What amperage can I expect?

[MrWild]

I have several large 12v. 20a. transformers. I could wire 3 of them in parallel/series so they run off of 115v and give me 36v before the rectifier. If I rewire one for higher volts such as 35~40v., what kind of drop should I expect in the amp capability? I'm assuming there is some sort of ratio involved with higher volts resulting in lower amps, but how much? Would a winding equivelant of 36v. (3 x the original 12v.) give me 1/3 (~6.5a) the original amps? I'd like 10a. due to the 4.6a steppers I'm using. Ganging these heavy transformers makes for a very heavy power supply and 20a capability seems like over kill.

[Al_The_Man]

The current capacity of each winding is going to remain the same when wired in series, in other words the total current capacity is still going to be 20amps. What you have done is increased the VA rating from 240va to 720va.
Al.

[H500]

If you tripled the voltage, it will get 1/3 the current. The requirement is that you use a big enough gage of wire so that all the space in the transformer is filled.

[Al_The_Man]

Originally Posted by H500 If you tripled the voltage, it will get 1/3 the current. The requirement is that you use a big enough gage of wire so that all the space in the transformer is filled.

He is talking about wiring three secondaries in series, at least that is what I got, so in that case the current of any secondary is not reduced and will remain at 20amps capacity, as will each one.
Al.

[MrWild]

H500 got my question right. I understand that if I used three 20a transformers in series to increase the voltage (input paralleled) I'd have 20a as the amperage would stay the same. I was wondering about the drop in amps if I rewound the secondary of one transformer and used just one rewound transformer for my power supply.

I may rewind two to increase volts to 24v. each and run these in series secondary/parallel primary for 48v. before rectification. The resulting 10a. is more than enough for three 4.6a steppers with chopper drives (according to research). Then again the lazy mans way would be to just do three transformers and have all the amps on tap I'll ever need. Because of their bulk, three transformers take up the entire floor of this reworked AT computer case I'm using.

I have three geckos and have quite a bit of leeway for volts.

Thanks to both of you for the quick replies. I'm stuck in a VA hospital at the moment so it will be a few days before I can start playing with the toys again.