Why Is My Microwave Sparking?

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It starts out calmly enough as you place a plate of leftovers in the microwave oven and set the timer for two minutes. But the situation very quickly turns alarming when you press the start button and begin to see sparks flying around inside the oven cavity. Why is my microwave sparking?! Repair Clinic has some likely answers, but first, turn the microwave off (if you haven’t already) and keep it off until you can fix the problem. In fact, unplug the unit’s power cord just to be on the safe side.

What causes a microwave to begin sparking or arcing?

Understanding what causes a microwave to begin sparking, or arcing as it is commonly referred to, requires knowing something about how the microwave actually heats up the food.

Alternating current is converted to direct current

Once a designated power level option and cooking time are selected, pressing the start button will cause the microwave’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. A high-voltage capacitor works with a diode to convert the alternating current power output of the transformer to direct current (DC), doubling the voltage to nearly 5,000 volts, which provides the voltage needed to power the appliance’s magnetron.

Electromagnetic waves generate the heat to cook the food

The magnetron emits electromagnetic waves which travel through a waveguide before entering the oven cavity. Once inside 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.

What is arcing and why does it happen inside a microwave oven cavity?

Electrical arcing is a form of electrical discharge where the current ionizes the air as it flows from one conductive point to another. You’ve seen this happen if you have inadvertently placed a metallic object such as a spoon or aluminum foil in the microwave and then turned the unit on (don’t try this at home!). The electromagnetic waves bouncing around inside the microwave oven cavity will strike the metal object, producing a spark and a crackling sound. This is why it’s important to only use microwave-safe plates and containers when heating up food or beverages. Decorative plates or mugs can potentially have metallic elements in the paint that will conduct electricity.

The three things you can do if your microwave is sparking

Provided you’ve confirmed that you haven’t left any metallic object in the microwave, there are three things you can do to troubleshoot why your microwave is sparking or arcing:

  1. Remove the waveguide cover – The microwave oven’s waveguide cover is a thin sheet, usually made of plastic or mica, which covers the opening on the top of the oven cavity, or on the right-side wall, where the electromagnetic waves enter. If food particles or grease build up on the cover, or the cover is damaged and shorting out, sparking can occur. Check the waveguide cover for residue or burn marks and confirm it is tightly secured. Even if the cover does not appear to be visibly damaged or loose, you should try removing the cover to see if that resolves the sparking or arcing problem. If it does, you should install a new waveguide cover to prevent the problem from reoccurring.
  2. Inspect the oven cavity for chipped paint – If there are any chips in the paint used to cover the interior of the microwave oven cavity, the exposed metal underneath will provide a conductive point for arcing to occur. Inspect the cavity for any chipped areas. If you find any, you should sand down the damaged section and use high-temperature microwave-safe touch-up paint to paint over the exposed area.
  3. Replace any damaged shelf supports – The most common place for the paint to chip is near a microwave shelf support, especially if the rack or shelf support is loose or damaged. Again, sand down the damaged area and repaint, but you should also replace any damaged rack or shelf supports with new ones.

What if the sparking is happening elsewhere in the microwave?

If you’re seeing sparking happening elsewhere in the microwave apart from the oven cavity, it could be caused by a shorted door switch or control board, or by a faulty high-voltage component such as the diode or magnetron. Repair Clinic recommends that only experienced professionals take on the repair since inspecting or testing these parts requires disassembling the microwave,

Safety first when repairing a microwave

Fortunately, replacing a waveguide cover or shelf support in your microwave doesn’t require you to disassemble the unit. Nonetheless, Repair Clinic always wants you to put safety first:

Because of the high voltage and high current used by microwave ovens, be aware that repairing this appliance poses a substantial risk of injury or death if precautions are not taken. You should always unplug the microwave before you attempt any disassembly. High-voltage capacitors used in microwaves have the potential to shock and this is the reason we stress that only experienced professionals access and replace internal components.

Make sure the microwave capacitor is discharged

If you do need to access internal components, you should make sure the capacitor is fully discharged immediately after removing the microwave’s shroud or cover. While 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, it’s always a good idea to confirm this by attempting to discharge the capacitor manually. This can be done by placing 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.

    Choose only genuine manufacturer microwave oven parts

    If you want to fix your microwave the right way (and keep it fixed), you should use the genuine manufacturer parts intended for your appliance. As your repair partner, Repair Clinic stocks only genuine OEM replacement parts that match the most popular microwave models such as those built by GE, Samsung, Whirlpool, LG, Kenmore, Frigidaire, and Panasonic. How can you find the right waveguide cover or rack support that fits your microwave? Enter the full model number of the unit in Repair Clinic’s “Search Parts Online & Get Answers” search bar to see a comprehensive list of compatible parts. You can then use the “Part Category” filter (“Shelf & Shelf Support”) along with the “Part Title” filter (“Rack Support”) to identify the specific part you need.

    Repair resources to fix your microwave

    More than just a parts warehouse, Repair Clinic has the repair resources to help you fix your microwave. Search the website’s “Videos & Articles” library to find schematics, step-by-step guides, and “how-to” videos that will show you the proper way to install a new diode in a Frigidaire Microwave (model FPMO227NUFA) or how to troubleshoot a microwave that’s not heating.

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