Microwave Ovens: Theory of Operation

The circuit of a typical microwave oven looks like this:

microwave oven circuit

microwave oven circuit

Note that I haven’t shown anything on the primary side of the transformer. This is all boring stuff; a timer, some interlock switches and the turntable and fan motors. I may mention it at a later date.

MAG1 is the magnetron. Electrically speaking, this is just a diode valve with a directly-heated cathode (it just happens to emit microwaves when it’s conducting; but the rest of the circuit doesn’t care about that, it just sees energy changing state). The anode is tied to the chassis, so the cathode must be pulled negative for the magnetron to conduct. This is a little odd, to most people’s way of thinking (unless you’re used to PNP transistors, in which case you will probably think valves and NPN transistors are odd), but it’s safer to have the anode earthed and the cathode at some dangerous voltage than the other way around. Diode CR1 is wired in inverse parallel with MAG1. The transformer has two secondaries: a low-tension winding which supplies power to the heater filament of MAG1, and a high-tension winding which supplies the anode of MAG1. The LT secondary needs good, thick insulation; but it’s only a few turns anyway. We also have a fuse, and a high-voltage capacitor.

microwave oven circuit

microwave oven circuit, showing current flows

During the positive half-cycle (red)
The left-hand plate of C1 charges to +2000V. The right-hand plate of C1 is held close to 0V by CR1, which is conducting. MAG1 is not conducting.
During the negative half-cycle (green)
The top of the HT winding is at -2000V so the right-hand plate of C1 drops to -4000V (the voltage across a capacitor cannot change instantaneously). CR1 is reverse biased. MAG1 is now forward-biased, and so conducts.

….. and that’s it! 20 milliseconds later, we go through the whole process again.