Put the variac on the primary of a transformer. e.g. a 24V
then the variac can be from 0v to say 24v depending on how
it is set up.

Martin

On 11/6/2010 10:48 PM, wrote:
On Fri, 05 Nov 2010 22:22:15 -0700, Bill Noble
wrote:

On 11/5/2010 10:04 PM, Jon Anderson wrote:
Want to make a very simple hot wire cutter to cut polyurethane foam.
Probably not more than 12-16" span, max. Found a simple setup on
Instructables using a 24v 2a transformer and a wall light dimmer.
Well, I've got a 12v 4a DC power supply, seems to me that this ought to
be enough, but thought I'd ask.

Thanks,

Jon

how are you going to control the voltage on the power supply - you don't
want the wire glowing white hot, you want it just warm enough to do the
job. Many 12V power supplies are regulated, so it would take a bit of
futzing around to make the supply adjustable
GENERALLY an AC transformer is a better choice - you know there is
no fancy regulation, and no rectifiers to pop.
A lamp dimmer can work for controll, but some do not like inductive
loads like transformers. A simple powerstat / variac controlling the
transformer works great - and you can use any transformer with
adequate current output because you have full control of the voltage.

A variac alone will do the job, but DO NOT be tempted to go that route
as it has no isolation - the cutter will be "live" in relation to
ground. You NEED an isolation transformer of some type in the circuit.