There are three main series of transistor codes used in the UK:
- Codes beginning with B (or A), for example BC108, BC478
The first letter B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. The rest of the code identifies the particular transistor.There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type, for example a higher current gain or a different case style. If a project specifies a higher gain version (BC108C) it must be used, but if the general code is given (BC108 any transistor with that code is suitable.
- Codes beginning with TIP, for example TIP31A
TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings.
- Codes beginning with 2N, for example 2N3053
The initial '2N' identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system.
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.
|BC107||TO18||100mA||45||110||300mW||Audio, low power||BC182 BC547|
|BC108||TO18||100mA||20||110||300mW||General purpose, low power||BC108C BC183 BC548|
|BC108C||TO18||100mA||20||420||600mW||General purpose, low power|
|BC109||TO18||200mA||20||200||300mW||Audio (low noise), low power||BC184 BC549|
|BC182||TO92C||100mA||50||100||350mW||General purpose, low power||BC107 BC182L|
|BC182L||TO92A||100mA||50||100||350mW||General purpose, low power||BC107 BC182|
|BC547B||TO92C||100mA||45||200||500mW||Audio, low power||BC107B|
|BC548B||TO92C||100mA||30||220||500mW||General purpose, low power||BC108B|
|BC549B||TO92C||100mA||30||240||625mW||Audio (low noise), low power||BC109|
|2N3053||TO39||700mA||40||50||500mW||General purpose, low power||BFY51|
|BFY51||TO39||1A||30||40||800mW||General purpose, medium power||BC639|
|BC639||TO92A||1A||80||40||800mW||General purpose, medium power||BFY51|
|TIP29A||TO220||1A||60||40||30W||General purpose, high power|
|TIP31A||TO220||3A||60||10||40W||General purpose, high power||TIP31C TIP41A|
|TIP31C||TO220||3A||100||10||40W||General purpose, high power||TIP31A TIP41A|
|TIP41A||TO220||6A||60||15||65W||General purpose, high power|
|2N3055||TO3||15A||60||20||117W||General purpose, high power|
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.
|BC177||TO18||100mA||45||125||300mW||Audio, low power||BC477|
|BC178||TO18||200mA||25||120||600mW||General purpose,low power||BC478|
|BC179||TO18||200mA||20||180||600mW||Audio (low noise), low power|
|BC477||TO18||150mA||80||125||360mW||Audio, low power||BC177|
|BC478||TO18||150mA||40||125||360mW||General purpose,low power||BC178|
|TIP32A||TO220||3A||60||25||40W||General purpose,high power||TIP32C|
|TIP32C||TO220||3A||100||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.
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.
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.
Maximum collector current.
Maximum voltage across the collector-emitter junction.
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.
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.
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.
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.