01 Nov 2021 - tsp
Last update 01 Nov 2021
So first off this is an article that requires an extensive warning:
So what is this blog article about? Building a cheap power supply out of some scrap or simple buyable components that delivers - depending on the configuration:
The transformer to be used is a standard microwave oven transformer (MOT) as it’s used in many different microwave ovens. Since these ovens use electron tubes - so called Magnetrons - in which electrons are emitted from a hot cathode which is achieved by heating a tungsten filament using around $3V$ and a rather low current (that’s derived by a second secondary winding on the transformer usually) and then accelerated towards an anode which requires a high negative voltage (electrons travel from a negative source to a more positive terminal - and at this voltage reach around 10.8% of the speed of light) since the target is designed to be at ground potential - they produce a somewhat higher voltage of typically around $2.1 kV$. Thus one can use these transformers as source for $2.1 kV$ alternating current. These devices are rather crude and have high losses - around 200W are typically burnt just to magnetize the core.
Side note: Why is the tube called Mangetron? In these devices electrons don’t travel on a straight line but are forced to travel along a curved path by an external magnetic field - these are the two black rings that you can see on the side of the magnetron. The electrons travel towards anodes that form the capacitor plates of an LC resonator and charge them alternatively depending on it’s previous state. That way they drive the resonance frequency of the LC tank circuits that is then also extracted via a single antenna and radiated into the microwave ovens working volume.
Note that the transformers usually have
Please note: Even though the ground wire is connected it won’t protect anyone from contact with the high voltage. The residual current detector won’t trigger since the power supply simply works as an isolated power supply. The only reason the ground wire is connected on the secondary side is to provide a reference potential. One could also operate the device fully open - it’s a little bit harder to assembly since the transformers case and transformer sheets are one of the connectors of the high voltage side but entirely possible. Keep in mind there is no way for an RCD to work when you reach into the transformer - and also no way for a miniature circuit breaker to save anyone from electric shock.
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The following parts are / might be used:
This is pretty simple. One should keep the filtering circuit on the line side if possible. Just attach the two leads of the primary winding to your line voltage and you’re good to go. That’s it.
Inside the microwave oven a simple trick is used as an AC voltage doubler. The capacitor is attached to one end of the transformer. During one half wave the capacitor is charged via the conductive diode, during the second half wave the whole circuit is basically biased up to 2.1 kV which doubles the available voltage. This is exactly what happens inside microwave ovens.
Note that when dismantling the microwave oven you usually don’t see the grounded side - the 0V side of the high voltage winding of the transformer is directly connected to the transformer casing and thus also to the microwave oven case. The other side is usually directly connected to the capacitor, the diode to the other side of the capacitor and wired directly to the case of the microwave oven again. This is done that way since the whole case also acts as target for electromagnetic radiation and is at the same potential as the target of the electrons which is also ground potential.
The simplest way is to build a half wave rectifier - just attach the capacitor via the diode on the high side and to ground on the other side. While the transformer is delivering negative output the circuit simply does nothing - during positive output the capacitor is charged up to $3 kV$ that are delivered at maximum by the transformer. The capacitor might be enough to smooth out the ripple of the charging cycle depending on the load.
One can get more stable output when adding a full wave rectifier in front of the single capacitor and get more smooth output by adding capacitors in parallel.
To get 6 kV DC one can build a full wave doubler circuit. In this case two capacitors are put in series. Their center tap is connected to the grounded side of the transformer. The high voltage side is attached via a diode once in forward and once in reverse direction to the high and low side of the assembly. This way the transformer charges the high side capacitor while delivering the positive half wave and the low side capacitor while delivering the negative half wave. Each capacitor is charged to $3 kV$ separately. Since they’re connected in series the voltages add up to $6 kV$.