Transistors 1

 

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.

 

Picture

 

A transistor may also 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.

 

 

Transistors are manufactured in different shapes but they have three leads (legs).

 

The BASE - which is the lead responsible for activating the transistor.

The COLLECTOR - which is the positive lead.

The EMITTER - which is the negative lead.

 

The diagrams below shows the leg layout of the transistors shown above .

 

Leg Layouts

 

The leads on a transistor may not always be in this arrangement. When buying a transistor, directions will normally state clearly which lead is the BASE, EMITTER or COLLECTOR.

 

SIMPLE USE OF A TRANSISTOR

 

The diagram below shows an NPN transistor which is often used as a type of switch. A small current or voltage at the base allows a larger voltage to flow through the other two leads (from the collector to the emitter).

 

Circuit diagram

 

The circuit shown below is based on an NPN transistor. When the switch is pressed a current passes through the resistor into the base of the transistor. The transistor then allows current to flow from the +9 volts to the 0vs, and the lamp comes on.

 

Transistor as a switch

 

The transistor has to receive a voltage at its ‘base’ and until this happens the lamp does not light. The resistor is present to protect the transistor as they can be damaged easily by too high a voltage / current. Transistors are an essential component in many circuits and are sometimes used to amplify a signal.

 

 

Types of transistor

 

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.

 

Transistor Types

 

The leads are labeled 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 FET's. They have different circuit symbols and properties and they are not (yet) covered by this page.

 

 

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.

 

Transistor Connections

 

The drawings above 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.

 

 

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.

 

Heat Sink Clip

 

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 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.

 

Heat Sink

 

 

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 (e.g. 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. Odd numbers are NPN, even numbers are PNP. 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.

 

Transistor Technical Data

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.

 

Notes

 

Notes

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.