Hotwire foam saw wiring

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

Reply to
Joseph Gwinn
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Guys... I gotta renege... :-(

I did find some information about my, um.... mistake.

I knew there was something to "not enough flux", but got it (what did Fonzie say?) "Wr...Wr....Wr............WRONG!

If the input voltage is below the "excitation voltage", the core losses will predominate over all other losses.

The excitation voltage can be as high as 60% of the rated primary voltage, depending upon the core style and style of winding (progressive stacked, or layered), but is usually about 25% of the rated primary voltage.

So, if you kept the RMS input above, say, 50% of the rating, core losses shouldn't be a problem.

Duh!

LLoyd

Reply to
Lloyd E. Sponenburgh

OK, but both core losses and total losses will be less than when running at rated voltage. It may be less efficient (Pout/Pin) but it will run cooler than normal.

Core losses predominate over all other losses even at rated voltage if there is no load on the secondary, because copper loss in the primary is minimal and copper loss in the secondary is zip.

Reply to
Don Foreman

Except for the fact that keeping the V constant works only if your heat loss is constant. As you move from cutting to not cutting foam, the P required to maintain a constant temp changes. What you need to do is to control either V or I and monitor the other, adjusting the controlled variable to keep the E/I ratio constant. [E/I=R]

[If keeping the V constant worked in this fashion, then incandescent lamps would not work either]

jk

Reply to
jk

Seems like a bit of overkill - but if it works, ok.

My hot wire saw is a whole lot more primative.

Just safety wire (Metal Content!) and a 12 V battery charger.

The "regulator" is a couple of feet of safety wire zig zagged between two rows of nails sticking out of a 2x4.

FWIW...

Richard

Reply to
cavelamb himself

========== If the transformer is not getting too hot and smoking or the dimmer switch blowing, then who cares?

The transformer provides mains isolation, so you should be good to go.

Unka' George ================ When we are planning for posterity, we ought to remember that virtue is not hereditary. Thomas Paine (1737-1809), Anglo-American political theorist, writer. Common Sense, ch. 4 (1776).

Reply to
F. George McDuffee

Seems like a simple workable idea, but man, an idea is worth nothing unless you over complicate the crap out of it. Overkill is my middle name.

Gregg

Reply to
greggspen

I disagree - nichrome has a resistance curve that rises with temp - so if it cools down, R goes down, current goes up.... do the math, I think you'll find that it works out about right for a reasonable range of temperatures

Reply to
Bill N

Mine was pretty simple, too. I had an old Variac core with an intact winding taped up to cover the bare winding. Then I put a few turns of #6 copper THW thru the window. Tied the #6 to a piece SS safety wire stretched between two nails for the cutter. Feed the variac winding with

120 V and add or subtract turns of #6 to get the cutter wire hot enough. It worked like a champ to cut up a 3' x 3' x 6' billet of styrofoam.

Randy

Reply to
Randal O'Brian

snip

True, but the temperature coefficient of nichrome (0.00017/deg C) is far too small for adequate compensation and this is further diluted because the part that is cooled by the foam is only small part of the total circuit resistance.

What is needed is a high temperature coefficient wire with as much as possible immersed in the foam. As the foam tries to cool the wire the resistance drops so that the power input increases. Pure nickel (0.0047/degC) is nearly thirty times better than nichrome for this purpose.

Not as good but readily available is copper (0.0039/deg C). Because of the self regulating temperature remarkably little power is needed. With 0.014" fuse wire, bow string tight, 0.5V at 10A cuts 2" polystyrene foam like butter.

Nickel is a better choice on all counts - higher temperature coefficient, higher resistivity. higher strength and higher temperature limit. The life of a copper wire is fairly short but its good enough for the odd foam shaping job.

For this scheme to work properly the wire must be driven from a constant voltage source either deliberately regulated or much low impedance than the hot wire load - I used a

6V 10A filament transformer fed from a variac. A variac or a dimmer plus a rewound 1V secondary would have been a better bet.

Jim

Reply to
pentagrid

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