Hii…
friends this is a tutorial on transistors one of the most basic and
important electronic components. In this article we will discuss on various
topics related to TRANSISTORS and their uses.
Introduction
Transistors are used most widely used in most of the
electronic projects and circuits. Transistors are used for amplification , like
amplifying small current output of any logic IC to drive a lamp or relays etc.
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.
Types
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.
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| Types of transistor |
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!
In addition to standard (bipolar junction) transistors, there are field-effect
transistors which are usually referred to as FETs. We will
discuss about them in any other article. 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!
How to
connect a transistor in circuit
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.
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| various type of transistor leads configurations |
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.
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 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
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| Transistor as a switch |
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 :
- Codes
beginning with B (or A), for example BC108, BC478
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 . 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, - 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.
Use of Transistor as an amplifier
Transistor are used to amplify small inputs to use them in various electronic applications. to use transistor as an amplifier just connect the transistor in the circuit as shown. now provide small input at input terminal and measure the output at output end with the use of multimeter and you will found that the output is amplified.
Transistor are used to amplify small inputs to use them in various electronic applications. to use transistor as an amplifier just connect the transistor in the circuit as shown. now provide small input at input terminal and measure the output at output end with the use of multimeter and you will found that the output is amplified.
But note that phase of output signal is reversed as compared to input signal.
To maximize
the current gain (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 example:
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| Darligton pair |
- For TR1 use BC548B with hFE1 = 220.
- For TR2 use BC639 with hFE2 = 40.
NOTE- image courtesy- http://www.electronics-tutorials.ws
