To save money, use a cheap shop soldering iron. Macro warehouse, Supercheap
and several of the cheap shops have a range of soldering irons on special
I have and use several cheap mains powered irons that were all under ten
dollars. They range from 25 watts, 40 watts to 80 watts and provide a good
range of heat capability for a wide range of joints. I also have a couple of
gas burners and a gas soldering iron that handle larger items like radiator
I did "High Reliability Hand Soldering" many years ago as part of my
apprenticeship which was soldering all sorts of little joints to NASA
spec's. I have used a lot of different irons, from tiny little temperature
controlled ones, to scope irons, to oxy gear for big jobs.
The 12 volt iron you have, is it a 12 volt hook it up and it stays hot, or
is it the "Scope" brand 12 volt iron that only heats as you push the ring
switch? The hook it up and leave it will run pretty well on a transformer,
but the scope ones really need a big transformer as they draw a lot of
current while switched on and nothing on standby. Scope transformers are
normally about 3 volts and the 12 volt iron they sell is the same one with a
longer cable to limit the current with 12 volts. They burn out the element
frequently on 12 volts.
Transformers are normally rated at the rms voltage they maintain at the
rated current they will supply. Some manufacturers will give the open
circuit voltage. You will often find this figure will vary a little and
sometimes this will give a "suck it and see" method of testing if it will
provide enough juice to run the iron at the temperature you want.
If you have enough to gear to test the transformer insulation, or run it
through a safety switch, you will have a little protection for when the
insulation breaks down and gives you 240 volts on the soldering iron barrel
and tip. In fact, I consider it good practice to use a safety switch on the
supply line to all of your test bench equipment, especially including
anything under test. ('cause you already know it has at least one problem!)
The soldering iron will be quite happy with ac going to it, so you don't
need any fancy circuit unless you want to make some sort of variable
temperature control, but the iron you have will not have a sensor included.
I have re-used many transformers from old equipment over the years, and find
they are often good, but watch for any damage to them, and be prepared for
them to go bang and scare the crap out of you!
Hope this helps,