor and remove it from the circuit, connect the alligator clips on the capacitor terminals and push the button. The capacitor analyzer will tell you if the capacitor is defective. In fact most of them will tell you the actual microfarad reading of the capacitor. If you see a microfarad reading in the display that matches your capacitor rating, it is good.
The bottom line is this, microwave oven capacitors can hold a powerful charge that can light you up. Always unplug the microwave oven and discharge a capacitor before touching the terminals.