Electrical – Changing Wattage on a Heat Pad

Two subjects are covered here. First, reality. Second, the questions posed.

First, Reality : About the most you can get from a U.S. 120 volt plug in is 1500 watts (generally speaking). So your heater element will draw more current (because of lower resistance) until the nichrome element heats up. During this transition time, one depends upon the circuit breaker characteristics to not shut off (trip off). Then, counting upon your forced air cooling (fan), the nichrome element should stabilize (temperature wise) at 9.6 ohms. That is as good as it gets for a constant applied voltage.

Second, your question : "How do I figure out the cold resistance". You need to get the manufacturer to supply you with the expected operating temperature where the resistance is 9.6 ohm ( or rather, at what temperature will the nichrome be at 1500 watts). From there, you can use the nichrome's temperature coefficient to calculate the resistance at room temperature.

An 82 ohm resistor would dissipate about 175 watts; indeed, a 15 watt bulb may dissipate that much power briefly. Something the size of a 15-watt bulb that dissipates 175 watts is going to get very hot, however, and when tungsten gets hot its resistance increases by an order of magnitude. You may have been expecting your 1K resistor to dissipate about 1.5 watts, but in practice it would have dissipated about 17.5 initially. Most resistors don't nicely increase their resistance with temperature the way a lightbulb filament does, however, so your resistor may have continued to dissipate that much power until it melted, whereupon its resistance might have actually decreased, causing it to try to dissipate even more power, until things went downhill very severely.

I would suggest that you might consider a Christmas decoration that's designed for a C7 bulb; such bulbs draw about 1 Watt. If you wanted to use a resistor that drew less power, two 33K resistors half-watt resistors in series would probably be reasonably safe if they were somewhat ventilated (the resistors should dissipate less than 1/8 watt each, but half-watt resistors should be used so that in case one fails shorted the other would be able to handle the 0.44-watt fault power) but might not let through enough power to operate the touch switch (if it does work, keep adding 33K resistors until it doesn't work reliably, and then remove one). The fact that incandescent bulbs have a much lower resistance when cold than when heated allows a much smaller energy-wasting element to be used for a given application than would otherwise be necessary.