This page describes some simple simulations exploring the use of microwave oven transformers (MOTs) and other microwave oven power supply components in high voltage power supplies like Tesla coils. The simulations only explore various configurations that may be implemented and are not to be considered as plans in themselves. The intent is to provide the experimenter with an overview of the options available.
First we will look at the basic design of a microwave oven high voltage power supply.
This is a schematic of the basic high voltage power supply circuit found inside the typical microwave oven. Now we will complete the circuit by adding the magnetron.
This is the complete circuit as found in microwave ovens. Note that the magnetron is represented in the diagram by a diode in series with a complex impedance. The impedance is complex, with a real component (the energy that is absorbed as heat by the load (food) and the oven itself) and an
imaginary component (the energy that is "bouncing around" inside the oven and has not yet been absorbed). But enough on that.
We will now redraw the circuit as a classic half wave voltage doubler.
We will now break the circuit into its functional groups.
Now for the first model.
The voltage markers are labeled with the colors of the corresponding traces in the voltage simulation below.
In this example the input voltage is 12VAC RMS at 60Hz. The transformer is modeled to have the same transformation ratio as a typical MOT. Note how the blue trace is "shifted" to the negative side of zero. This is where the level shifter stage of the voltage doubler gets its name. Now we will look at some current waveforms for the same model. The first will be for the no load condition. Current markers are set up like this:
The no load condition:
Now the same model with a load on it.
Note the waveform (yellow) coming from the transformer. The current is drawn in short bursts, which are short in duration but high in amplitude. This is the mode of operation that the MOT is designed to be used in. That is why they do not seem to have much current limiting when compared to a NST in a standard (AC) configuration. A look at the input waveform is next.
In this model the transformer is linear, and no provision is made to account for the leakage inductance. It is apparent from the magnitude of the input current waveform that a current limited transformer is definitely necessary. Next we have the voltage traces for the same model under load.
Now exactly the same model but with the polarities of the diodes reversed.
Now on to more fun stuff.