X varactor diode

Y power rectifier 20 Z zener diode

In the case of diodes for specialized applications, the third letter does not generally have any particular significance. Zener diodes have an additional letter (which appears arrerthe numbers) which denotes the tolerance of the zener voltage. The following letters are used:

25 A±1% В ±2% С ±5% D±10%

Zener diodes also have additional characters which indicate the zener 30 voltage (e.g. 9V1 denotes 9.1V).

Example

Identify each of the following diodes:

(i) AA113

(ii) BB105

35 (Hi) BZY88C4V7

Diode (i) is a general-purpose germanium diode.

Diode (ii) is a silicon diode fortuning applications (sometimes referred

to as a varicap).

Diode (iii) is a silicon zener diode having 5% tolerance and 4.7V zener 40 voltage.

Writing Linking facts and ideas, 1

Study these statements about resistors:

1. Resistors are electronic components.

2. Resistors are used to add resistance to a circuit.

We can link the statements like this:

Resistors are electronic components which add resistance to a circuit.

which add resistance to a circuit is a relative clause. This clause helps to define resistors. It is an essential part of the sentence.

Study these statements.

3. Very accurate resistors are used in instruments.

4. These resistors are expensive.

We can link the statements like this:

Very accurate resistors, which are expensive, are used in instruments.

which are expensive is also a relative clause, but it contains information that is not essential to the sentence. Relative clauses that carry inessential information are separated from the rest of the sentence by commas.

Study these statements:

5. Each resistor is marked with colours.

6. The colours indicate the value of the resistor.

Statement 6 explains the purpose of the colours. We can link these statements like this:

Each resistor is marked with colours to indicate its value. Task 6 Study this diagram of a carbon resistor and consider how it is made.

ceramic j resistive element

housing I (carbon rod)

Л connecting wire

end cap forced on end-sealing

to metal-sprayed end compound

Now join the following groups of statements to make longer sentences. Use the words printed in italics above each group. You may omit words and make whatever changes you think are necessary in the word order and punctuation of the sentences.

which

A resistor is a component.

A resistor is used to add resistance to a circuit.

which

Carbon resistors are made of compressed graphite. The graphite is formed into small tubes.

to

A ceramic coating is applied over the graphite. The ceramic coating insulates the graphite.

4 to

The ends of the graphite are sprayed with metal. This forms contacts.

5 which

End caps are forced on the metal-sprayed ends. The caps have connecting wires attached.

6 to

The ceramic is marked with colour bands. The bands indicate the value and tolerance.

7 which

Resistors are made in a range of preferred values. These values meet all the needs of circuit designers.

_Batteries

Tuning-in

Study this statement:

Twenty billion batteries are sold every year.

Why do you think this is so? What different kinds of batteries are there? List some of the things you use which contain batteries. Compare your list with someone else's.

Try to complete this table of the differences between two kinds of cells. Use these terms:

secondary manganese dioxide cadmium primary

nickel zinc portable phones torches

Zinc-carbon cell NiCad cell

Type of cell

Positive electrode

Negative electrode

Example of use

Now listen to the tape to check your answers.

Now read this text. Note any further information about these cells.

Zinc-carbon cell

NiCad cell

Electrolyte EMF

Zinc-carbon cell

It has a zinc negative electrode, a manganese dioxide positive electrode, and the electrolyte is a solution of ammonium chloride. The carbon rod is in contact with the positive electrode (but is not involved in the chemical reaction) and is called the current collector. The EMF is 5 1.5V and the internal resistance about 0.5Q. This is the most popular cell for low-current or occasional use, e.g. in torches.

Nickel-cadmium cell (NiCad)

The electrodes are of nickel (+) and cadmium (-) and the electrolyte is potassium hydroxide. It has an EMF of 1.2V and is made in the same sizes as primary cells, e.g. HP2, PP3; button types are also available. Ю High currents can be supplied. Recharging must be by a constant current power supply because of the very low internal resistance.

Label this diagram of a Zinc-carbon cell with these terms. More than one term can refer to the same part of the diagram.

a zinc can

b current collector

с jacket

d carbon rod

e positive electrode

f electrolyte

2

з

4

Language Study Describing components

Two questions we may need to answer when we describe components are:

1. What is it called?

2. What does it do?

In other words, we need to be able to:

1. label components.

2. describe their function.

We can use these ways of labelling components:

It is called a Zinc-carbon cell. It is known as a NiCad cell.

We can describe the function of components like this:

A cell provides electricity.

Cells change chemical energy into electricity.

Task 6 Here are some circuit symbols. Label them and describe their function. For

example:

5 h It's called a transformer. It steps AC voltages up or down. This list of functions may help you.

a'	varies capacitance in a circuit	f	protects a circuit
b	rectifies alternating current	g	varies the current in a circuit
с	adds resistance to a circuit	h	steps AC voltages up or down
d	measures very small currents	i	receives RF signals
e	breaks a circuit	i	measures voltages

Speaking practice

Task 7 Work in pairs, A and B. You have some details, but not all, about two kinds of

cells. Find out the missing details from your neighbour so that you can complete your table.

Student A: Your table is on page 174. Student B: Your table is on page 181.

Word Study Verbs and related nouns

Task 8

Each of these verbs has a related noun ending in -er or -or which refers to an instrument or component. Complete the column of nouns. You have met these nouns in this and earlier units.

Verb

Example record

1. oscillate

2. transmit

3. transform

4. charge

5. rectify

6. process

7. amplify

8. collect

9 detect 10 tune

Noun

recorder

Technical reading Battery charger

Task 9 Study this circuit diagram of a battery charger and try to name all the

components.

-o constant

current ■o output

Now read this text to check your answers:

The power to drive an electronic circuit is normally provided by an AC mains power supply but batteries are often used for portable equipment. Secondary cells can be recharged to their original voltage and can therefore be used many times over.

stabilized DC

output

AC

mains input

5 Recharging is done using a battery charger which consists of a mains power supply with a DC output slightly larger than the required battery EMF. A current is driven through the battery in the opposite direction to its normal output current. The block diagram of a battery charger is shown in Fig. 1.

				smoothing circuit		stabilizing circuit
transformer	—»—	rectifier	>■

Fig. 1