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Microwave Transformer

I have salvaged a old microwave transformer that i am going to use for a SLA battery charger i am building. I have rewound the secondary with 50amp wire and i get an AC output of 12v what is what i want but im confused with working out how many amps it puts out. I have run it through a bridge rectifier and a capacitor and i get about 18V DC. I was testing the amps by adding 12v 50watt light bulbs to it 1 at a time and when i get to 200watts the voltage starts to drop to about 10v DC. My question is does that mean the transformer is being overloaded? and if so is there a way to increase the amp output by winding secondary with different wire? I have seen online videos of Microwave transformers being rewound and they give out 50amps at 12v.

There is more to it than replacing a coil ;)
1. The transformer can't really get more output than what it had before - you are limited by the primary coil.
2. A MOT also has added pieces to reduce the magnetic flux - they have to be removed for full power but should stay in place if you want some extra protection against overload.
3. To even get a chance of proper performance (if cut open) the MOT needs to be welded close again, just clamping won't do it.

Considering you are in the 15A range on the output you should have enough power to charge a battery, although I use a proper charging circuit for it and not just a transformer with a rectifier...

JamieG14 (author)  Downunder35m1 year ago

Thx for the fast reply, I am building a proper circuit and using an arduino to control it so i can select battery voltage and amps for charging. Is there a way to test the max current a transformer will put out?

Sure, just connect a proper ampere meter and a adjustable, resistive load.
Increase the load and measure the amps while doing so.