Rewound Microwave Transformer

Thread Starter


Joined Mar 4, 2004
I am trying to rewind a microwave oven transformer secondary using 10 gauge wire on the inside, to get approximently 45 volts from 110 volts on the primary. The transformer is two E core sections, with a current limiter. I get approximently 1 volt per turn, and the E section for the secondary is too small. Could I wind the secondary outside the core? Could I combine winding the inside and outside of the core?


Joined Jan 22, 2004

could youclarify these questions?

1.Could I wind the secondary outside the core?

2.Could I combine winding the inside and outside of the core?

3. What was the original voltage output ofthe secondary?


Joined Aug 20, 2004

I'm working on pretty much exactly the same thing. Mine is not done to my satisfaction just yet, however I can give you a couple of hints...

Weigh your transformer core, and assume a power rating of approximately 100 watts per pound. Above that you'd need active cooling (fan, heatsink, etc). The transformer can be warm but not hot to the touch - otherwise it's overloaded. I have a 10 lb transformer that should be good for a kilowatt, I'm trying for 30V/30A on mine.

If your transformer is the same size and you want 45 volts, you can draw 22 amps max. The voltage drops under a load, so aim for a higher open-circuit voltage. (how much higer? no idea, try it and see ;) You should be ok using 12 or 14 gauge wire for that current (see wire ratings, 16 gauge would be marginal, and bigger is of course better as long as it fits. Use solid wire only, stranded will run hotter and produce less current due to internal losses.

No, you can't wind outside the core, since such a winding gets equal numbers of flux lines in opposite directions and hence zero overall voltage (actually sometimes that is done deliberately to reduce EM interference). The thinner wire should do the trick.

The transformer will absolutely not be short-circuit-safe after it's rewound, so you should fuse the output as well as the input. A couple of auto fuses should do the trick. The core itself should be tied to mains ground (green wire).

Watch out for the killer voltage spike when you turn it off... a toroidal choke between the line and the primary (both hot and neutral wound on it together) and a big MOV across the primary (rated 50-100 joules) should suppress sending most of that back into the line. The spike appears on *both* the primary and secondary, and is *very* energetic given the size of the transformer.

If you want a test load, try a hot water heater element (put it in water when you run it or it will self-destruct ;). You can probably get one from the hardware store for $10, it's rated at something like 3000W / 240V but in your case it'll run at ~ 600W / 45V / 13A.

If you want to pot the secondary but don't have transformer laquer, you could use GE Silicone II (other silicones are corrosive, that one should be ok).

If you want more power, you can use two or more transformers with the primaries in parallel and the secondaries in series with thicker wire and fewer windings (like this guy at has done). It is a very cool idea... however, if you use a series-parallel combo with more than two transformers, do not tie their midpoints together like he did... then you can even use mis-matched transformers.

Other people who have done this: and

Where I'm at so far... 40 turns of 12 gauge wire, 45 volts with no load. Still working on spike supression, I think I'll end up doing a triac controller to avoid spikes entirely since my intended load cannot handle that. Eventually I'll do a uC-controlled current-source switching supply with IRF540 MOSFETs on the output end... bass-ackwards I know but that's what I need.

Good luck and let me know how it goes.
First, I'm trying to do an inverter. So perhaps the ordinary transformers may not be all that effective, but I'm yet to give it an actualy try. I'm in the process of making it.

But then I just wanted help in rewinding a microwave transformer to get a 12V to 220V from a 12V either 4A or a 10A battery source. Is it even possible to even light a few tube lights with them...?
And then if I need to rewind the Micro wave transformer, how do I do it? What type of wire do I use?