01 November 2010

Choosing a transistor


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.

NPN transistors
Code Structure Case
style
IC
max.
VCE
max.
hFE
min.
Ptot
max.
Category
(typical use)
Possible
substitutes
BC107 NPN TO18 100mA 45V 110 300mW Audio, low power BC182 BC547
BC108 NPN TO18 100mA 20V 110 300mW General purpose, low power BC108C BC183 BC548
BC108C NPN TO18 100mA 20V 420 600mW General purpose, low power  
BC109 NPN TO18 200mA 20V 200 300mW Audio (low noise), low power BC184 BC549
BC182 NPN TO92C 100mA 50V 100 350mW General purpose, low power BC107 BC182L
BC182L NPN TO92A 100mA 50V 100 350mW General purpose, low power BC107 BC182
BC547B NPN TO92C 100mA 45V 200 500mW Audio, low power BC107B
BC548B NPN TO92C 100mA 30V 220 500mW General purpose, low power BC108B
BC549B NPN TO92C 100mA 30V 240 625mW Audio (low noise), low power BC109
2N3053 NPN TO39 700mA 40V 50 500mW General purpose, low power BFY51
BFY51 NPN TO39 1A 30V 40 800mW General purpose, medium power BC639
BC639 NPN TO92A 1A 80V 40 800mW General purpose, medium power BFY51
TIP29A NPN TO220 1A 60V 40 30W General purpose, high power  
TIP31A NPN TO220 3A 60V 10 40W General purpose, high power TIP31C TIP41A
TIP31C NPN TO220 3A 100V 10 40W General purpose, high power TIP31A TIP41A
TIP41A NPN TO220 6A 60V 15 65W General purpose, high power  
2N3055 NPN TO3 15A 60V 20 117W General purpose, high power  
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.
PNP transistors
Code Structure Case
style
IC
max.
VCE
max.
hFE
min.
Ptot
max.
Category
(typical use)
Possible
substitutes
BC177 PNP TO18 100mA 45V 125 300mW Audio, low power BC477
BC178 PNP TO18 200mA 25V 120 600mW General purpose, low power BC478
BC179 PNP TO18 200mA 20V 180 600mW Audio (low noise), low power  
BC477 PNP TO18 150mA 80V 125 360mW Audio, low power BC177
BC478 PNP TO18 150mA 40V 125 360mW General purpose, low power BC178
TIP32A PNP TO220 3A 60V 25 40W General purpose, high power TIP32C
TIP32C PNP TO220 3A 100V 10 40W General purpose, high power TIP32A
Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data.

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. This information is also available in suppliers' catalogues.
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.
The gain is often quoted at a particular collector current IC which is usually in the middle of the transistor's range, for example '100@20mA' means the gain is at least 100 at 20mA. Sometimes minimum and maximum values are given. Since the gain is roughly constant for various currents but it varies from transistor to transistor this detail is only really of interest to experts.
Why hFE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here.
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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