Microwave Blows Fuse When Started? Here’s Why

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To help keep you safe, most modern microwaves have several different fuses. To prevent microwaves from leaking out, door fuses will blow if the door is not closed properly. Thermal fuses will blow to prevent the microwave from overheating and potentially causing a fire. A fuse will also blow if there is a surge in electricity, which can cause a short circuit. If the microwave blows a fuse when started, the most likely cause is that the high-voltage capacitor has shorted out and needs to be replaced.

High-Voltage Capacitor

The high-voltage capacitor stores or condenses electricity before releasing it to the microwave’s magnetron. When the electricity is released, it is released very quickly for immediate use. As its name suggests, the high-voltage capacitor stores large amounts of electricity. To keep you safe, if the high-voltage capacitor becomes defective, it will cause a fuse to blow when you try to start the microwave.

There are several reasons why the high-voltage capacitor may have shorted and become defective. If the microwave is new, the capacitor may have been defective when installed or may have become damaged during transport or installation. If the microwave is old, the capacitor has most likely become defective because of normal wear and tear. A surge in electricity may have also caused the high-voltage capacitor to short out.

How to Replace the High-Voltage Capacitor

To access the high-voltage capacitor, the microwave’s cabinet will need to be removed. Removing the cabinet will depend on the type of microwave. You may need to remove several access panels with screws located in various locations, including underneath the microwave. You may also need to remove the grille and/or control panel.

Before disconnecting wires inside the microwave, take a digital picture to remember how to reconnect them.

  1. Disconnect the power to the microwave.
  2. If the microwave is installed in a cabinet, remove the mounting screws and move the microwave to a safe location to perform the repair.
  3. Remove the appropriate access panel(s) to remove the microwave cabinet.
  4. Disconnect the wire harness.
  5. Discharge the high-voltage capacitor (see safety warning).
  6. Use needle-nose pliers to disconnect the capacitor wires.
  7. Remove the screws that secure the capacitor mounting bracket.
  8. Remove the capacitor from the microwave.
  9. Test the capacitor with a multimeter to determine if it needs to be replaced.

Other Causes

If the microwave was recently repaired, a fuse may have blown because it was installed incorrectly or because the wrong type of fuse was installed. When replacing a fuse, the connectors must face the right way, and the replacement fuse should have the same ampere rating as the original. In addition to the fuse blowing, installing a fuse with a different ampere rating could damage the wiring and cause a fire. Therefore, when replacing a fuse, it is best to use the same type of fuse, with the same ampere rating.

How to Replace a Blown Fuse

If the fuse has blown when starting the microwave, it is most likely the main fuse, also called the line fuse, that has blown. Microwave fuses can be located in several different locations depending on the make and model of the microwave. To locate the fuse, refer to the microwave’s wiring diagram or manual. The main fuse is usually a narrow, rectangle shape, two inches in length, and either a ceramic fuse or a glass fuse with a filament.

To test and replace the fuse:

  1. Disconnect the microwave from the power source.
  2. Remove the relevant access panel. Depending on the make and model, you may need to unscrew the control panel or microwave frame.
  3. Unscrew or unclip the fuse and remove it from the microwave.
  4. Test the fuse with a multimeter for continuity.
  5. If defective, replace the fuse with a new one that matches the fuse you removed.
  6. Reassemble the microwave and restore the power.
  7. Test the new fuse by microwaving a cup of water for thirty seconds.

If the fuse blows again, either it was the wrong type of fuse, the fuse was installed incorrectly, another microwave part is faulty, or there is an issue with the residence’s electrical system.

Safety Warning

Repairing a microwave is more dangerous than repairing most other household appliances. The high-voltage capacitor inside the microwave can store a lethal amount of electricity, even after the microwave has been disconnected from the power for several months. If the high-voltage capacitor or another electrical component is causing the fuse to blow, the capacitor will need to be discharged.

The capacitor can be discharged by touching both the positive and negative terminals of the capacitor with a metal screwdriver blade. Precautions must be taken to insulate yourself against electrocution. A screwdriver with a rubber handle, or needle-nose pliers with rubber handles, could also be used to discharge the capacitor. Due to the risk, repairing the microwave and discharging the capacitor should be left to a trained microwave repair professional.

Repair or Replace?

While microwave fuses are relatively inexpensive to replace, the safety concerns and the cost of replacing the high-voltage capacitor or another component may make replacing the microwave the better option. Purchasing a new microwave with a warranty also supports the argument that replacing the microwave is the better option.

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