Transistor with heat sinks

Function

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
h_FE.
see different types of transistors layout and picture of transistors

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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.

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.

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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.

Transistor leads for some common case styles.

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Soldering

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.

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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.

Heat sink Photograph ©
Rapid Electronics

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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.

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 h_FE.


A simple switching circuit

to test an NPN transistor

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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.
  

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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 I_C
and the current gain h_FE. To make selection easier most suppliers
group their transistors in categories determined either by their typical use
or maximum power rating.