Heat Sinks

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.

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-Sink compound can be used to improve heat flow from the transistor to the heat sink, this is especially important if an insulation kit is used. 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.