On December 6, 2010 at 13:39, Ernie Bornn-Gilman said...
If you do the calcs, you'll find that 10K throws off only 1.44 watts of heat when it sees a full 120 VAC RMS. This does not seem to be much power at all.
Don't let that make you think it won't get plenty hot, though. We're really not familiar with how much heat that can be, especially if it's in a confined space. I've burned my finger on a 1/2 watt resistor that's got 1/4 watt of power going through it. You will definitely need to provide heat sinking for that resistor and the system will have to be comfortable with a small hot spot in it.
Ernie's calculation of the power dissipation is correct.
An important concept to remember is that resistor power ratings are based on a temperature rise above 25C. (Room temp for those who don't speak SI units) So if you don't want a resistor to get very hot, buy a resistor rated for about 5x the power you intend to dissipate.
So, in this case you would need to select a resistor that is capable of 1.44W x 5 = 7.2W The next highest standard wattage is 10Watts, so I would use a 10K ohm 10 Watt resistor.
The following would be a good choice:
[Link: search.digikey.com]You would need to solder two leads to the terminals, and then insulate the connections with electrical tape or heat shrink tubing. I would then suggest mounting it in a metal junction box using the two "ears" on the anodized aluminum body to mount it using a couple self tapping screws.
