Giáo trình Tiếng Anh chuyên ngành Y - Phần 1

Giáo trình Tiếng Anh chuyên ngành Y - Phần 1

Unit 1: Power supplies

Diodes have many uses in electronics Perhaps their most obvious use is in the area of power supplies Power uses diodes as rectifies, to convert alternating current to direct current They also use diodes as regulators, to stabilize voltage This chapter covers the circuits which use diodes in these ways

Power supplies also rely on other components that you have already studied Most electronic circuits use a great variety of devices It is very important

to understand how they can perform together as a system

As you can begin to understand how systems such as power supplies work

in electronics, you can begin to understand the principles of troubleshooting Troubleshooting is an analytical process by which you can determine the cause of circuit problems It is much easier to repair circuits when you know how and why they work

1 THE POWER SUPPLY

This chapter marks your beginning study of electronic circuits The power supply is the most fundamental circuit of any electronic system The power supply changes the available electric energy (usually ac) to the form required by the various circuits cannot function One of the first steps in troubleshooting any electronic device is to check the supply voltages at various stages in the circuitry

Power supplies can be very simple or very complicated depending on the requirements of the system A simple power supply may be required to furnish 12

V dc A more complicated power supply may provide several voltages, some positive and some negative with respect to the chassis ground Some power supply may have a wide tolerance regarding voltage

The actual voltage output may vary ± 20 percent Another power supply may have to keep its output voltage within ± 0.01 percent Obviously, the strict tolerance complicates the design of the supply

Figure 5-1 (4-1) shows a power supply as part of an electronic system.Drawings of this type are called block diagrams Block diagrams are very useful for showing the various major circuits within electronic systems The power supply in Fig 5-1 (4-1) occupies one of the blocks within the system It is the most critical since it energizes the rest of the system For example, if a problem develops in the power supply, the fuse might “blow” (open)

In that case, none of the voltages could be supplied to the other circuits Another type of the problem might involve the loss of only one of the outputs of the power supply Suppose the +12V dc out drops to zero because of a component failure in the power supply Circuits A and circuits B would no longer work

The second output of the power supply shown in Fig 5-1 (4-1) develops both positive and negative dc voltages with respects to the common point (usually the metal chassis) This out put could fail, too It is also possible that only the negative output could fail In either case, circuit C would not work normally under such conditions.

Troubleshooting electronic system can be made much easier with block diagrams If the symptoms indicate the failure of one of the block, the technician can devote most attention to that part of the circuit Since the power supplyenergizes most or all the other blocks, it is one of the first things to check when troubleshooting

SELF TEST Complete the missing word in each statement.

1 Power supply will usually change ac to

2 Power supply voltages are usually specified by using the chassis as

a reference

3 Drawings such as Fig 1 are called diagrams

4 On the block diagram, the circuit that energizes most or all the other blocks

is called the

2 RECTIFICATION

Most electronic circuits need dc AC is supply by the power companies The purpose of the power supply is to change ac to dc rectification Alternating current flows in both directions, and direct current is only one direction Since diodes will conduct in only one direction, they are good rectifies

Typical ac voltages supplied by the power companies to residential and small commercial customers are 11V, 120V, 208V and 240V Electronic circuits often require much lower voltages Transformers are used to step down the voltage

to the level needed Fig 1-2 could be an electronic circuit, a battery being charged,

or some other device In this chapter, the power supply circuits will be shown with load resistors (RL)

The transformer in Fig 1-2 has a voltage ratio of 10:1 With 120V across the primary, 12V ac is developed across the secondary If it were not for the diode, there would be 12V ac across the load resistor The diode will allow the current to flow through the load only in the direction of the arrow Since current is flowing in only one direction, it can be called direct current When direct current flows through a load, a dc voltage appears across the load

Note the polarity across the load in Fig 1-2 Electrons always move from negative to positive through a load The positive end of the load is connected to the cathode end of the rectifier In any rectified circuit, the positive end of the load will be that end which contacts the cathode of the rectifier It can also be stated

that the negative end of the load will be in contact with the anode of the rectifier Fig 1-3 shows the negative side of a load resister connected to the anode end of the resister.

It was stated that to forward bias a diode, the anode must be made positive with respect to the cathode It was also note that diode manufactures often mark the cathode with a plus (+) sign This may seem confusing When the diode acts as

a rectifier, the function of the plus sign becomes clear The plus sign is placed on the cathode end to show the technician which end of the load will be positive Look at Fig 1-2 to see if this so

Fig 1-4(a) shows the input waveform to the rectifier circuits of Figs 1-2 and 1-3 Two complete cycles are shown In Fig 1-4(b) the waveform that appears across the load resistor of Fig 1-2 is shown The negative half of the cycle is missing since the diode blocks it This waveform is called half-wave, pulsating dc

It represents only the positive half of the ac input to the rectifier

In Fig 1-3 the diode has been reversed This caused the positive half of the cycle to be blocked Fig 1-4(c) The waveform is also half-wave, pulsating dc Both circuits, Figs 1-2 and 1-3 are classified as half-wave rectifiers

The ground reference point will determine which way the waveform will be shown for a rectifier circuit For example, in Fig 1-3 the positive end of the local is grounded If an oscilloscope is connected across the load, the ground lead of the oscilloscope will be positive and the probe top will be negative Most oscilloscopes show positive as ‘up’ and negative as ‘down’ The actual waveform seen on the screen will then appear as that shown in Fig 1-4(c) Waveform can appear up or down depending on circuit polarity, instrument polarity, and the connection between instrument and circuit

Half-wave rectifies are usually limited to low power applications They take useful output from the ac source for only half the input cycle They are actually not supplying any load current half the time This limits the amount of electric energy they can deliver over a given period of time High power means delivering large amounts of energy in given time A half-wave rectifier is a poor choice in high power applications

TASK 1

Determine whether each statement is true or false.

1 Current that flows in both directions is called alternating current

2 Diodes make good rectifiers because they will conduct in only one direction

3 A rectifier can be used in a power supply to step up voltage

4 In a rectifier circuit, the positive end of the load will be connected to the cathode of the rectifier.

5 A half-wave rectifier supplies load current only 50 percent of the time

6 Half-wave rectifiers are usually used in high power applications

1 A repeated noun becomes a pronoun.

Springs become they.

2 A word replaces an earlier expression.

Force in engineering becomes one.

3 A word replaces a whole sentences or clause.

Spring exert more force the more they are stretched becomes this property.

When the AC voltage is applied to the search coil, a magnetic field is produced around it If there is a metal object under ground, the field induces an electric current in the object The induced current in turn creates a magnetic field around the object This induces a voltage in the search coil The induced voltage is converted into an audible note by the circuitry in the box This sound guides the treasure hunter to the buried object

Look back in the text and find the reference for the word in italics.

1 … , they are good rectifiers

2 … If it were not for the diode.

3 … it can be called direct current.

4 … end which contacts the cathode of the rectifier.

5 … This may seem confusing.

6 … It represents only the positive half of …

7 … This causes the positive half of the …

8 … They take useful output from…

9 … electric energy they can deliver.

TASK 3

Put the word in bracket in the correct word form.

1 Alternating current flows in both (direct)

2 Electronic circuits often require much voltage (low)

3 Transformers are used to step down the voltage to the level (need)

4 The positive end of the load to the cathode end of the rectifier (connection)

5 Wave-form can appear up or down on circuit polarity, instrument polarity, and the between instrument and circuit (depend/connect)

6 A half-wave rectifier is a poor in high power applications (choose)

Unit 2: Electronics in the home

- Find out the meaning of these abbreviations: IC, CD, Hi-fi

- Read quickly through the text and write down the items mentioned

Electronics began at the start of the twentieth century with the invention of the vacuum tube The first devices for everyday use were radios, followed by televisions, record players, and tape recorders These devices were large and used

a lot of power

The invention of the transistor in 1947 meant that much smaller, low powered devices could be developed A wide variety of electronic devices such as hi-fi (high fidelity) units and portable radios become common in the home

It was not until 1958 that microelectronics began with the development of ICs (Integrated Circuits) on silicon chips This leads to a great increase in the use

of electronics in everyday items The introduction of the microprocessor allowed electronics to be used for the control of many common processes

Microprocessors are now used to control many household items such as automatic washing-machines, dishwashers, central heating systems, sewing machines and food processors Electronic timers are found in digital alarm clocks, water heaters, electric cookers, and microwave ovens Telephones use electronics

to provide automatic dialing and answer phone facilities New entertainment devices have been developed such as video recorders and CD (Compact Disc) players

In the future, electronics are likely to become even more common in the home multimedia entertainment systems and computer controlled robots are developed

TASK 1

Fill the gaps in this table with the help of the text.

DATE INVENTION APPLICATIONS IN THE

TASK 2

Use the space below to make a list of ways in which you think electronics may be used in the home in the future.

TASK 3. Adjective and noun formation

Use the suffixes in the boxes to write the adjective formed from the nouns and nouns formed form the verbs.

Centre

Electricity

Portability

Digit

Constant

Use

Invent

Record

Complete the sentence with verb and adjective or a noun form the box.

1 People change their mind for a of reasons

2 Microelectronics began with the of ICs on silicon chips

3 New entertainment devices have been developed such as video

and CD (Compact Disc) player

4 New devices have been developed such as video recorder and

CD (Compact Disc) players

5 Electronics timers are found in alarm clocks and water heater

6 The EMF is 1.5V and the internal about 50 Ω

7 Have characteristics that make them very in electronic circuits

8 Batteries are often used for equipment

9 An current, voltage, or charge is one that is produced by

10 The carbon rod is in contact with the positive electrode is called the current

11 NiCad batteries have such as a small internal resistance that the charger must produce a current output

12 John is a regular of Holland’s health-care system

Unit 3: Operational amplifiers

Integrated circuits technology has had quite an impact on modern electronic design Integrated circuits have given designers high performance for low cost In the linear field, perhaps the best example is the modern operational amplifier

There are no perfect amplifiers through modern operational amplifiers approach perfection They have characteristics are as follows:

1 Common-mode rejection This gives them the ability to reduce hum and noise.

2 High input impedance They will not ‘load down’ a high-impedance signal

source

3 High gain They have ‘gain to burn’ which can be reduced by using feedback.

4 Low output impedance They are able to deliver a signal to a low-impedance

load

No signal-stage amplifier circuit can rate high in all the above characteristics An operational amplifier is actually a combination of several amplifier stages Refer to Fig 3-1

The first section of this combination circuit is a differential amplifier Differential amplifiers have common-mode rejection and high input impedance Some operational amplifiers may use field-effect transistors in this first section for even better input impedance

The second section of Fig 3-1 is another differential amplifier This allows the differential output of the first section to be used Thus, common-mode rejection, maximum signal swing, and additional gain are all realized

The third section of Fig 3-1 is a common collector, or emitter-follower stage This configuration is known for its low output impedance Notice that the output is a single terminal No differential output is possible This is usually referred to as a single-ended output Most electronic applications require only a single-ended output

A single-ended output terminal can show only one phase with respect to ground This is why in Fig 3-1 one input is marked non-inverting and the other is marked inverting The non-inverting input will be in-phase with the output terminal The inverting input will be 180 out of phase with the output terminal

Figure 3-2 shows the amplifier in a simplified way Notice the triangle Often triangles are used in schematic diagram to represent amplifiers Also notice that the inverting input is marked with a minus (-) sign and the non-inverting input

is marked with a plus (+) sign This is standard practice in linear diagrams

Many circuits today are integrated Everything is in one little package A technician can not see inside the package or make any internal measurements Therefore, it is seldom necessary to show the schematic details of the internal circuit Fig 3-3 is an example of the standard way of showing an operational amplifier.

Modern operational amplifiers approach perfection They have the desirable characteristics listed earlier Such amplifiers were originally used in analog computes They performed mathematical operations This is where they got the name “operational amplifiers”, or “op-amps” Op-amplifiers are very popular, mainly because they are available in compact, integrated form at very reasonable costs

TASK 1

Answer the following questions.

1 Refer to Fig 1 which section of the amplifier operated as an emitter follower to produce low output impedance?

2 Refer Fig 1A signal is applied to the inverting input terminal What is the phase

of the signal that appears at the output terminal as compared to the input signal?

3 Refer to Fig 1 is the output of the amplifier differential or single-ended?

TASK 2

Match a line in A with a line in B

1 An operational amplifier Have a combination of several amplifier

stage

2 Most electronic applications Will be b common-mode rejection and a high

input impedance

3 A technician can not see

inside the package or make any

internal measurements

Is c a single-ended output

4 Op-amps are very popular Require d in-phase with the output terminal

5 The non-inverting input Therefore e it is seldom necessary to show the

schematic details of the internal circuitry

6 Differential amplifiers Because f they are available in compact

integrated form at very reasonable costs

TASK 3

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

What does a fuse do? It protects a circuit

We can emphasize function by using this pattern:

The function of a fuse is to protect a circuit

This list of function may help you

a Adds capacitance to a circuit f protect a circuit

b Rectifies alternating currents g Varies the current in a circuit

c Adds resistance to a circuit h Steps AC voltage up or down

d Measures very small currents i Receiver RF signals

Language focus

Enumeration

Enumeration is useful when you want to list or state a number of factors:

Reasons, purposes, causes, results, symptoms, advantages, disadvantages, recommendations, changes, etc.

In this part of the lesson we will learn how to link sentences using discourse markers for Enumerating

1 Adverbial connectives

- First, second, third, last, finally

- First, then, next, lastly

Example:

The cathode ray tube (CRT) operated as follows:

- First electrons are emitted from a heated cathode

- Then these electrons are accelerated to give them a high velocity

- Next they are formed into a beam which can be deflected vertically and horizontally

- Finally they are made to strike a screen on its inner surface with a phosphor

2 Determiners

Nuclear reaction can occur by two different processed:

The first is nuclear fission, the second is nuclear fusion

Or the first process is fission, the second process is fusion

Exercise 1: Using the different markers to write sentences of Enumeration

1 According to one professor, there have been two changes in psychology in the past 15 years

- A movement from a behaviorist to a cognitive approach,

- A increased interaction between psychology and other sciences which study people

2 Common cold has several symptoms:

- Feel very unpleasant

- Your head aches

- You sneeze and cough

- Your nose is all stuffed up and it keeps running

3 The order of a filter determines how sharply out-of-band signals are attenuated

We shall see examples of filters

- a first-order, high-pass filter,

- a low-pass filter

- to show how the two filters dealt with above can be made into a band pass filter

Unit 4: Test and repair instruments

Logic probe

This instrument is used for measuring voltage levels and pulses in digital logic circuits When the probe is placed on the pin of a logic IC, small colored LEDs light up to indicate if a pulse is detected or whether the pin is at high or a low logic level

Oscilloscope

This instrument is used to measure fast - moving signals It shows how a signal varies with time or relative to another signal It used a cathode ray tube to display the waveform of the measured signal on a screen

Function generator

This instrument contains a triangular wave oscillator which can be switched

to produce triangular, square, or sine waves over a range of frequencies It used ti test and adjust a variety of electronic equipment such as audio amplifiers The function generator provides a known signal which can be injected into a circuit Often it is used with an oscilloscope so that a visual display of the waveform can

be seen

TASK 2

Which of the instruments would you use to do the following?

1 To check a fuse

2 To determine the frequency response of an audio amplifier

3 To test for the presence of a control signal on the output pin of computer chip

4 To determine the value of the current through a transformer

5 To measure the frequency of an oscillator.

TASK 3. Compound nouns.

Study these examples of compound nouns.

A signal generator = equipment for generating signals

A cassette player – equipment for playing cassettes

1 Playing CDs?

2 Receiving batteries (signals)?

3 Charging batteries?

4 Amplifying aerial (signals)?

5 Filtering (out) noise?

6 Synthesizing speech?

7 Cleaning cassette heads?

8 Amplifying (the) power (of a signal)?

9 Sensing vibration?

10 Scanning (the human) body (for disease)?

TASK 4

Read the text and put the verbs in bracket in to the correct verb form.

For fault – finding you must have at least a multi-meter, either analogue or digital

An oscilloscope (not/be) absolutely essential but you will (find) yourself very restricted without one It’s like trying to repair (try/repair) a car while (wear) a blindfold

For audio equipment, a signal source (need) Clearly, a function generator is useful but simpler and cheaper alternative work well in most cases You only need a (fix) frequency source, say 400 or 1000 Hz sine or square wave For cassette recorders a tape with a constant 4000 Hz wave .(record) on both channels is adequate for most fault-finding However, for .(check) playback levels and frequency response and aligning the tape head, proper test tapes, which are expensive (require)

For serious work, a collection of test leads and audio connectors is essential Most modern audio equipment uses phono sockets so it’s worthwhile (invest) in cables which terminate in phono plugs For other types of sockets, adaptors are available