You would use the variac on the 120 v side of the stepdown transformer so
it is feeding thtransformer a variable input voltage. Never on the output.

That said, you are still taking the wrong approach, the transformer in that
battery charger is still to high a voltage output and to low a current. You
want like a big asses old4-5 v filament transformer with a 100 amp output!!

On 1 Jan 2006 13:06:45 -0800, "mj" wrote:

I would like to
solder up to around .025 brass sheets together too. I found a 12/30/75
amp charger at Menards that is probably my next step. But I would like
to be able to vary the current coming out. Do I need a variac for this?
Does this go on the input (110v) or the output (12v) side of the
charger? What specs (volts/amps/watts) should I look for on this? I
also know that I could use a filament transformer of around 6.3v 6-20
amps on the secondary in place of the battery charger. Any suggestions
on which is better? I KNOW that using a light dimmer switch for the
adjustment setting is a bad idea.