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25th April 2021

8 MOT Power Supply

Introduction

High voltage hobbyists all around the world are usually fooling around with so called "pole pig" transformers. 

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These transformers usually supply a single neighborhood or even a single house. They aren't too heavy and they can run on a mains voltage with ease.

Here in Czech Republic we usually have just a single huge transformer for the whole village/city. 

Of course it would not be practical to have a huge 500kg transformer just to make some arcs. So this is where microwave oven transformers come in handy. 

A pole pig transformer

Microwave oven transformers are quite powerful high voltage transformers. They step up the mains voltage to almost 2.3kV and combined with a high voltage capacitor and a high voltage diode in a voltage doubler configuarion it produces well over 4kV to run the magnetron which creates the microwaves. 

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Microwave oven transformer

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Microwave oven capacitor

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Microwave oven magnetron

These transformers can deliver well over 700W of power. The current on the secondary high voltage winding can reach half an Amp! Always make sure you cannot come in contact or near proximity with the high voltage output of this transformer. 

The transformer has quite a few problems though. The transformer's core is not big enough for the power rating. This is made so that the transformer is cheap to make and not as heavy. MOTs (microwave-oven-transformer) weigh roughly 5kg. Well designed transformers usually weigh over 10kg for these power levels and are twice as big. 

The cheap design causes high power losses because it runs very close to saturation. They are not meant to be short-circuited not even for a second. At full mains voltage they overheat in a few seconds when in short circuit condition. 

2 phase supply

A partial solution to the saturation problems is to run the MOT at lower voltage. If we have more MOTs and 2 phase mains voltage available, then we can wire 2 MOTs in series and run them between 2 phases. Here in Europe our mains voltage is 230Vrms and 400Vrms between two phases. 

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2 phase MOTs setup

This way the primaries only get 200V and so they can run below the point of saturation. But then again, this is only partial solution. The transformers still heat up a lot. In this configuration the MOTs can run in short circuit for a few minutes. Still not a continuous power supply, but for some arcs it will do.

Resonant setup

To reach the maximum possible output, we can introduce a resonance to our setup, because let's be honest, resonance is always the solution! We can make use of microwave oven capacitors as they are designed to be rated for this kind of abuse. Their nominal voltage is 2.1kVAC, but before they are approved and used in a commercial product, they have to be tested up to 10kV. This way they can survive resonant voltage rise and they also do not complain when a few Amps is flowing through them.

8 resonant MOTs

So to get the absolute maximum from a 32A 2phase outlet I put together 8 MOTs and 16 capacitors in the following configuration:

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8 MOTs and 16 capacitors setup

MOTs in this configuration are wired 4 in parallel and two of these parallel combinations in series (4P2S). The capacitors are two sets of 8 capacitors in parallel wired in series (8P2S). Finished setup with some 3D printed parts ended up looking like this:

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Finished setup

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Finished setup, 3 phase 16A connector and some spare capacitors

First arcs with 4 MOTs (16kVA)

So unfortunately I only have a 16A breaker available for now, so I had to disconnect 4 MOTs. In this setup the transformer consists of 2MOTs in parallel and 2 of these parallel combinations in series (2P2S). Let's make some arcs!

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Over a meter long arc

Some nice hot arcs

I have made a whole long video explaining this setup a bit more with some current measurements:

I can't wait to test this setup with 32A breakers. With all 8 MOTs connected, I expect the arc to be able to stretch to almost 2 meters.

No words can really put into justice how lethal this thing is! Microwave oven transformers are the biggest high voltage hobbyists killers. Touching the output of a single MOT will certainly end up with a death. Combining these transformers in series or parallel combinations only makes it more dangerous. Do not try to replicate this unless you know very well what are you doing!