Here's a GE microwave schematic. There's a lot of typical microwave stuff going on here, but we're going to focus in on one component today: the monitor switch.

What exactly is this switch's purpose, and how does it affect the circuit?

The purpose of the monitor switch is to act as a safety to prevent the magnetron from being energized while the door is open. The door latch and switches in microwaves are designed so that, if everything is working correctly, the monitor switch should never be closed at the same time as the interlock switches. In fact, the only situation in which it would be possible for the monitor switch to be closed at the same time as the door switches is if there's been a mechanical failure of the door latch/switches.

So what happens if the monitor switch is closed at the same time as the door switches? Well, first let's take a look at how things look during normal operation. This markup shows the power supply to the high voltage transformer. Note that the monitor switch is open here, as it should be during normal operation.

Nothing too complicated there. But what happens if that monitor switch is closed?

Well, that does look like it would be a problem! If that monitor switch is closed during operation, it creates a dead short -- and that's by design! The amps in that circuit will immediately jump far above the rated spec on the microwave's fuse, which will go open and kill the whole unit. A simple, but effective way of making sure the magnetron can never run with the door open.

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