I have a 120 >12 volt power supply and would like to be able to adjust the out-put for use in foam cutting Answered

You need to put a proper dimmer at the output of the power supply. Search for 12V dimmer on eBay.

Got a 12-24 volt dimmer at Fry's electronics works great Thanks go out to everyone that worked on this

OK so if I use a dimmer (120 volt) it of course change the output at the power supply. my power supply is just a simple 120 ac >12 volt dc

I have done a fair bit of foam cutting back in the day and I still wonder how people like to complicate things for no reason ;)
There is a very basic rule that helps to avoid problems with the power supply:
You select the wire lenght and diameter according to your power supply and heating requirements.
If the wire is getting far too hot then it is too short or too thin.
If the power supply overheats or fails then the wire is too thick or too short.
Quite simple math according to Ohm's law.
Depending on how you design your cutter you can simply leave some extra wire to put the clamp on and adjut the output that way....

What is your power supply ? All you need is a transformer and a dimmer to drive the transformer.

OK if I had a variac I would not need to adjust the out put of my 120 volt ac to 12 volt dc thanks for the answer. can't I just put a potentiometer at the output

https://www.google.co.uk/search?q=variable+current...

https://www.google.co.uk/search?q=variable+voltage...

Here is a very simple circuit, how to use it, how it is wired and examples.

I use gloves to avoid burns.

Click on the pics to see the entire Image.