On most of the time and V CE may be about half the supply voltage. However, power transistors used in circuits such as an audio amplifier or a motor speed controller will be partly Switch because when 'full on' V CE is almost zero. The heat is not a problem if I C is small or if the transistor is used as a Power is determined by the collector current I C and the voltage V CE across the transistor: P = I C × V CE Usually the base current I B is too small to contribute much heat, so the thermal The rate of producing waste heat is called the thermal power, P. The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for transistors passing large currents. They may have a different case style so take care when placing on the circuit board. Possible substitutes - transistors with similar electrical properties which will be suitable There may be separate tables for different categories. Transistors operating as amplifiers, the power is roughly I C × V CE.įor transistors operating as switches the maximum collector current (I C max.) is more important.Ĭategory - typical use for the transistor, a good starting point when looking for a substitute. Heat sink will be required to achieve the maximum rating. maximum total power which can be developed in the transistor, note that a Transistor to transistor this detail is only really of interest to experts. Since the gain is roughly constant for various currents but it varies from Sometimes minimum and maximum values are given. Which is usually in the middle of the transistor's range, for example means the gain is at least 100 at 20mA. The gain is often quoted at a particular collector current I C Note that current gain is just a number so it has no units. The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! H FE - the current gain (strictly the DC current gain). maximum voltage across the collector-emitter junction, ignore this for low voltage circuits. Structure - type of transistor, NPN or PNP, a substitute must be the same type. These are some of the terms you are likely to see: They contain a great deal of useful information but can be difficult to understand if you are not familiar with the To make a final choice you may need to consult tables of technical data provided in catalogues, books and on-line.

To make selection easier most suppliers group their transistors in categoriesĭetermined either by their typical use or maximum power rating. The most important properties to look for are the maximum collector current I CĪnd the current gain h FE. Most projects will specify a particular transistor but you can usually substitute an equivalent transistorįrom the wide range available.

Test transistor base emitter collector