Rewinding microwave oven transformer

If it's a car amp, it is setup for 12V DC. A transformer won't provide DC.

Ed

Hmm, ?

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I understand that a car amp needs dc to opperate. I am going to use the diodes from an old alternator to get the required dc voltage. It would be much more easy for me to just buy a large power supply but short of an arc welder i don't think I will be able to find one that can supply

100A Also I don't want to pay anything for it. I allready have the transformer, the diodes and the jumper cables But I don't want to wire it all up until I know if it is ok to use multicore cable to rewind a transformer.

Not unless you get off on the stench of burning plastic! Your idea's a non-starter.

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No problems with multicore cable in a transformer, the only reason solid is used is that it's easier to insulate and cheeper. You must have some pretty flimsy jumper leads there if you're expectring to get 10-20 turns on the transformer.

microwave oven transformers aren't built for continuous running without a cooling fan blowing over them, they can be modified by adding some extra turns to the primary,

Don't forget to fuse it, something like that could start a fire.

Bye. Jasen

If you would really like to use a car amplifier, I would suggest getting an old car battery and an old car battery charger. The kind you use to charge a dead battery inside your house, they plug into household 120V outlets and continuously charge the car battery. (Lead acids like being charged constantly). If you haven't lost me here is the method to my madness:

More than likely, the car amp only uses 150A when it needs to drive a really hard bass hit at full volume. Usually it is only using a fraction of that. No transformer that isn't $$$ is going to provide that. However, hooking your amplifier to a car battery that is being continuously charged can handle that. When the amp draws alot of power the battery acts like a huge capacitor and provides it, otherwise most of the power comes from the battery charger and a little from the battery. Of course in the end the net charge into the battery must be greater than the net discharge of the battery but the battery will be recharged whenever you're not using the amplifier, which if your normal will be at least 8 hours a day.

The problem isn't using stranded wire - in fact, in those "make a spot welder with your MOT" articles, they recommend welder cable, which is stranded.

The problem is a MOT won't give you 100A either, at lest not for very friggin' long. And they're rated for intermittent duty, and _that_'s with forced air cooling!

Very very bad idea - you'd be better off to look at something like surplus BMF transformers, or an industrial battery charger.

Good Luck! Rich

Thanks for all your help it has given me a few good ideas. I gave up on the jumper cables as they where to large but I am going to try rewinding the transformer with 80A car wire. I will let you know what happens. The amp is only 300w so I don't think that the microwave transformer will have any problem powering it.

I set it up as above and it appears to be working fine but after running for 1/2 an hour or so the transformer gets very hot. I have kept this under control with a couple of fans but I am wondering if it is damaging to the transformer running it like this? What could I do to get the heat down other than adding more fans? Would replacing the magnetic shunts stop it heating up or does it need some form of current limiting?

As long as temp stays below, say, 100°C, it should be fine.

Technically. Personally, I would try to improve that!

Nah. Here's what you have to do. Look at the size of the primary winding's wire. Find some laying around of the same size (I'm guessing 14 to 16AWG). Put on about 20 turns (yes this will take up some of your secondary's space, you'll have to live with that) and connect it in series with the primary. Make sure to match winding direction, otherwise you'll overload it worse.

You'll need to add a few more secondary turns.

Alternately, Google "buck transformer" and drop primary volts by 10-20V or so. Same idea, you need more secondary turns.

The problem is, MOTs are wound with as little copper as possible. This overloads (saturates) the iron core, causing high peak currents in the primary (the secondary load doesn't matter!). High currents means more heat in the primary copper, causing it to heat up. A rather unhappy situation, so by adding primary turns (or reducing the voltage), you give the iron core an easier life and everything is happy in Transformer Land.

Yes, to fit more turns probably means smaller wire means less current handling. That's the price you pay for electrical happiness.

Tim

-- Deep Fryer: a very philosophical monk. Website:

Removing the magnetic shunts and using some of that space to put some additional primary turns would help a lot.

A way to effectively double the primary winding is to use two identical transformers, but put the two primaries in series and the secondaries in parallel.

Thanks for all the help. I think I will try putting two transformers together with the primaries in series and the secondarys in parallel. However I don't have two MOTS the same. Will it matter if they are a little bit different? One is a bit smaller than the other.

Add more turns to the primary, (this will reduce the current in the primary, and therfore the heating...) and also the voltage in the secondary, so you may need to add turns there too)

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Bye.
   Jasen

Thanks to everyone that posted replies. I found a similar transformer to the one I all ready had at work and I set the two up with the primaries in series and the secondary in parallel and with this setup the transformers will happily power a 100w bulb continuously with only a little heat output from one transformer and no heat output from the other one. I think I could improve this by using two identical transformers but I hardly ever see two identical microwaves.