There is about always several variacs on eBay. gl, John

"Gary Coffman" wrote in message
...
On 10 Mar 2004 20:40:18 -0800, (Andres) wrote:
I'm trying to rebuild a thermal evaporator. We need a power supply to
heat the boats (only precious metals and aluminum). I've seen very
high prices for power supplies at some vendors, and I read that a
VARIAC can be a good, and cheap choice. Now, what is a variac? Can it
be used to evaporate gold, silver, aluminum? Can I have a variac to
control 3 boats? Not at the same time but one after the other? Any
help is appreciated.

A variac is just an autotransfromer with a mechanical sliding tap.
By turning a knob, you change the tap point, hence the output
voltage. Variacs over about 500 watts tend to be expensive.
Used carelessly, they can also easily be damaged (burnt contact
finger, smoked windings, etc). They are best used as set and
forget devices. Adjusting them under heavy load tends to rapidly
burn the contact finger.

I don't know how precise temperature control needs to be for
your application, nor how many watts of heat you need. But I'd
guess that you'd be better served by an electronic phase angle
type of control, using some sort of feedback to maintain set
point temperature.

If the set point temperature doesn't have to be tightly held,
then a simple bang bang type control (either all the way on
or all the way off, switching according to measured temperature
feedback) will be all you'll need. If the allowable deviation from
set point temperature is small, then a proportional type control
will be required.

But if your wattage requirements are modest, and manual
adjustment is sufficient, then an ordinary lamp dimmer control
might be all you need. This would have the same user interface
as a Variac, ie a knob, but at the same power levels it would
cost 10 to 20 times less.

Gary