What is the purpose of a heat sink in transistors?

Answer 1

Heat sinks are needed for transistors passing large currents.

Waste heat is produced in transistors due to the current flowing through them. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate the heat by transferring it to the surrounding air.

The rate of producing waste heat is called the thermal power, P. Usually the base current IB is too small to contribute much heat, so the thermal power is determined by the collector current IC and the voltage VCE across the transistor:

P = IC × VCE

The heat is not a problem if IC is small or if the transistor is used as a switch because when 'full on' VCE is almost zero. However, power transistors used in circuits such as an audio amplifier or a motor speed controller will be partly on most of the time and VCE may be about half the supply voltage. These power transistors will almost certainly need a heat sink to prevent them overheating.

Power transistors usually have bolt holes for attaching heat sinks, but clip-on heat sinks are also available. Make sure you use the right type for your transistor. Many transistors have metal cases which are connected to one of their leads so it may be necessary to insulate the heat sink from the transistor. Insulating kits are available with a mica sheet and a plastic sleeve for the bolt. Heat-conducting paste can be used to improve heat flow from the transistor to the heat sink, this is especially important if an insulation kit is used.