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The transformer pictured above was found by Joe, KI4GIP. It came from a welder. The picture does not show how big it really is. He made a 150 amp power supply at 230 volts on the primary.
Transformers can be found at most Ham Radio swap meets. Sometimes they can be found at electronic swap meets or surplus stores. They come in all sizes and shapes. You can also look for old 120 volt stick welders, which have nearly a 50 amp secondary. Another would be to look for old battery chargers. These are the best of all because if the cabinet is still there, a lot of times the rating is on the cabinet.
It is the easiest, if you know the voltage and amps, that it is rated. If not, then you have to hook it up and measure the secondary voltage. An ideal voltage is about 5 more volts than you want to regulate. Let's say that it is 13.8 volt finished. Then and ideal secondary voltage would be 18 or 19 Volts AC on the secondary.
The Amp rating can be estimated by the wire size on the Secondary. It is important to mainly consider fairly heavy transformers. Let's say, at least 5 to 10 pounds, as this is a good guide to estimate the overall usability. Usually, the heavier, the better. If the Secondary windings are 10 gauge or larger, you can safely say that it could produce 10 to 20 amps on a Surge. It depends on a lot of other things too. Such as Eddy Currents and so on. But that is too technical for what I want to do here. The heavier the transformer the more current that can be drawn on a continuous basis.
I hope you find a nice one !
Primary - AC Input side of transformer
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