Transistor codes

  • Sharebar
Home > Electronics > Basic Electronics > Transistor codes

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.

NPN transistors

Code Case
style
IC
max.
VCE
max.
V
hFE
min.
Ptot
max.
Category
(typical use)
Possible
substitutes
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  

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 Case
style
IC
max.
VCE
max.
V
hFE
min.
Ptot
max.
Category
(typical use)
Possible
substitutes
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.

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.