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This page covers practical matters such as precautions when soldering and identifying leads. The operation and use of transistors is covered by the Transistor Circuits page.

Types | Connecting | Soldering | Testing | Codes | Choosing | Darlington pair | Heat sinks

Also see: Transistor Circuits


Transistors 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!

A 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 or website 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.

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
Code Structure Case
(typical use)
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
(typical use)
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.

Darlington pair

Darlington pair This 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.

Heat sinks for transistors

Heat sinks are needed for transistors passing large currents.

Heat sink
Heat sink

Photograph © Rapid Electronics

Waste heat is produced in transistors due to the current flowing through them. 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.

The rate of producing waste heat is called the thermal power, P. Usually the base current IB is too small to contribute much heat, so the thermal power is determined by the collector current IC and the voltage VCE across the transistor:

P = IC × VCE   (see diagram below)

Insulation kit
Insulation kit
Heat-conducting paste
Heat-conducting paste

Photographs © Rapid Electronics


The heat is not a problem if IC is small or if the transistor is used as a switch because when 'full on' VCE is almost zero. However, power transistors used in circuits such as an audio amplifier or a motor speed controller will be partly on most of the time and VCE may be about half the supply voltage. These power transistors will almost certainly need a heat sink to prevent them overheating.

Power transistors usually have bolt holes for attaching heat sinks, but clip-on heat sinks are also available. Make sure you use the right type for your transistor. Many transistors have metal cases which are connected to one of their leads so it may be necessary to insulate the heat sink from the transistor. Insulating kits are available with a mica sheet and a plastic sleeve for the bolt. Heat-conducting paste can be used to improve heat flow from the transistor to the heat sink, this is especially important if an insulation kit is used.

Heat sink ratings

Heat sinks are rated by their thermal resistance (Rth) in °C/W. For example 2°C/W means the heat sink (and therefore the component attached to it) will be 2°C hotter than the surrounding air for every 1W of heat it is dissipating. Note that a lower thermal resistance means a better heat sink.

This is how you work out the required heat sink rating: NPN transistor with load

  1. Work out thermal power to be dissipated, P = IC × VCE
    If in doubt use the largest likely value for IC and assume that VCE is half the supply voltage.
    For example if a power transistor is passing 1A and connected to a 12V supply, the power P is about 1 × ½ × 12 = 6W.
  2. Find the maximum operating temperature (Tmax) for the transistor if you can, otherwise assume Tmax = 100°C.
  3. Estimate the maximum ambient (surrounding air) temperature (Tair). If the heat sink is going to be outside the case Tair = 25°C is reasonable, but inside it will be higher (perhaps 40°C) allowing for everything to warm up in operation.
  4. Work out the maximum thermal resistance (Rth) for the heat sink using: Rth = (Tmax - Tair) / P
    With the example values given above: Rth = (100-25)/6 = 12.5°C/W.
  5. Choose a heat sink with a thermal resistance which is less than the value calculated above (remember lower value means better heat sinking!) for example 5°C/W would be a sensible choice to allow a safety margin. A 5°C/W heat sink dissipating 6W will have a temperature difference of 5 × 6 = 30°C so the transistor temperature will rise to 25 + 30 = 55°C (safely less than the 100°C maximum).
  6. All the above assumes the transistor is at the same temperature as the heat sink. This is a reasonable assumption if they are firmly bolted or clipped together. However, you may have to put a mica sheet or similar between them to provide electrical insulation, then the transistor will be hotter than the heat sink and the calculation becomes more difficult. For typical mica sheets you should subtract 2°C/W from the thermal resistance (Rth) value calculated in step 4 above.

If this all seems too complex you can try attaching a moderately large heat sink and hope for the best. Cautiously monitor the transistor temperature with your finger, if it becomes painfully hot switch off immediately and use a larger heat sink!

Why thermal resistance?

The term 'thermal resistance' is used because it is analagous to electrical resistance:
  • The temperature difference across the heat sink (between the transistor and air) is like voltage (potential difference) across a resistor.
  • The thermal power (rate of heat) flowing through the heat sink from transistor to air is like current flowing through a resistor.
  • So R = V/I becomes Rth = (Tmax - Tair)/P
  • Just as you need a voltage difference to make current flow, you need a temperature difference to make heat flow.

Rapid Rapid Electronics have kindly allowed me to use their images on this page. Rapid stock a wide range of components, tools and materials for electronics. I am happy to recommend them as a supplier for individuals and education. In my experience their standard delivery really is rapid!


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