That's simple. Go to your local automotive parts store or local farm supply store and buy a battery charger rated for 20 amps or more output and take the transformer out of that.

Or buy one on line.

And then take it apart and count the turns on the primary and secondary.

and count the E's and ends of the iron.

Or just use it before you wreck it.

TTYL

Jens

Sorry I just couldn't holds this back.

DUAWAE.

http://uk.rs-online.com/web/p/din-rail-panel-mount-transformers/0504149/

This is starting to sound like homework.

Yes, if you backtrack his posts he's getting a degree (or maybe not) based on answers from CR4.

If I recall correctly (it was 30+ years ago now), transformer design was a set of two semester level courses, and that was just for general information for Electrical Engineers. If you REALLY wanted to design transformers for a living, you would want to take more courses that were tailored to the details.

So if you are wanting to design your own because you think it will be cheaper than buying one, guess again. It will be FAR less expensive to buy one, the transformer business is not all that profitable any longer but because it is now largely automated, the economies of scale keep the manufacturers alive.

If you want this information to satisfy a homework assignment, maybe you should develop your reseaerch skills a little more. At this rate you will never be a proper EE, but you may be trying to become a useless manager, because you seem to think it OK to get other people to do your work for you.

That is the truth. You are really familiar with transformer business.

Get a transformer out of a microwave oven (not the one the wife uses!).

Use a grinder with a cutoff disc to get the laminations apart, by grinding the welds holding the Es and Is together. Don't go too deep, about 1/8 inch max.

Remove the copper wire. This is usually a solid mess of copper and epoxy glue. Use lots of wood blocks and a mallet, so you don't damage the laminations.

Wind 20 turns of wire, say no thinner than #28 wire, on to the central leg of the transformer.

Tie the Es and Is of the transformer together with a radiator hose clamp of suitable size.

Connect a Variac to the coil thru a one ohm resistor, and start cranking up the volts.

Plot the voltage drop across the resistor against the voltage delivered by the Variac. The curve will be linear for a while, then suddenly the drop across the resistor will increase much more rapidly. Note the point where the curve is no longer linear (knee of the curve). Select a value of Variac Volts just below this knee.

20/Variac Volts, gives turns per volt you need to wind the transformer (around 3 turns/Volt?).

The typical microwave transformer will handle a bit more than the 240 Watts you want out of it, but this is a cheap and conservative design.

The wire you use for winding your transformer windings should carry less than 2.5A per sq mm of cross-section. Ask your mates for an FPS value, I can't remember it this far along. So for primary 220V winding, you need a little more than 1 Amp rating (> 0.4 sq mm x-section), and for secondary 20A rating (>8.0 sq mm x-section).

Do not use the transformer primary winding that you removed. It uses turns/volt that are inadequate, and the current draw would be excessive.

Enjoy the exercise.

Phaddy, a good answer from me.

your answer contains helpful information for the OP.

I see no reference to INSULATION on the winding wire... I sure hope he is NOT going to use bare copper wire.

Bill