Choosing a transistor

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    Most projects will specify a particular transistor, but if necessary you can usually substitute an equivalent transistor from the wide range available. The most important properties to look for are the maximum collector current IC and the current gain hFE. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating.

    To make a final choice you will need to consult the tables of technical data which are normally provided in catalogues. They contain a great deal of useful information but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows the most important technical data for some popular transistors, tables in catalogues and reference books will usually show additional information but this is unlikely to be useful unless you are experienced. The quantities shown in the table are explained below.

    Structure = This shows the type of transistor, NPN or PNP. The polarities of the two types are different, so if you are looking for a substitute it must be the same type.

    Case style = There is a diagram showing the leads for some of the most common case styles in the Connecting section above.

    IC max. = Maximum collector current.

    VCE max. = Maximum voltage across the collector-emitter junction.
    You can ignore this rating in low voltage circuits.

    hFE = This is the current gain (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor – even for those of the same type! Note that current gain is just a number so it has no units.

    Ptot max. = Maximum total power which can be developed in the transistor, note that a heat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifiers, the power is roughly IC × VCE. For transistors operating as switches the maximum collector current (IC max.) is more important.

    Category = This shows the typical use for the transistor, it is a good starting point when looking for a substitute. Catalogues may have separate tables for different categories.

    Possible substitutes = These are transistors with similar electrical properties which will be suitable substitutes in most circuits. However, they may have a different case style so you will need to take care when placing them on the circuit board.

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