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Transistors

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 | Heat sinks |
Testing | Codes |
Choosing | Darlington pair

Also see: Heat sinks |
Transistor Circuits

Function

Transistors amplify current, for example they can be used to amplify the small output
current from a logic chip 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

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 for some common case styles.

Connecting

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.

Soldering

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
Photograph © 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 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.

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 10k
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
style

IC
max.

VCE
max.

hFE
min.

Ptot
max.

Category
(typical use)

Possible
substitutes

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
style

IC
max.

VCE
max.

hFE
min.

Ptot
max.

Category
(typical use)

Possible
substitutes

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

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
(B, C 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.

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