4200V single-phase 2-MOT resonant supply

Providing output of a kilowatt long-term and in excess of 4 kVA short-term, undoubtedly it is the most powerful single-phase power supply I’ve made, and the scariest of them all! Whether you want to power up medium size Tesla coils, arc-weld through any sensitive electronic evidence or commit suicide using one of the most horrendous ways, this beast always gets the job done.

4200V Killer supply, arc 1Hi there

Shame that living in a flat gives no possibility of obtaining three phase power. In such a case I’d go for 1.5 meter long arcs with a beastly 15 kVA stack 🙂

4200V Killer supply, arc 240cm arc

Just what sorcery is inside? None! Purely, just parts from 4 microwave ovens. Yes, you’ve guessed it right – two microwave oven transformers (MOTs) with their primaries paralleled and secondaries in anti-phase, to double the voltage of a single MOT (2100 volts ac) to 4200 V. To put both transformers in resonance, a capacity of 2-3 microfarads rated at least 3kV= is put in series with each transformer output (center tapped to ground, yielding two 2kV~ outputs against ground). In my case I used four oil-filled microwave oven capacitors rated 1uF each in a 2×2 configuration. Don’t omit these – you’ll thank me later.

4200V Killer supply, gutsWhat’s inside

A MOT is, however, a fine example of maximum savings on materials. Some are even wound with aluminum primaries, lacquered orange to look like copper… (remember, chinks are masters when it comes to fakes!) So even these 1000W-rated MOTs pictured above are really just 250-300 watt transformers long-term and cannot be used for their “rated” powers, or God forbid arcing, without active cooling. That’s why there’s a fan inside the guts of your microwave. For this setup to survive, you will need at least two of them fans, in close proximity to your MOTs.
What I did above is far from enough! Should have used a bigger cage…

4200V Killer supply, plasma climbing upPlasma climbing up from the arc

Unless you have a circuit breaker rated 25A and more, don’t try connecting more than one microwave oven transformer to your mains directly, or the huge inrush current will cause it to trip most of the cases. I used a soft-start circuit obtained from one of the microwaves I disassembled, and works like a charm. Basically, it connects those MOTs for a fraction of second to mains, through a few-ohm 30W resistor. Then a relay shorts it out, connecting them directly. Despite drawing 10 amps at idle, this method never trips my class B “10A” breaker. Or, well, at least the power up itself doesn’t. 🙂

4200V Killer supply, frontIt used to be an oscilloscope decades ago…

If you don’t want to use fans to cool those transformers down, you can limit output current with stacks of chokes out of mercury or sodium lamps. And, as mentioned above, I used microwave oven capacitors to get those MOTs in resonance. However, if you plan using different capacitors rated the minimum voltage as described above, discharge them before playing inside. Most oil filled microwave oven capacitors discharge themselves within few seconds as they have an internal bleeder resistor encapsulated.
My setup draws a juice of 10.7A~ rms from 240V, at idle. Arcing and fiddling with the output made the ammeter to jump between the 20 and 25A markers, so watch your electricity bill 😉

That’s all folks – I suppose there’s no need to draw a schematic for this, as the wiring and the soft-start solution have been documented thoroughly above. Contrary, if you really needed a circuit schematic for a setup like this, you should swiftly click on “Low voltage circuits” on the side menu…

5 Replies to “4200V single-phase 2-MOT resonant supply”

  1. Hi Jozef,

    I messaged you before on another great blog post of yours about the CFL circuits which blow up in just a few seconds, forget that because I have gathered 2 of those big high pressure sodium lamp chokes so I can now build a reliable high voltage power supply I hope.

    However, the bulky chokes are not transformers… they have just one winding which is center tapped and I have a 45uF capacitor and one ignitor circuit that I connected on the first choke as it should be done original and that gives me a 4.5kV output at the end of the winding.

    I connected the 4.5kV output to the 2nd choke and my own hand made capacitors in series plus my flyback primary winding and that negative lead is connected back to the first choke common negative.

    I think it’s connected the right way but I know that I’m playing with a lot of potential power and because it’s directly connected to mains, half of the system is live and only my flyback high voltage output is DC which is isolated from the mains so it’s very dangerous and I don’t allow anyone to come close to any part of it.

    Can you please verify that I have it connected correctly before I flip the switch because I’m a little scared to just do it since I’m not really 100% sure of what I’m doing.


    Ricardo Penders CEO

  2. Hi there, idk if you still reply but I guess I’ll try. I tried this, it works, but there are some differences. Firstly, I do not have four capacitors, I only have two. Secondly, one of the transformers is smaller than the other. Despite this, I get huge, beautiful arcs. But, the smaller transformer gets very hot quite quickly , usually from ~1-2 minutes of use. The larger one is fine. I am using two fans, one on the bottom of the case blowing air in and one on the top blowing out. Do you have any suggestions as to how I can prevent the transformer from getting so hot? A method to do so that will decrease power output as little as possible is requested.

  3. hi
    I search for a high voltage circuit that is used in portable xray equipment
    this circuit is a kind of voltage clamp circuit that its voltage input is 220 volt and the output is 10 kilo volt
    Do you know this circuit design?

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