This page covers practical matters such as precautions when soldering and identifying leads. For information on the operation and use of transistors in circuits please see the transistor circuits page.

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 (one of many parameters for transistors, each with their own symbol).


Types of transistor

NPN and PNP transistor symbols

There are two types of standard (bipolar junction) transistors, NPN and PNP, with different circuit symbols as shown. 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 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 covered by this page.


Transistors have three leads which must be connected the correct way round. Take care because a wrongly connected transistor may be damaged instantly when you switch on.

The orientation of the transistor may 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 show the leads for some common transistor case styles.

Note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC pin diagrams which show the view from above.

Transistor leads


using a crocodile clip as a heatsink

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. You can buy a special tool, but a standard crocodile clip (without a plastic cover) works just as well and is cheaper.

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:

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 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

Testing an NPN transistor

2. Testing in a simple circuit

Connect the transistor into the simple circuit shown. The supply voltage is not critical, anything between 5V 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.

testing a transistor

A simple switching circuit
to test an NPN transistor

Transistor codes

There are three main series of transistor codes used in the UK:

Codes beginning with B (or A), e.g. BC108

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, e.g. 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, e.g. 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 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 may need to consult tables of technical data provided in catalogues, books and on-line. They contain a great deal of useful information but can be difficult to understand if you are not familiar with the terms and abbreviations used.

These are some of the terms you are likely to see:

Structure - type of transistor, NPN or PNP, a substitute must be the same type.

Case style - layout of the leads.

IC max. - maximum collector current.

VCE max. - maximum voltage across the collector-emitter junction, ignore this for low voltage circuits.

hFE - 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.

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 - typical use for the transistor, a good starting point when looking for a substitute. There may be separate tables for different categories.

Possible substitutes - transistors with similar electrical properties which will be suitable substitutes in most circuits. They may have a different case style so take care when placing on the circuit board.

Heat sinks for transistors

Heat sink

Heat sinks are needed for transistors passing large currents.

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.

Photograph © Rapid Electronics

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

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.

Insulation kit

Insulation kit

Heat-conducting paste

Heat-conducting paste
Photographs © Rapid Electronics

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.

Working out the required heat sink rating:

  1. First find the thermal power to be dissipated:
    P = IC × VCE
    (if in doubt use the largest likely value for IC and assume VCE is half the supply voltage).
    Example: a transistor is passing 1A and connected to a 12V supply so the power is about 1 × ½ × 12 = 6W.
  2. Find the maximum operating temperature (Tmax) for the transistor if possible, otherwise assume Tmax = 100°C.
NPN transistor with load
  1. 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.
  2. 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.
  3. 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).
  4. 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.

Or use trial and error!

If the steps above seem 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 Electronics have kindly allowed me to use their images on this website and I am very grateful for their support. They stock a wide range of transistors and other components for electronics and I am happy to recommend them as a supplier.