Re: Hotwire foam saw wiring

In article <8rjp23980p0gb8nng2olnai2bv0as7ppav@xxxxxxx>,
Bruce L. Bergman <blnospambergman@xxxxxxxxxxxxxxxxx> wrote:

On Mon, 23 Apr 2007 07:37:34 -0400, "Lloyd E. Sponenburgh"
<lloydsp@xxxxxxxxxxxxxx> wrote:
"greggspen" <greggspen@xxxxxxxxx> wrote in message
news:1177325685.379014.162230@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

A bit off topic I know, but related to LFMC.
I've built hotwire saws before and controlled the temperature of the
wire with a domestic light dimmer to regulate the power to a heavy
duty 12V wound transformer.
Every now and then I read that this isn't a sound idea but never read
why.
Anyone got an opinion on this?

An _ordinary_ step-down transformer becomes quite inefficient at
below-normal primary voltage. Core losses soar, and the transformer tends
to overheat, shortening its life -- sometimes to "immediate death".

That, and the step-down transformer windings don't like seeing the
chopped waveform from the switching transistor in the dimmer, and the
transistor isn't thrilled about the big inductive load either.

It would be simple and pretty inexpensive to build a "dimmer" for use on the
low-side, so you don't vary the transformer input.

High-watt wirewound rheostat, ceramic body. 25W and 50W readily
available. You can also cut the power output of the secondary in half
rather crudely with a few diodes to form a half-wave rectifier

Or get a Variac (variable transformer) and connect the secondary
directly to the hot wire. Might want to plug into a GFCI receptacle
for personnel protection, just in case someone touches the hot wire
and a ground at the same time.

Lutron makes solid-state dimmers intended to run at line voltage and
drive a transformer, used to dim low-voltage (12v or 24v) lamp strings.
Typical application being the underside of kitchen cabinets.

Lutron has a fairly clear explanation and list of models on their
website.

I would imagine that such a dimmer would have no problem controlling a
hot wire knife.

Joe Gwinn
.

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