what transformer should i use or can i use to make a 220V AC to 12V/40A DC converter?

Question

i as actually asking the spes of transformer i know it is going to be a big one but any ways

Answer 1

You're absolutely right - it's going to be a big one followed by big rectifiers with big heat sinks.

The specs for your 220V AC will be based on the primary windings - and obviously, they must be 220V.

The secondary windings should be rated at least 50% higher than you plan to use it for - and, if you plan to use it at a 100% duty cycle at full 40A output, I'd recommend at least 100% or an 80 Amp rating.

A fan and good ventilation is an excellent idea, also.

Sounds like an interesting project.

Answer 2

You have pretty much written the specs in your question. You need a 220V primary and want 480 VA (12V * 40A) of power. You need 12 VAC output. You need to rectify the output since transformers are AC devices. At 40 amps, the rectifier will be large but not expensive.

Answer 3

You may want to get a transformer that produces 24VAC center-tapped so that you can use just 2 BIG diodes rather than 4 in a Full Bridge. This should reduce your loses some.

I have a Professional Garage Battery Charger that is capable of 12Volts (and higher) at 40Amps, but it runs off of 115VAC. Maybe you just want to buy one already made? Try Ebay look for it used.

Note that if you are charging batteries, you actually want higher than 12Volts, at least 15V. My charger has a boost mode that will put out 18V for sulfated batteries.