Following a side discussion in another thread here, where someone warned me that a 2-watt resistor would get very hot if one actually ran 2 watts of electricity through it, I decided to test the hypothesis. I found a 75Ω, 3W resistor in my junk box, connected it to my trusty bench supply, measured the current through it just to be sure (yep, 200mA), and ran 15 volts through it.
Sure enough, it got toasty. Then it got really, really hot. Too hot to touch. Ouch!
So now I'm puzzled: how the hell does one rate resistors, power-wise? I had thought up to now that a 3 watt resistor could withstand 3 watts continuous with no ill effects. Apparently not.
How do you specify a big-enough resistor (power-wise) so that it doesn't get scorching hot? Any rules of thumb? Let's say one has a current-limiting resistor that dissipates 2 watts: what size resistor would be adequate? (Assuming typical construction techniques, no combustible material adjacent to resistor, a little warm is OK.)