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.

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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 ormaximum 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.