a heat sink

A heat sink is the finned metal block bolted to a hot component to carry its heat away into the air before the component cooks itself. A power transistor or linear regulator wastes energy as heat in a tiny chip; left alone, that chip's temperature climbs until it fails. The heat sink gives the heat a large surface to escape from — like spreading a hot dish across a wide cool tray instead of leaving it in a small cup, so it sheds warmth far faster.

The flow of heat is modelled just like Ohm's law for current, with temperature difference playing the role of voltage and thermal resistance (in degrees C per watt) the role of resistance. The chip's temperature equals the ambient temperature plus the power dissipated times the total thermal resistance from junction to air: Tj = Tamb + P times (Rjc + Rcs + Rsa), the three resistances being junction-to-case, case-to-sink (through the insulating pad and grease), and sink-to-air. Example: dissipating 5 W into a sink path totalling 8 °C/W from a 25 °C room gives a junction at 25 + 5 times 8 = 65 °C — safe. Halve the heat sink's effectiveness (double Rsa) and the chip may exceed its 125 to 150 °C limit and shut down or die.

Why it matters: thermal design is where many linear-regulator and power circuits live or die. You compute the worst-case dissipation, look up the chip's maximum junction temperature, and choose a heat sink whose thermal resistance keeps you safely below it with margin, often derating to 80 percent of the limit. Honest details: the insulating pad and the thin film of thermal grease both add thermal resistance, still air is far worse than moving air (a fan transforms a sink), and a heat sink that is too small simply lets the part run hot and age fast.