LIBRARYPower and heat.

Power is the rate a part uses energy, measured in watts. For a resistor it takes three equivalent forms, shown below. Whatever form you use, the power a resistive part burns leaves as heat, and that heat is what sets the part you buy.

Where the power goes

A part carrying current at a voltage is turning electrical energy into heat at a rate of V x I watts. A voltage regulator is the clearest case. On a One Thousand Drones L1.01 board the AP2112K regulator takes the USB 5 V input down to 3.3 V; the 1.7 V it drops, times the current the board draws, becomes heat in the regulator. Draw more current and it runs hotter.

How hot does the part actually get?

The watts a part burns raise its internal temperature above the air around it. How far above is set by the part's junction-to-ambient thermal resistance, a figure in degrees C per watt from the datasheet: the die sits that many degrees hotter for every watt it dissipates. A small package like the AP2112K's SOT-23-5 has a high thermal resistance, so the copper plane it solders to is, quite literally, its heatsink. More copper pulls the number down and the part runs cooler (Diodes AP2112 datasheet).

Picking a part rated for the heat

A resistor's power rating is the point where it sits near its maximum temperature in still air. A common thick-film chip resistor is rated at 70 C ambient and must not exceed a 155 C film temperature (Vishay CRCW e3 datasheet). Run one at its rating and it is hot, drifting, and short-lived, so pick a part rated above the power it dissipates, with margin, and check the datasheet's derating curve, which pulls the allowed power down as the ambient rises.