NPN vs PNP
Teacher note: give presentation using powerpoint
A transistor has three legs: a collector, an emitter and a base. Below is the symbols for an NPN and a PNP transistor.
Transistors as switches
The easiest way to understand transistors is to think of them as switches. You can switch a big current (between the collector and emitter) with a much smaller current (in the base). Lets look at an example:
NPN transistor as a switch (on)
NPN transistor as a switch (off)
Transistors is also handy to convert between different voltages (5V and 12V in the example above.)
While an NPN transistor conducts when a current flows into the base, a PNP transistor will conduct when no current flows into the base and stop conducting when a current flows into the base.
PNP transistor as a switch (on)
PNP transistor as a switch (off)
FunctionTransistors 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
Types of transistor
The leads are labelled base (B), collector (C) and emitter (E).
Transistor currentsThe diagram shows the two current paths through a transistor. You can build this circuit with two standard 5mm red LEDs and any general purpose low power NPN transistor (BC108, BC182 or BC548 for example).
The small base current controls the larger collector current.
When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.
When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off.
A transistor amplifies current and can be used as a switch.
This arrangement where the emitter (E) is in the controlling circuit
(base current) and in the controlled circuit (collector current) is called common emitter mode. It is the most widely used arrangement for
transistors so it is the one to learn first.
The last bit is what confuses a number of people. Let's say you have a 9V battery connected to a 100 ohm resistor and then connected to and NPN transistor and finally to ground. Let's further assume that the beta (Hfe) of the transistor is 250. This is shown below in figure 3.
What is the maximum current that can flow through the transistor? If we replace the transistor with a piece of wire, then using Ohm's law we can calculate the current flow to be:
I = 9 volts/100 ohms
I = 9/100 or .09 Amps (90 mA).
Now when the current is present, if it has a current of 2mA flowing into the base the maximum current the transistor will conduct is 2mA * Hfe, substituting in 250 for Hfe gives us 2 mA * 250 for a total of 500 mA.
And yet we know that if the transistor was a wire the most we would see is 90mA. So the actual answer is this "The amount of current that flows through the transistor is the lesser of the available current (90mA) and Ibe*Hfe. 500mA "
Generally if you apply enough base current that the maximum current possible will always flow, then the transistor is operating like a switch (rather than operating like an amplifier). In this mode, the transistor is said to be saturated.
ConnectingTransistors 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.
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.
SolderingTransistors 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 sinksWaste 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.
Testing a transistorTransistors 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 multimeterUse 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):
2. Testing in a simple switching circuitConnect 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
Transistor codesThere are three main series of transistor codes used in the UK:
Choosing a transistorMost 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.