Quote
cdru
P = I^2 * R
STP75NF75 - 11 amps^2 * .0095 ohms= 1.1495 watts
IRLB8743PBF - 11 amps^2 * .0025 ohms= .3025 watts
STP55NF06L - 11 amps^2 * .014 ohms = 1.69400 watts
Take each of the numbers above, multiply by 62.5°C/W, and add to an ambient temperature of 25°C. That will give you how hot each transistor will get
STP75NF75 - 1.1495 watts * 62.5 + 25 = 96.84375°C (~206°F)
IRLB8743PBF - .3025 watts * 62.5 + 25 = 43.90625°C (~111°F)
STP55NF06L - 1.69400 watts * 62.5 + 25 = 130.875°C (~276°F)
Thanks a lot for showing how simple to calculate this is. I took the freedom to add this to the wiki: [reprap.org]