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A diode is made from semiconducting materials. It has two terminals, called the anode and the cathode. Current can flow easily from anode to cathode.
CURRENT AND VOLTAGE
When there is a voltage (a pd) across a resistor, the two quantities are related as in the equation pd/current = resistance (see opposite). Ohm's Law applies.
This is not true for diodes. The circuit at top right has a milliammeter (mA) to measure the current through the diode, and a voltmeter (V) to measure the voltage across it. The power source is a power supply unit (PSU) that produces a DC voltage, variable from 0 V up to, say, 10 V.
The voltage is set to different values and the current is measured for each voltage. A graph of the results (the V-I graph) would be a straight line for a resistor, but is curved for a diode.
The diode in the circuit above is connected so that current flows freely through it, from anode (a) to cathode (k). It is said to be forward biased.
The V-I graph obtained with this circuit is shown below. Its main feature is that no current flows through the diode until the forward bias of more than about 0.7 V. As the voltage is increased above 0.7 V, the current increases, slowly at first then more rapidly.
If a diode is connected the other way round, so that its cathode is more positive than its anode, it is reverse biased.
Only a few nanoamps (1 nA = 10-9 A = 0.0000000001 A) flow through a reverse biased diode. With a signal diode or rectifier diode, the reverse voltage may be up to several hundred volts, depending on the type. But, if the voltage is too big, the diode breaks down and is destroyed.
An LED has a small breakdown voltage. Typically, a reverse bias of about 5 V will destroy it. Check that the LEDs are connected the right way round before switching on the power supply to a newly-built circuit.
LEDs may also break down when forward biased if the current through them is too big. The way to prevent this is described on p. 6.
When reverse biased, a Zener diode breaks down at a fixed voltage, its Zener voltage, VZ (see top right). This may be only a few volts, Zener breakdown does not destroy the diode. This property is used in voltage regulator circuits.
Photodiodes are nearly always connected with reverse bias. The small leakage current through the photodiode is proportional to the amount of light falling on it.
QUESTIONS ON CIRCUITS
1 Draw a schematic diagram of a door alert circuit built from a battery of 4 cells, a push-button, and a buzzer (see the chart of symbols on p. 357). On the diagram mark the point in the circuit that is at the highest potential. Show the direction of flow of current.
2 Draw a schematic diagram of a door alert circuit that has two push-buttons, A and B. The buzzer is to sound when either A or B is pressed. What happens if both buttons are pressed at the same time?
3 A DC electric motor is powered by a 4.5 V battery and controlled by a SPST (single pole single throw) switch. There is a light emitting diode which comes on when the power supply is switched on. The diode needs a resistor to limit the current through it. Draw the schematic diagram of the circuit.
4 List the energy conversions that occur when the circuit of Question 3 is switched on.
5 Name four good conductors of electric current. Give an example of the use of each and state which one of the four is the best conductor.
6 Name four electrical insulators, giving an example of the use of each.
7 The pd across a torch lamp is 2.4 V and the current through it is 700 mA. What is the resistance of the lamp?
8 What is the current through a 220 resistor if the pd across it is 5 V?
9 What is the power of the torch lamp referred to in question 7?
10 What is the power of the resistor referred to in question 8?
11 A 2 V car battery has a 100 resistor connected across its terminals. What is the current through the resistor? At what rate is electrical energy being converted and to what form is it converted?
QUESTIONS ON DIODES
1 Name the terminals of a diode. In which direction does the current flow when the diode is forward biased?
2 Describe, with the help of a graph, how the current through a forward biased diode varies as the pd across the diode is varied from 0 V to 5 V.
3 Using a data sheet or information obtained on the Web, list and comment on the characteristics of a named signal diode.
4 Repeat 3 for a high-power rectifier diode.
5 List the features of six different types of LED, and state an application for each.
6 Draw the symbols for (a) a signal diode, (b) a Zener diode, (c) a photodiode, and (d) a light-emitting diode. Label the anode and cathode of each symbol.
7 Search this book, or another electronics book, or the WWW, for a simple electronic circuit containing a diode. Draw the circuit and describe how the properties of the diode are made use of in this circuit.
MULTIPLE CHOICE QUESTIONS
1 An example of a conductor is:
A nylon. B aluminium. BLDBLD rubber. D glass.
2 An example of an insulator is:
A copper. B steel. BLDBLD PVC (polyvinyl chloride). D carbon.
3 The pd across a resistor is 15 V and its resistance is 120 σ. The current through the resistor is:
A 125 mA. B 8 A. BLDBLD 1.25 A. D 105 mA.
4 A silicon diode is forward biased so that its anode is 0.5 V positive of its cathode. The current through the diode is:
A 0.5 A. B 0 A. BLDBLD not known. D enough to burn out the diode.
Transistors are used in one of two ways:
Transistor amplifiers are described later in the book. Transistor switches are described in this topic, using three different types of transistor.
As we shall explain, the purpose of most transistor switches is to control an electrical device such as a lamp, a siren or a motor.
The switched device usually requires a current of several milliamperes, possibly several amperes. The current needed for operating the transistor switch is a lot smaller, often only a few microamperes. This makes it possible to control high-current devices from sensors, logic gates and other circuits with low-current output. The main limitation is that only devices working on direct current can be switched, but not devices powered by alternating current. These points are illustrated by the examples below.
SWITCHING A LAMP
The switching component in this circuit is a metal oxide silicon field effect transistor. This name is usually shortened to MOSFET. The words 'metal oxide silicon' refer to the fact that the transistor consists of a metal conductor, an insulating layer of oxide (actually silicon oxide), and a semiconducting layer of silicon.
The words 'field effect' refer to the way in which this type of transistor works. It works by placing an electric charge on the 'metal' part of the transistor, known as the gate. The charge on the gate results in an electric field, and the effect of this field is to control the amount of current flowing through the semiconductor layer. Current flows from the drain terminal to the source terminal.
Drain current (ID) flows from the drain to the source, if:
the drain terminal is positive of the source terminal and,
the gate is charged to a voltage that is sufficiently positive of the source.
In this way, the MOSFET acts as a voltage-controlled switch. The voltage at which the transistor switches on is called the threshold voltage.
A MOSFET of the type just described is known as an n-channel enhancement MOSFET. There are other types (pp. 16 and 69), but this type is by far the most commonly used.
The light-sensitive component in this circuit is a photodiode. Like any other diode, it conducts in only one direction.
Usually a photodiode is connected with reverse bias. This would mean that no forward current flows through it. However, a small leakage current passes through it. The leakage current is only a few nanoamps in darkness, but rises to several microamps when light falls on the photodiode.
This circuit uses a MOSFET transistor for switching a lamp. We might use this to control a low-voltage porch lamp which is switched on automatically at dusk.
The sensor is a photodiode, which could be either of the visible light type or the type especially sensitive to infra-red radiation. It is connected so that it is reverse-biased. Only leakage current passes through it. The current varies according to the amount of light falling on the diode. In darkness, the current is only about 8 nA but it rises to 3 mA or more in average room lighting.
The current change is converted into a voltage change by resistor R1. In light conditions, the current is relatively large, so the voltage drop across the resistor is several volts. The voltage at the point between R1 and the diode is low, below the threshold of the transistor. The transistor is off and the lamp is not lit.
At dusk, light level falls, the leakage current falls, and the voltage drop across R1 falls too. This makes the voltage at the gate of the transistor rise well above its threshold, which is in the region of 2.4 V. The transistor turns fully on. We say that it is saturated. When the transistor is 'on', its effective resistance is only 5 ω. Current flows through the lamp and transistor, lighting the lamp.
The current flowing down through the sensor network (R1 and D1) is exceedingly small — no more than a few microamps. Little current is available to charge the gate of Q1. However, a MOSFET has very high input resistance, which means that it requires virtually no current to turn it on. It is the voltage at the gate, not the current, that is important.
Because the gate requires so little current, a MOSFET is the ideal type of transistor for use in this circuit. If we were to use a BJT, we might find that the sensor network was unable to provide enough current to turn it on.
Excerpted from Electronics Circuits and Systems by Owen Bishop Copyright © 2011 by Owen Bishop. Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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