My first idea to be able to have a custom size heating pad is mylar sheeting. The problem would be getting the electrical current into the metal film.
The film has just about the perfect resistance for moderate voltage application. It is almost perfectly dispersed across the sheet, too.
Resitive Heating Mat
I have a 2 inch wide roll of polyester coated with a layer of metal. It appears to have been for making capacitors. It is not coated with a very consistent layer, and has a resistance of about 10 ohms per foot.
The mylar sheets I've seen have a much more consistent-looking coating, sealed between two polyester layers, though. I have not determined a way to reliably access the metal layer. Thus my post here.
The actual amount of current is not very important. The key is the power dissipation per unit of area, which may be as high as 1w/in^2 when in contact with air. (Pressed against a heat sink, it could be one (or two) order(s) of magnitude higher.
The mylar sheets I've seen have a much more consistent-looking coating, sealed between two polyester layers, though. I have not determined a way to reliably access the metal layer. Thus my post here.
The actual amount of current is not very important. The key is the power dissipation per unit of area, which may be as high as 1w/in^2 when in contact with air. (Pressed against a heat sink, it could be one (or two) order(s) of magnitude higher.
I remember the old CD platers which had some spikes which penetrated into a ribbon connector. Spikes are the only way I though of which I would think could make reliable contact. They would have to penetrate the film and some type of backing material.
How thick is the metal on a film resistor? As small as they are, they have ratings into the watt range.
How thick is the metal on a film resistor? As small as they are, they have ratings into the watt range.
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