# Charger Project MOT step up or down?



## nimblemotors (Oct 1, 2010)

I'm working on my battery charger. I'm going to use a "MOT", or a Microwave Oven Transformer that I removed from my old microwave a few years ago. This charger should cost me about $100...
The microwaves are rated at 1kw of power, so I assume the transformer about the same power rating. I am obviously going to replace the secondary winding that produces 2,000 volts for the microwave with another winding.
This output AC will be rectified into DC, and then a more precision DC-DC conversion on this input to control current and voltage to the pack.

My question is really should I step UP the voltage from the MOT, or would be better to step DOWN the voltage? I want 50 volts to charge the pack.
I could have it output 100 volts and then step it down to 50, or wind it to 25 and step it up. Seems to me let the MOT handle the larger current of lower voltage output and then step it up in the DC-DC, but honestly seems to make no different really, only that step UP DC-DC doesn't need a high-side driver.


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## piotrsko (Dec 9, 2007)

Fwiw: I believe that's peak, not rms, and don't know for how [email protected] peak since I've never run mine at full on for longer than 30% duty cycle. Tends to make food blackened.

I'd just tap a winding for the voltage I want, YMMV.


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## nimblemotors (Oct 1, 2010)

I would want this charger to work for both 120vac and 240vac,
so what I'm thinking is making it do both step up and down,
step up 35vdc at 120vac and step down 70vdc at 240vac.


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## PStechPaul (May 1, 2012)

MOTs are often made cheaply and are not very efficient. It may be better to use a PFC front end boost converter like the one in the EMW charger, although it needs some design tweaks. It will provide about 350 VDC from 85-264 VAC. You may even be able to use the front end of one or more computer PSUs to get 1 kW. If you do not need isolation, a buck converter can be made with a current regulated output, and for isolation you need only add a transformer and switching circuitry similar to the computer PSU.


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## alvin (Jul 26, 2008)

nimblemotors said:


> I'm working on my battery charger. I'm going to use a "MOT", or a Microwave Oven Transformer that I removed from my old microwave a few years ago. This charger should cost me about $100...
> The microwaves are rated at 1kw of power, so I assume the transformer about the same power rating. I am obviously going to replace the secondary winding that produces 2,000 volts for the microwave with another winding.
> This output AC will be rectified into DC, and then a more precision DC-DC conversion on this input to control current and voltage to the pack.
> 
> ...


I have rewound a couple of the MOT's but when charging they run too hot .

I did use one of the cores to make the inductor for a Bonn charger. That is the only charger I have been using for the last 5 years. The last 2 years it has been charging LiFePo cells.

On the transformer I had tried two primary coils from MOT's just to get isolation. The solder melted on the secondary leads. Another one I wound with magnet wire I had and brought out several taps to try different voltages. But it runs pretty hot too.

I have also made one of those arc welders with a couple of the transformers. It does work but gets hot fast. So you have to cool it down to continue.

If you get something working I would like to know.


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## nimblemotors (Oct 1, 2010)

The details of the $100 25amp charger are on my new website,
electric-boat-forum.com

All the source code is there, and guess what, no 'free'kin communist license restrictions.


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