Microwave micro switches

Question

The wires connecting to the normally closed switch have a short circuit across them, which is causing the switch to stay closed all the time! The clock works, but when you try to cook something the fuse blows. What could be causing the short?

Answer 1

Typical microwave ovens will have multiple micro switches that are designed to provide safe use of the unit. Safe use translates to making sure the door is closed before the unit is able to produce the microwave energy, and magnetron cavity energy is dissipated if the door is opened while the microwave is operating, to prevent operator exposure to microwave energy.

The safety feature is normally designed so that the multiple switches have to sequence on/off properly when the when the microwave door is opened and closed. If the switch sequencing fails or changes (worn or broken mechanism, misaligned switches or user attempted defeat of safety feature), that short circuit becomes a short across the AC power line at the microwave. In this case, contacts in one of the microswitches can fuse together. The short circuit will normally trip the circuit breaker in the house electrical panel, requiring reset. If one of the microswitches is fused, this will happen repeatedly. This prevents operation of the oven in the unsafe mode.

In your case the problem is likely to be one of the microswitches has failed, or the interlock mechanism that operates these switches is worn, broken or misaligned. The door latch closure also changes switch operation timing or sequence.

You may want to reference this older question and additional links therein:

Microwave door interlock switch keeps failing

Pay particular attention to comments regarding OEM replacement switch costs versus buying switches from an electronics supplier.