# Current input and output of transformer

I'm trying to make a welder-like transformer, but I'm having the problem that apparently I'm drawing too much current from the primary side of the transformer as the variac I'm using to supply about 10V is getting too hot. The variac is a chinese model rated for 20A, and unfortunately I have a 30A circuit and 30A fuse installed so the variac is smoking and trying to catch fire instead of fuse-blowing.

How can I limit the current on the primary side to 20A?

I did some calculations on the reactance of that coil and determined its current limiting capability is a very high current limit (293A, 0.001 H). The formula I'm using is 2 x PI x 60Hz x L = Ohms impedance. Whats puzzling to me is that this formula doesn't account for the magnetic permeability of the core or anything. Or is that already accounted for in the L reading? I measured the L via an LCR gauge across the coil wound around the transformer.

Another thing I'm wondering about is will an increase in current on the secondary side of a transformer (due to low resistance) cause an increase in current in the primary side?

I know the ratio of turns is inversely related to current (high voltage, low current : low voltage, high current), but isn't current determined by resistance of the load? Does a high current on secondary from low resistance cause a proportionately higher current on the primary?

Is a transformer like a set of 3 gears, with gear size being turns/voltage, the middle gear being flux, and the speed of the first and last gear being current? I like to visualize electronic components like water or physical machines.

Sorry for all the questions but this will sure help me out a bit.

My current course of action is to get an AC welding transformer and rewind the secondary with thick wire (2 ga) for high current output. I'm hoping that the primary's inductance is high enough to limit primary current to 20A (same as the welder's spec).

Thanks

• A variac is not suitable as it does not provide isolation between the primary and secondary. "The variac is ... rated for 20A, and ... I have a 30A circuit and 30A fuse installed". Well that's not going to end well. Fix it! Commented Jul 23, 2018 at 19:41
• 10V @ 30A is nowhere near enough power for welding. A typical welder is about 10-15V @ 80-100A. Commented Jul 23, 2018 at 23:05
• If you have measued the inductance L, the core permeability has already been included there. Usually it’s just L=n^2*Al where Al is proportional to the core area, a constant and your permeability. Commented Jul 23, 2018 at 23:37
• @Jules thats only the primary current and voltage not the secondary. Its a step down transforner so current goes up on the secondary. Commented Jul 24, 2018 at 1:56