Testing A Microwave Capacitor

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At first, the microwave appeared to be working as you placed the bowl of chicken noodle soup on the glass turntable tray and set the timer for two minutes to reheat your lunch. However, when the beeping timer alert went off, you found the soup no hotter than it had been two minutes earlier. What happened? A microwave that is not heating properly can be caused by a defective door switch, a blown line fuse, or even a faulty control board, but it can also be caused by a malfunctioning high voltage circuit component such as the capacitor. In this article, Repair Clinic will address how the microwave’s high voltage circuit works and two different ways you can test a microwave capacitor to determine if the part is malfunctioning.

How the capacitor works with a microwave’s high voltage circuit

A capacitor stores an electrical charge and then releases to send voltage to components such as compressors and motors found in air conditioners, furnaces, refrigerators, and even washing machines. The capacitor is also a critical component of a microwave’s high voltage circuit.

The capacitor works with a diode to covert alternating current to direct current

Pressing the microwave’s start button will cause the appliance’s control board to send 120 volts of alternating current (AC) through a line fuse, a cavity thermostat, and multiple door switches, before the current reaches a high voltage transformer. The microwave’s capacitor works with a diode to convert the alternating current power output of the transformer to direct current (DC), increasing the voltage to nearly 5,000 volts. This additional voltage is necessary to power the microwave’s magnetron, the component chiefly responsible for cooking the food.

The magnetron emits electromagnetic waves to heat the food

Once the voltage reaches the magnetron, the component emits electromagnetic waves which travel through a waveguide before entering the oven cavity. As the food rotates on a turntable in the oven cavity, the electromagnetic waves cause water molecules within the food to vibrate intensely, generating heat that cooks the food from the outside in.

A malfunctioning capacitor will cause the high voltage circuit to stop working

If the capacitor is unable to hold a charge or the charge is too weak, the microwave may still “turn on” and the turntable may still rotate, but the alternating current sent to the transformer will not be converted to direct current, so no high voltage will reach the magnetron which will be unable to send out the electromagnetic waves to heat up the food. In short, a malfunctioning capacitor will cause the entire high voltage circuit to stop working. There are two tests you can perform to determine if the microwave capacitor is functioning normally, but because there is some disassembly involved to reach the capacitor for testing, a necessary safety warning is required…

A necessary microwave safety warning

Depending on the microwave model, you may be able to reach the capacitor by simply uninstalling the control panel. For other models, you will need to remove the appliance’s entire shroud or cover and this may require fully uninstalling the microwave if it’s installed above a range and acts as a venting system for the range’s cooktop. Regardless of the microwave model, safety should always come first.

Because of the high voltage and high current used by microwave ovens, the appliance poses a substantial risk for injury or death if precautions are not taken. You should always unplug the microwave before you attempt any disassembly. However, the microwave capacitor has the ability to deliver a dangerous shock and for this reason, we recommend that only experienced professionals access and replace internal components.

Discharging the capacitor

Any time any portion of the microwave is disassembled, you should immediately make sure the capacitor is fully discharged, even if you suspect the component has lost its ability to hold an electrical charge. As a safety precaution, most high voltage capacitors used in recent microwave models have a bleeder resistor in them that should automatically discharge the capacitor after the voltage has been discontinued. That’s a welcome feature, but it’s still recommended you confirm this by attempting to discharge the capacitor manually. How can you do this? Simply place a screwdriver blade or needle-nose pliers across each set of capacitor terminals. The handle of the screwdriver or pliers should be insulated, and you should avoid touching the metal portion of the tool when the tool is in contact with the terminals.

How to test a microwave capacitor

There are two tests you can perform to help determine if a microwave capacitor is functioning normally. The first uses an analog-style ohm meter to test if the capacitor is capable of storing and then releasing its electrical charge; the second will determine if the component is operating with the proper capacitance rating and requires a capacitor tester or a multimeter with a capacitor testing function.

Testing the microwave capacitor’s ability to store and release an electrical charge

  1. Disconnect the wires from the capacitor terminals and isolate or uninstall the capacitor from the microwave.
  2. If necessary, calibrate the analog-style ohm meter by contacting the ends of the meter leads together while adjusting the needle to read “zero”, then rotate the meter’s range selection dial to a setting of 1,000 ohms or higher.
  3. Contact one meter lead to one of the capacitor terminals and the second lead to the other terminal.
  4. Observe the meter display. When the second lead comes into contact with the terminal, the meter’s needle should deflect toward zero ohms and return to infinite resistance (this will happen very quickly). Reverse the meter leads and you should observe the same result.
  5. Did you not see the needle move or did the needle stay near zero ohms? That means the capacitor is not able to store and properly release an electrical charge and is faulty.

Determining if the capacitor has shorted

A standard ohm meter can also be used to determine if the capacitor has shorted. Place one meter lead on a terminal and the other lead against the capacitor’s casing, then move the first lead to the other terminal, keeping the second on the casing. Neither terminal should display electrical continuity. If the meter display does indicate electrical continuity, you’ll know the capacitor has shorted and will need to be replaced.

Measuring the microwave capacitor’s capacitance rating

  1. Note the microfarad rating printed on the capacitor itself. Example: 1.00 µF.
  2. Choose the appropriate setting on the tester or meter.
  3. Contact a meter lead to one of the capacitor’s terminals and the second lead to the other terminal.
  4. Observe the meter display. The reading on the display should be close to the microfarad rating on the capacitor. If the display indicates the amount of microfarads is lower than the rating on the capacitor, the component does not have the suitable capacitance to help power the microwave’s high voltage circuit and a new capacitor will need to be installed to fix the heating problem.    
Microwave Capacitor Testing

Microwave repair information found at Repair Clinic

To assist you in accessing the capacitor or any other microwave component, Repair Clinic provides free microwave repair help on its website in the “Videos & Articles” section. It’s there that you’ll find model-specific schematics for microwaves manufactured by GE, Samsung, Whirlpool, LG, Kenmore, Frigidaire, Panasonic and more. You’ll also find procedural videos like this one showing how to replace a capacitor on a Whirlpool Microwave/Vent Hood Combo (Model WMH73521CS6) or this one which takes you step-by-step through replacing a line fuse on a Frigidaire Microwave (Model FPMO227NUFA).

Choose genuine OEM microwave parts from Repair Clinic

Generic replacement parts are not a good fit for your microwave since manufacturers design parts properly calibrated and tested to work with all the other the components in the unit. This is why Repair Clinic stocks genuine original equipment manufacturer (OEM) parts to ensure the appliance, lawn, and garden equipment, or heating and cooling unit can be fixed correctly. To find all of the compatible OEM parts for your microwave, enter the full model number of the unit in the Repair Clinic website search bar. From there, you can use the “Part Category” navigation filter (“Capacitor”, “Fuse”) followed by the “Part Title” filter (“High Voltage Capacitor”, “Line Fuse”) to locate the exact part you need for the repair.

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