On Sun, 1 Jan 2006 19:51:46 -0600, "Ron Moore"
wrote:

Definitely on the primary (110v) side. Just remember that the Variac does
NOT isolate the output from the 110v input. I'm sure your aware but thought
I'd mention it. The output of the soldering gun would be AC but the
charger, of course, is DC. I had considering building one but also using a
DC cap in the circuit to give it just a bit more "punch". I've not looked
inside one of the commercial units to verify if those are done that way or
not.
Respectfully,
Ron Moore

They're not. They're AC. AC heats just as well as DC, why rectify
it?

You're right that DC with a cap would have more punch -- if you want
divots in your sheetmetal.