transistorsTransistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.
A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).
The amount of current amplification is called the current gain, symbol hFE.
For further information please see the Transistor Circuits page. 

Types of transistor

NPN and PNP transistor symbols
Transistor circuit symbols
There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors.
The leads are labelled base (B), collector (C) and emitter (E).
These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels!
Darlington pair is two transistors connected together to give a very high current gain.
In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Transistor leads
Transistor leads for some common case styles.


Transistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on.
If you are lucky the orientation of the transistor will be clear from the PCB or stripboard layout diagram, otherwise you will need to refer to a supplier's catalogue to identify the leads.
The drawings on the right show the leads for some of the most common case styles.
Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above.
Please see below for a table showing the case styles of some common transistors. 

Crocodile clip, photograph © Rapid Electronics
Crocodile clip
Photograph © Rapid Electronics.


Transistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink.
Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation. 

Heat sink
Heat sinkPhotograph © Rapid Electronics

Heat sinks

Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air.
For further information please see the Heat sinks page. 

Testing a transistor

Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it:
testing a transistor
Testing an NPN transistor

1. Testing with a multimeter

Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range.
Test each pair of leads both ways (six tests in total):
  • The base-emitter (BE) junction should behave like a diode and conduct one way only.
  • The base-collector (BC) junction should behave like a diode and conduct one way only.
  • The collector-emitter (CE) should not conduct either way.
The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used. 
testing a transistor
A simple switching circuit
to test an NPN transistor

2. Testing in a simple switching circuit

Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboard for example. Take care to include the 10kohm resistor in the base connection or you will destroy the transistor as you test it!
If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released.
To test a PNP transistor use the same circuit but reverse the LED and the supply voltage.
Some multimeters have a 'transistor test' function which provides a known base current and measures the collector current so as to display the transistor's DC current gain hFE

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
(typical use)
BC107NPNTO18100mA45V110300mWAudio, low powerBC182 BC547
BC108NPNTO18100mA20V110300mWGeneral purpose, low powerBC108C BC183 BC548
BC108CNPNTO18100mA20V420600mWGeneral purpose, low power 
BC109NPNTO18200mA20V200300mWAudio (low noise), low powerBC184 BC549
BC182NPNTO92C100mA50V100350mWGeneral purpose, low powerBC107 BC182L
BC182LNPNTO92A100mA50V100350mWGeneral purpose, low powerBC107 BC182
BC547BNPNTO92C100mA45V200500mWAudio, low powerBC107B
BC548BNPNTO92C100mA30V220500mWGeneral purpose, low powerBC108B
BC549BNPNTO92C100mA30V240625mWAudio (low noise), low powerBC109
2N3053NPNTO39700mA40V50500mWGeneral purpose, low powerBFY51
BFY51NPNTO391A30V40800mWGeneral purpose, medium powerBC639
BC639NPNTO92A1A80V40800mWGeneral purpose, medium powerBFY51
TIP29ANPNTO2201A60V4030WGeneral purpose, high power 
TIP31ANPNTO2203A60V1040WGeneral purpose, high powerTIP31C TIP41A
TIP31CNPNTO2203A100V1040WGeneral purpose, high powerTIP31A TIP41A
TIP41ANPNTO2206A60V1565WGeneral purpose, high power 
2N3055NPNTO315A60V20117WGeneral 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
(typical use)
BC177PNPTO18100mA45V125300mWAudio, low powerBC477
BC178PNPTO18200mA25V120600mWGeneral purpose, low powerBC478
BC179PNPTO18200mA20V180600mWAudio (low noise), low power 
BC477PNPTO18150mA80V125360mWAudio, low powerBC177
BC478PNPTO18150mA40V125360mWGeneral purpose, low powerBC178
TIP32APNPTO2203A60V2540WGeneral purpose, high powerTIP32C
TIP32CPNPTO2203A100V1040WGeneral purpose, high powerTIP32A
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.

StructureThis 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 styleThere 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.
hFEThis 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.
CategoryThis 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 substitutesThese 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.

Darlington pair

Darlington pairThis is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. Darlington pairs are sold as complete packages containing the two transistors. They have three leads (BC and E) which are equivalent to the leads of a standard individual transistor.
You can make up your own Darlington pair from two transistors.
For example:
  • For TR1 use BC548B with hFE1 = 220.
  • For TR2 use BC639 with hFE2 = 40.
The overall gain of this pair is hFE1 × hFE2 = 220 × 40 = 8800.
The pair's maximum collector current IC(max) is the same as TR2.