aec lab basics

8/8/2019 AEC Lab Basics

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PESIT

Department ofTelecommunication

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AE LAB Supporting Notes

FACULTY:KRSSession :- Aug07 Dec 07

S A M 3 K S 1 of 4026-Sep-07


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>>>>>Circuit Symbols

Circuit symbols are used in Circuit diagrams which show how a circuit is connectedtogether. The actual layout of the components is usually quite different from the circuitdiagram..

Wires and connections

Component Circuit Symbol Function of Component

WireTo pass current very easily from one part of a circuit

to another.

Wires joined

A 'blob' should be drawn where wires are connected(joined), but it is sometimes omitted. Wiresconnected at 'crossroads' should be staggered

slightly to form two T-junctions, as shown on the

right.

Wires not joined

In complex diagrams it is often necessary to drawwires crossing even though they are not connected. I

prefer the 'bridge' symbol shown on the right

because the simple crossing on the left may bemisread as a join where you have forgotten to add a

'blob'!Power Supplies


Cell

Supplies electrical energy.

The larger terminal (on the left) is positive (+).A single cell is often called a battery, but strictly a battery is two

or more cells joined together.

Battery

Supplies electrical energy. A battery is more than one

cell.The larger terminal (on the left) is positive (+).

DC supplySupplies electrical energy.

DC = Direct Current, always flowing in one direction.

AC supply

Supplies electrical energy.

AC = Alternating Current, continually changingdirection.

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Transformer

Two coils of wire linked by an iron core.

Transformers are used to step up (increase) and step

down (decrease) AC voltages. Energy is transferredbetween the coils by the magnetic field in the core.

There is no electrical connection between the coils.

Earth(Ground)

A connection to earth. For many electronic circuits

this is the 0V (zero volts) of the power supply, but formains electricity and some radio circuits it reallymeans the earth. It is also known as ground.

Resistors


Resistor

A resistor restricts the flow of current, for

example to limit the current passing through anLED. A resistor is used with a capacitor in a

timing circuit.Some publications still use the old resistor symbol:

Variable Resistor

(Rheostat)

This type of variable resistor with 2 contacts (a

rheostat) is usually used to control current.

Examples include: adjusting lamp brightness,adjusting motor speed, and adjusting the rate of

flow of charge into a capacitor in a timing circuit.

Variable Resistor

(Potentiometer)

This type of variable resistor with 3 contacts (a

potentiometer) is usually used to control voltage.

It can be used like this as a transducer converting position (angle of the control spindle) to an

electrical signal.

Variable Resistor

(Preset)

This type of variable resistor (a preset) is

operated with a small screwdriver or similar tool.It is designed to be set when the circuit is made

and then left without further adjustment. Presets

are cheaper than normal variable resistors so theyare often used in projects to reduce the cost.

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Capacitors


C apacitor

A capacitor stores electric charge. A capacitor

is used with a resistor in a timing circuit. It can

also be used as a filter, to block DC signals butpass AC signals.

C apacitor,

polarised

A capacitor stores electric charge. This type

must be connected the correct way round. A

capacitor is used with a resistor in a timingcircuit. It can also be used as a filter, to block

DC signals but pass AC signals.

V ariable Capacitor A variable capacitor is used in a radio tuner.

Diodes


D iodeA device which only allows currentto flow in one direction.

L EDLight Emitting Diode

A transducer which converts

electrical energy to light.

Z ener DiodeA special diode which is used tomaintain a fixed voltage across its

terminals.

Photodiode A light-sensitive diode.

Transistors


T ransistor NPNA transistor amplifies current. It can be used with other

components to make an amplifier or switching circuit.

T ransistor PNPA transistor amplifies current. It can be used with other

components to make an amplifier or switching circuit.

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Meters and Oscilloscope


V oltmeterA voltmeter is used to measure voltage.The proper name for voltage is 'potential difference', but

most people prefer to say voltage!

A mmeter An ammeter is used to measure current.

G alvanometer

A galvanometer is a very sensitive meter which

is used to measure tiny currents, usually 1mAor less.

O hmmeterAn ohmmeter is used to measure resistance.Most multimeters have an ohmmeter setting.

O scilloscope An oscilloscope is used to display the shape ofelectrical signals and it can be used to measure

their voltage and time period.

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>>>>>AC, DC and Electrical signals

Alternating Current (AC)

Alternating Current (AC) flows one way, then the other way, continually reversingdirection.

An AC voltage is continually changing between positive (+) and negative (-).

The rate of changing direction is called the frequency of the AC and it is measured inhertz (Hz) which is the number of forwards-backwards cycles per second.

Mains electricity in the INDIA has a frequency of 50Hz.

An AC supply is suitable for powering some devices such as lamps and heaters butalmost all electronic circuits require a steady DC supply (see below).

Direct Current (DC)

Direct Current (DC) always flows in the same direction, but it may increase anddecrease.

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AC from a power

supply

This shape is called a

sine wave.

This triangular signal AC because it change

between positive (+) an

negative (-).

Steady DC from a battery or regulatedpower supply, this is ideal for electronic

circuits.

Smooth DC from a smoothed powersupply, this is suitable for some electronics.

Varying DC from a power supplywithout smoothing, this is not suitable for

electronics


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A DC voltage is always positive (or always negative), but it may increase and decrease.

Electronic circuits normally require a steady DC supply which is constant at one valueor a smooth DC supply which has a small variation called ripple.

Cells, batteries and regulated power supplies provide steady DC which is ideal forelectronic circuits.

Power supplies contain a transformerwhich converts the mains AC supply to a safe low

voltage AC. Then the AC is converted to DC by a bridge rectifier but the output isvarying DC which is unsuitable for electronic circuits.

Some power supplies include a capacitor to provide smooth DC which is suitable forless-sensitive electronic circuits, including most of the projects on this website.

Lamps, heaters and motors will work with any DC supply.

Properties of electrical signals

An electrical signal is a voltage or currentwhich conveys information, usually it means avoltage. The term can be used for any voltageor current in a circuit.

The voltage-time graph on the right showsvarious properties of an electrical signal. Inaddition to the properties labelled on the graph,there is frequency which is the number of cycles per second.

The diagram shows a sine wave but these properties apply to any signal with aconstant shape.

Amplitude is the maximum voltage reached by the signal.It is measured in volts, V.

Peak voltage is another name for amplitude. Peak-peak voltage is twice the peak voltage (amplitude). When reading an

oscilloscope trace it is usual to measure peak-peak voltage.

Time period is the time taken for the signal to complete one cycle.It is measured in seconds (s), but time periods tend to be short so milliseconds(ms) and microseconds (s) are often used. 1ms = 0.001s and1s = 0.000001s.

Frequency is the number of cycles per second.It is measured in hertz (Hz), but frequencies tend to be high so kilohertz (kHz)

and megahertz (MHz) are often used. 1kHz = 1000Hz and 1MHz = 1000000Hz.

frequency =1

and time period =1

time period frequency

Mains electricity in the India has a frequency of 50Hz,so it has a time period of1/50 = 0.02s = 20ms.

Root Mean Square (RMS) Values

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The value of an AC voltage is continually changingfrom zero up to the positive peak, through zero to thenegative peak and back to zero again. Clearly formost of the time it is less than the peak voltage, sothis is not a good measure of its real effect.

Instead we use the root mean square voltage (VRMS)which is 0.7 of the peak voltage (Vpeak):

VRMS = 0.7 Vpeak and Vpeak = 1.4 VRMS

These equations also apply to current.They are only true for sine waves (the most commontype of AC) because the 0.7 and 1.4 are different values for other shapes.

The RMS value is the effective value of a varying voltage or current. It is theequivalent steady DC (constant) value which gives the same effect.

For example a lamp connected to a 6V RMS AC supply will light with the samebrightness when connected to a steady 6V DC supply. However, the lamp will bedimmer if connected to a 6V peak AC supply because the RMS value of this is only

4.2V (it is equivalent to a steady 4.2V DC).

You may find it helps to think of the RMS value as a sort of average, but pleaseremember that it is NOT really the average! In fact the average voltage (or current) of anAC signal is zero because the positive and negative parts exactly cancel out!

What do AC meters show, is it the RMS or peak voltage?

AC voltmeters and ammeters show the RMS valueof the voltage or current. DC metersalso show the RMS value when connected to varying DC providing the DC is varyingquickly, if the frequency is less than about 10Hz you will see the meter readingfluctuating instead.

What does '6V AC' really mean, is it the RMS or peak voltage?If the peak value is meant it should be clearly stated, otherwise assume it is the RMSvalue. In everyday use AC voltages (and currents) are always given as RMS valuesbecause this allows a sensible comparison to be made with steady DC voltages (andcurrents), such as from a battery.

For example a '6V AC supply' means 6V RMS, the peak voltage is 8.6V. The UK mainssupply is 230V AC, this means 230V RMS so the peak voltage of the mains is about320V!

So what does root mean square (RMS) really mean?

First square all the values, then find the average (mean) of these square values over acomplete cycle, and find the square root of this average. That is the RMS value.Confused? Ignore the maths (it looks more complicated than it really is), just accept thatRMS values for voltage and current are a much more useful quantity than peak values.

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>>>>>Components

Breadboard

Uses of Breadboard

A breadboard is used to make up temporary circuits for testing or to try out an idea.No soldering is required so it is easy to change connections and replace components.Parts will not be damaged so they will be available to re-use afterwards.

The photograph shows a typical small breadboard which is suitable for beginnersbuilding simple circuits with one or two ICs (chips). Larger sizes are available and youmay wish to buy one of these to start with.

Connections on BreadboardBreadboards have many tiny sockets (called 'holes') arranged on a 0.1" grid. The leadsof most components can be pushed straight into the holes. ICs are inserted across thecentral gap with their notch or dot to the left.

Wire links can be made with single-core plastic-coated wire of 0.6mm diameter (thestandard size). Stranded wire is not suitable because it will crumple when pushed into ahole and it may damage the board ifstrands break off.

The diagram shows how thebreadboard holes are connected:

The top and bottom rows are linkedhorizontally all the way across asshown by the red and black lines onthe diagram. The power supply isconnected to these rows, + at the topand 0V (zero volts) at the bottom.

The other holes are linked verticallyin blocks of 5 with no link across thecentre as shown by the blue lines onthe diagram.

Building a Circuit on Breadboard

Converting a circuit diagram to a breadboard layout is not straightforward because thearrangement of components on breadboard will look quite different from the circuitdiagram.

When putting parts on breadboard you must concentrate on theirconnections, not theirpositions on the circuit diagram. The IC (chip) is a good starting point so place it in the

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centre of the breadboard and work round it pinby pin, putting in all the connections andcomponents for each pin in turn.

The best way to explain this is by example, sothe process of building this 555 timer circuit onbreadboard is listed step-by-step below.

The circuit is a monostable which means it willturn on the LED for about 5 seconds when the'trigger' button is pressed. The time period isdetermined by R1 and C1 and you may wish to try changing their values. R1 should bein the range 1k to 1M .

Time Period, T = 1.1 R1 C1

Capacitance

Capacitance

Capacitance (symbol C) is a measure of a capacitor's abilityto store charge. A large capacitance means that morecharge can be stored. Capacitance is measured in farads,symbol F. However 1F is very large, so prefixes (multipliers)are used to show the smaller values:

(micro) means 10-6 (millionth), so 1000000F = 1F

n (nano) means 10-9 (thousand-millionth), so 1000nF = 1F

p (pico) means 10-12 (million-millionth), so 1000pF = 1nF

There are many types of capacitor but they can be split into two groups,polarisedand unpolarised.

Polarised capacitors (large values, 1F +)

Examples:

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the correctway round, at least one of their leads will be marked + or -. They are notdamaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads areattached to each end (220F in picture) and radial where both leads are at thesame end (10F in picture). Radial capacitors tend to be a little smaller and theystand upright on the circuit board.

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Monostable Circuit Diagram

unpolarised capacitor symbol

polarised capacitor symbol


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It is easy to find the value of electrolytic capacitors because they are clearlyprinted with their capacitance and voltage rating. The voltage rating can be quitelow (6V for example) and it should always be checked when selecting anelectrolytic capacitor. It the project parts list does not specify a voltage, choose acapacitor with a rating which is greater than the project's power supply voltage.25V is a sensible minimum for most battery circuits.

Unpolarised capacitors (small values, up to 1F)

Examples: Circuit symbol:

Small value capacitors are unpolarised and may be connected either way round. Theyare not damaged by heat when soldering, except for one unusual type (polystyrene).

They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult tofind the values of these small capacitors because there are many types of them andseveral different labelling systems!

Charge and Energy Stored

The amount of charge (symbol Q) stored by a capacitor is given by:

Charge, Q = C V where:Q = charge in coulombs (C)C = capacitance in farads (F)

V = voltage in volts (V)

When they store charge, capacitors are also storing energy:

Energy, E = QV = CV where E = energy in joules (J).

Note that capacitors return their stored energy to the circuit. They do not 'use up'electrical energy by converting it to heat as a resistor does. The energy stored by acapacitor is much smaller than the energy stored by a battery so they cannot be usedas a practical source of energy for most purposes.

Capacitive Reactance Xc

Capacitive reactance (symbol Xc) is a measure of a capacitor's opposition to AC(alternating current). Like resistance it is measured in ohms, , but reactance is morecomplex than resistance because its value depends on the frequency (f) of the electricalsignal passing through the capacitor as well as on the capacitance, C.

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Capacitive reactance, Xc =1

where:

Xc = reactance in ohms ( )

f = frequency in hertz (Hz)

C = capacitance in farads (F)2 fC

The reactance Xc is large at low frequencies and small at high frequencies. For steadyDC which is zero frequency, Xc is infinite (total opposition), hence the rule thatcapacitors pass AC but block DC.

For example a 1F capacitor has a reactance of 3.2k for a 50Hz signal, but when thefrequency is higher at 10kHz its reactance is only 16 .

Note: the symbol Xc is used to distinguish capacitative reactance from inductivereactance XL which is a property of inductors. The distinction is important because X Lincreases with frequency (the opposite of Xc) and if both XL and Xc are present in acircuit the combined reactance (X) is the difference between them. For furtherinformation please see the page on Impedance.

Capacitors in Series and Parallel

Combined capacitance (C) of

capacitors connected in series:1

=1

+1

+1

+ ...C C1 C2 C3

Combined capacitance (C) of

capacitors connected in parallel:C = C1 + C2 + C3 + ...

Charging a capacitor

The capacitor (C) in the circuit diagram is being chargedfrom a supply voltage (Vs) with the current passingthrough a resistor (R). The voltage across the capacitor(Vc) is initially zero but it increases as the capacitorcharges. The capacitor is fully charged when Vc = Vs.The charging current (I) is determined by the voltageacross the resistor (Vs - Vc):

Charging current, I = (Vs - Vc) / R (note that Vc is

increasing)At first Vc = 0V so the initial current, Io = Vs / R

Vc increases as soon as charge (Q) starts to build up(Vc = Q/C), this reduces the voltage across the resistor andtherefore reduces the charging current. This means that therate of charging becomes progressively slower.

time constant = R C

where:

time constant is in

seconds (s)

R = resistance inohms ( )

C = capacitance in

farads (F)

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For example:If R = 47k and C = 22F, then the time constant, RC =47k 22F = 1.0s.If R = 33k and C = 1F, then the time constant, RC =33k 1F = 33ms.

A large time constant means the capacitorcharges slowly. Note that the time constant is a

property of the circuit containing thecapacitance and resistance, it is not a propertyof a capacitor alone.

The time constant is the time taken for thecharging (or discharging) current (I) to fall to 1/eof its initial value (Io). 'e' is the base of naturallogarithms, an important number in mathematics(like ). e = 2.71828 (to 6 significant figures) sowe can roughly say that the time constant is thetime taken for the current to fall to 1/3 of its initial

value.After each time constant the current falls by 1/e(about 1/3). After 5 time constants (5RC) thecurrent has fallen to less than 1% of its initial value and we can reasonably say that thecapacitor is fully charged, but in fact the capacitor takes for ever to charge fully!

The bottom graph shows how the voltage (V) increases as the capacitor charges. Atfirst the voltage changes rapidly because the current is large; but as the currentdecreases, the charge builds up more slowly and the voltage increases more slowly.

After 5 time constants (5RC) the capacitor is almost fully charged with its voltage almostequal to the supply voltage. We can reasonably say that the capacitor is fully charged

after 5RC, although really charging continues for ever (or until the circuit is changed).

Time 0RC 1RC 2RC 3RC 4RC 5RC

Voltage 0.0 5.7V 7.8V 8.6V 8.8V 8.9V

Charge 0% 63% 86% 95% 98% 99%

Discharging a capacitor

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Graphs showing the current and

voltage for a capacitor chargingtime constant = RC


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The top graph shows how the current (I)decreases as the capacitor discharges. Theinitial current (Io) is determined by the initialvoltage across the capacitor (Vo) and resistance

(R):

Initial current, Io = Vo / R.

Note that the current graphs are thesame shape for both charging anddischarging a capacitor. This type ofgraph is an example of exponentialdecay.

The bottom graph shows how thevoltage (V) decreases as the capacitor

discharges.

At first the current is large because the voltage is large, so charge is lost quickly and thevoltage decreases rapidly. As charge is lost the voltage is reduced making the currentsmaller so the rate of discharging becomes progressively slower.

After 5 time constants (5RC) the voltage across the capacitor is almost zero and we can

reasonably say that the capacitor is fully discharged, although really dischargingcontinues for ever (or until the circuit is changed).

Time 0RC 1RC 2RC 3RC 4RC 5RC

Voltage 9.0 3.3V 1.2V 0.4V 0.2v 0.1V

Charge 100% 37% 14% 5% 2% 1%

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Graphs showing the current and

voltage for a capacitor dischargingtime constant = RC


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Capacitor Coupling (CR-coupling)

Sections of electronic circuitsmay be linked with a capacitorbecause capacitors pass AC(changing) signals but block DC(steady) signals. This is calledcapacitor coupling or CR-coupling. It is used between thestages of an audio system topass on the audio signal (AC)without any steady voltage (DC)

which may be present, forexample to connect aLoudspeaker. It is also used forthe 'AC' switch setting on anOscilloscope.

The precise behaviour of acapacitor coupling is determinedby its time constant (RC). Notethat the resistance (R) may beinside the next circuit section

rather than a separate resistor.

For successful capacitorcoupling in an audio system the signals must pass through with little or no distortion.This is achieved if the time constant (RC) is larger than the Time period (T) of the lowestfrequency audio signals required (typically 20Hz, T = 50ms).

Output when RC >> TWhen the time constant is much larger than the time period of the input signal thecapacitor does not have sufficient time to significantly charge or discharge, so the signalpasses through with negligible distortion.

Output when RC = TWhen the time constant is equal to the time period you can see that the capacitor hastime to partly charge and discharge before the signal changes. As a result there issignificant distortion of the signal as it passes through the CR-coupling. Notice how thesudden changes of the input signal pass straight through the capacitor to the output.

Output when RC


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positive and negative. This can be useful in a system which must detect when a signalchanges suddenly, but must ignore slow changes.

>>>>>Resistors:

Example: Circuit symbol:

Function

Resistors restrict the flow of electric current, for example a resistor isplaced in series with a light-emitting diode (LED) to limit the currentpassing through the LED.

Connecting and soldering

Resistors may be connected either way round. They are not damaged

by heat when soldering.

Resistor values - the resistor colour code

Resistance is measured in ohms, the symbol for ohm is an omega .1 is quite small so resistor values are often given in k and M .1 k = 1000 1 M = 1000000 .

Resistor values are normally shown using coloured bands.Each colour represents a number as shown in the table.

Most resistors have 4 bands:

The first band gives the first digit. The second band gives the second digit. The third band indicates the number of zeros. The fourth band is used to shows the tolerance (precision) of the resistor, this

may be ignored for almost all circuits but further details are given below

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The Resistor

Colour Code

Colour Number

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Grey 8

White 9


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Examples:

A 470 resistor with 10V across it, needs a power rating P = V/R = 10/470 =0.21W.In this case a standard 0.25W resistor would be suitable.

A 27 resistor with 10V across it, needs a power rating P = V/R = 10/27 =3.7W.

A high power resistor with a rating of 5W would be suitable.

Rheostat

This is the simplest way of using a variable resistor. Twoterminals are used: one connected to an end of the track,the other to the moveable wiper. Turning the spindle

changes the resistance between the two terminals from zeroup to the maximum resistance.

Rheostats are often used to vary current, for example to control the brightness of alamp or the rate at which a capacitor charges.

If the rheostat is mounted on a printed circuit board you may find that all three terminals are connected!However, one of them will be linked to the wiper terminal. This improves the mechanical strength of themounting but it serves no function electrically.

>>>>>Connectors and Cables

Crocodile clips

The 'standard' crocodile clip has no cover and ascrew contact. However, miniature insulatedcrocodile clips are more suitable for many purposesincluding test leads. They have a solder contact andlugs which fold down to grip the cable's insulation,

increasing the strength of the joint. Remember tofeed the cable through the plastic coverbeforesoldering! Add and remove the cover by fully opening the clip, a piece of wood can beused to hold the jaws open.

BNC plugs

These are designed for screened cables carrying high frequency signals where anundistorted and noise free signal is essential, for example Oscilloscope leads. BNC

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Rheostat Symbol

Crocodile clips


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plugs are connected with a push and twist action, to disconnect you need to twistand pull.

>>>>>Cables

Cable... flex... lead... wire... what do all these terms mean?

A cable is an assembly of one or more conductors (wires) with some flexibility.

A flex is the proper name for the flexible cable fitted to mains electricalappliances. A lead is a complete assembly of cable and connectors. A wire is a single conductor which may have an outer layer of insulation (usually

plastic).

Single core equipment wire

This is one solid wire with a plastic coating available in a widevariety of colours. It can be bent to shape but will break ifrepeatedly flexed. Use it for connections which will not be disturbed, for example links

between points of a circuit board.

Typical specification: 1/0.6mm (1 strand of 0.6mm diameter), maximum current 1.8A.

Stranded wire

This consists of many fine strands of wire covered by an outerplastic coating. It is flexible and can withstand repeated bendingwithout breaking. Use it for connections which may be disturbed, for example wiresoutside cases to sensors and switches. A very flexible version ('extra-flex') is used fortest leads.

Typical specifications:10/0.1mm (10 strands of 0.1mm diameter), maximum current 0.5A.7/0.2mm (7 strands of 0.2mm diameter), maximum current 1.4A.16/0.2mm (16 strands of 0.2mm diameter), maximum current 3A.24/0.2mm (24 strands of 0.2mm diameter), maximum current 4.5A.55/0.1mm (55 strands of 0.1mm diameter), maximum current 6A, used for test leads.

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'Figure 8' (speaker) cable

'Figure 8' cable consists of two stranded wiresarranged in a figure of 8 shape. One wire isusually marked with a line. It is suitable for low

voltage, low current (maximum 1A)signals where screening fromelectrical interference is not required.It is a popular choice for connectingloudspeakers and is often called'speakercable'.

Signal cable

Signal cable consists of several colour-coded cores of stranded wire housed within anouter plastic sheath. With a typical maximum current of 1A per core it is suitable for lowvoltage, low current signals where screening from electrical interference is not required.

The picture shows 6-core cable, but 4-core and 8-core are also readily available.

Co-axial cable

This type of screened cable (see above) is designed to carry high frequency signalssuch as those found in TV aerials and Oscilloscope leads.

Mains flex

Flex is the proper name for the flexible cable

used to connect appliances to the mains supply.It contains 2 cores (for live and neutral) or 3cores (for live, neutral and earth). Mains flex hasthick insulation for the high voltage (230V in UK)and it is available with various current ratings: 3A, 6A and 13A are popular sizes in theUK.

Mains flex is sometimes used for low voltage circuits which pass a high current, butplease think carefully before using it in this way. The distinctive colours of mains flex

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should act as a warning of the mains high voltage which can be lethal; using mains flexfor low voltage circuits can undermine this warning.

Meters

Analogue and Digital Systems

Analog systems

Analogue systems process analog signals which can take any value within a range, forexample the output from an LDR (light sensor) or a microphone.

An audio amplifieris an example of an analog system. The amplifier produces an outputvoltage which can be any value within the range of its power supply.

An analog meter can display any value within the range available on its scale.

However, the precision of readings is limited by our ability to read them. For examplethe meter on the right shows 1.25V because the pointer is estimated to be half waybetween 1.2 and 1.3. The analogue meter can show any value between 1.2 and 1.3 butwe are unable to read the scale more precisely than about half a division.

All electronic circuits suffer from 'noise' which is unwanted signal mixed in with thedesired signal, for example an audio amplifier may pick up some mains 'hum' (the 50Hzfrequency of the UK mains electricity supply). Noise can be difficult to eliminate from

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Analogue signal

Analogue meter display
http://www.kpsec.freeuk.com/meters.htm#analoguehttp://www.kpsec.freeuk.com/meters.htm#analogue


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analogue signals because it may be hard todistinguish from the desired signal.

Digital systems

Digital systems process digital signals which can take only a limited number of values(discrete steps), usually just two values are used: the positive supply voltage (+Vs) andzero volts (0V).

Digital systems contain devices such as logic gates flip-flops, shift registers andcounters. A computer is an example of a digital system.

A digital meter can display many values, but not every value within its range. Forexample the display on the right can show 6.25 and 6.26 but not a value between them.This is not a problem because digital meters normally have sufficient digits to showvalues more precisely than it is possible to read an analogue display.

Logic signals

Most digital systems use the simplest possible type of signal which hasjust two values. This type of signal is called a logic signal because the

two values (or states) can be called true and false. Normally the positivesupply voltage +Vs represents true and 0V represents false. Other labelsfor the true and false states are shown in the table on the right.

Noise is relatively easy to eliminate from digital signals because it is easyto distinguish from the desired signal which can only have particular values. Forexample: if the signal is meant to be +5V (true) or 0V (false), noise of up to 2.5V can beeliminated by treating all voltages greater than 2.5V as true and all voltages less than2.5V as false.

Analogue display

Analogue displays have a pointer which moves over agraduated scale. They can be difficult to read because of theneed to work out the value of the smallest scale division. Forexample the scale in the picture has 10 small divisionsbetween 0 and 1 so each small division represents 0.1. Thereading is therefore 1.25V (the pointer is estimated to be halfway between 1.2 and 1.3).

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Digital (logic) signal

Digital meter display

Logic states

True False

1 0

High Low

+Vs 0V

On Off


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The maximum reading of an analogue meter is called full-scale deflection orFSD (it is5V in the example shown).

Analogue meters must be connected the correct way round to prevent them beingdamaged when the pointer tries to move in the wrongdirection. They are useful for monitoring continouslychanging values (such as the voltage across a capacitor

discharging) and they can be good for quick roughreadings because the movement of the pointer can beseen without looking away from the circuit under test.

Taking accurate readings

To take an accurate reading from an analogue scale youmust have your eye in line with the pointer. Avoid lookingat an angle from the left or right because you will see a reading which is a little too highor too low. Many analogue meters have a small strip ofmirroralong the scale to helpyou. When your eye is in the correct position the reflection of the pointer is hidden

behind the pointer itself. If you can see the reflection you are looking at an angle.

Instead of a mirror, some meters have a twisted pointerto aid accurate readings. Theend of the pointer is turned through 90 so it appears very thin when viewed correctly.

Digital display

Values can be read directly from digital displays so they are easyto read accurately. It is normal for the least significant digit (on theright) to continually change between two or three values, this is afeature of the way digital meters work, not an error! Normally you

will not need great precision and the least significant digit can be ignored or rounded up.

Digital meters may be connected either way round without damage, they will show aminus sign (-) when connected in reverse. If you exceed the maximum reading mostdigital meters show an almost blank display with just a 1 on the left-hand side.

All digital meters contain a battery to power the display so they use virtually no powerfrom the circuit under test. This means that digital voltmeters have a very highresistance (usually called input impedance) of 1M or more, usually 10M , and theyare very unlikely to affect the circuit under test.

For general use digital meters are the best type. They are easy to read, they may beconnected in reverse and they are unlikely to affect the circuit under test.

Connecting meters

It is important to connect meters the correct way round:

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Correctreflection hidden

Wrongreflection visible


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The positive terminal of the meter, marked + or coloured red should beconnected nearest to + on the battery or power supply.

The negative terminal of the meter, marked - or coloured black should beconnected nearest to - on the battery or power supply.

Voltmeters

Voltmeters measure voltage. Voltage is measured in volts, V. Voltmeters are connected in parallel across components. Voltmeters have a very high resistance.

Measuring voltage at a point

When testing circuits you oftenneed to find the voltages atvarious points, for example thevoltage at pin 2 of a 555 timerchip. This can seem confusing -where should you connect thesecond voltmeter lead?

Connect the black(negative -) voltmeterlead to 0V, normally thenegative terminal of thebattery or power supply.

Connect the red (positive+) voltmeter lead to the point you where you need to measure the voltage.

The black lead can be left permanently connected to 0V while you use the redlead as a probe to measure voltages at various points.

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Connecting a voltmeter in parallel


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You may wish to use a crocodile clip on the black lead to hold it in place.

Voltage at a point really means the voltage difference between that point and 0V (zerovolts) which is normally the negative terminal of the battery or power supply. Usually 0Vwill be labelled on the circuit diagram as a reminder.

Analogue meters take a little power from the circuit under

test to operate their pointer. This may upset the circuit andgive an incorrect reading. To avoid this voltmeters shouldhave a resistance of at least 10 times the circuit resistance(take this to be the highest resistor value near where themeter is connected).

Ammeters

Ammeters measure current. Current is measured in amps (amperes), A.

1A is quite large, so mA (milliamps) and A(microamps) are often used. 1000mA = 1A, 1000A= 1mA, 1000000A = 1A.

Ammeters are connected in series.To connect in series you must break the circuit and put the ammeter across thegap, as shown in the diagram.

Ammeters have a very low resistance.

The need to break the circuit to connect in series means that ammeters are difficult to use onsoldered circuits. Most testing in electronics is done with voltmeters which can be easily connectedwithout disturbing circuits.

Galvanometers

Galvanometers are very sensitive meters which are used tomeasure tiny currents, usually 1mA or less. They are used tomake all types of analogue meters by adding suitableresistors as shown in the diagrams below. The photographshows an educational 100A galvanometer for which various multipliers and shunts areavailable.

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Connecting an ammeter in series


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Making a VoltmeterA galvanometer with a high

resistance multiplier in series tomake a voltmeter.

Making an AmmeterA galvanometer with a low

resistance shunt in parallel tomake an ammeter.

Galvanometer with multiplier and Maximum meter current 100A (or 20A re

This meter is unusual in allowingreverse readings to be shown.

Ohmmeters

An ohmmeter is used to measure resistance in ohms ( ).Ohmmeters are rarely found as separate meters but allstandard Multimeters have an ohmmeter setting.1 is quite small so k and M are often used.

1k = 1000 , 1M = 1000k =1000000 .

Multimeters

Multimeters are very useful testinstruments. By operating a multi-position switch on the meter they can bequickly and easily set to be a voltmeter,an ammeter or an ohmmeter. Theyhave several settings (called 'ranges') for each type of meter and the choice of AC orDC.

Some multimeters have additional features such as transistor testing and ranges formeasuring capacitance and frequency.

Analogue multimeters consist of a Galvanometer with various resistors which can beswitched in as multipliers (voltmeter ranges) and shunts (ammeter ranges).

Measuring resistance with a multimeter

To measure the resistance of a component it must not be connected in a circuit. Ifyou try to measure resistance of components in a circuit you will obtain false readings

(even if the supply is disconnected) and you may damage the multimeter.

The techniques used for each type of meter are very different so they are treatedseparately:

Measuring resistance with a DIGITAL multimeter

1. Set the meter to a resistance range greater than you expect the resistanceto be.

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Analogue Multimeter Digital Multimeter


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Notice that the meter display shows "off the scale" (usually blank except for a 1 on the left). Don'tworry, this is not a fault, it is correct - the resistance of air is very high!

2. Touch the meter probes together and check that the meter reads zero.If it doesn't read zero, turn the switch to 'Set Zero' if your meter has this and try again.

3. Put the probes across the component.Avoid touching more than one contact at a time oryourresistance will upset the reading!

Measuring resistance with an ANALOGUE multimeter

The resistance scale on an analogue meter is normally at the top, it is an unusual scalebecause it reads backwards and is not linear(evenly spaced). This is unfortunate, butit is due to the way the meter works.

1. Set the meter to a suitable resistance range.Choose a range so that the resistance you expect will be near the middle of the scale. Forexample: with the scale shown below and an expected resistance of about 50k choose the 1k

range.2. Hold the meter probes together and adjust the control on the front of the

meter which is usually labelled "0 ADJ" until the pointer reads zero (on the

RIGHT remember!).If you can't adjust it to read zero, the battery inside the meter needs replacing.3. Put the probes across the component.

Avoid touching more than one contact at a time oryourresistance will upset the reading!

Testing a diode with a multimeter

The techniques used for each type of meter are very different sothey are treated separately:

Testing a diode with a DIGITAL multimeter

Digital multimeters have a special setting for testing adiode, usually labelled with the diode symbol.

Connect the red (+) lead to the anode and the black (-)to the cathode. The diode should conduct and the meterwill display a value (usually the voltage across the diode in mV, 1000mV = 1V).

Reverse the connections. The diode should NOT conduct this way so the meterwill display "off the scale" (usually blank except for a 1 on the left).

Testing a diode with an ANALOGUE multimeter

Set the analogue multimeter to a low value resistance range such as 10. It is essential to note that the polarity of analogue multimeter leads is reversed on

the resistance ranges, so the black lead is positive (+) and the red lead isnegative (-)! This is unfortunate, but it is due to the way the meter works.

Connect the black (+) lead to anode and the red (-) to the cathode. The diodeshould conduct and the meter will display a low resistance (the exact value is notrelevant).

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Diodes

a = anodek = cathode


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Reverse the connections. The diode should NOT conduct this way so the meter

will show infinite resistance (on the left of the scale).

Oscilloscopes

An oscilloscope is a test instrument which allows you to look at the 'shape' of electricalsignals by displaying a graph of voltage against time on its screen. It is like a voltmeterwith the valuable extra function of showing how the voltage varies with time. A graticulewith a 1cm grid enables you to take measurements of voltage and time from the screen.

The graph, usually called the trace, is drawn by a beam of electrons striking the

phosphor coating of the screen making it emit light, usually green or blue. This is similarto the way a television picture is produced.

Oscilloscopes contain a vacuum tube with a cathode (negative electrode) at one end toemit electrons and an anode (positive electrode) to accelerate them so they moverapidly down the tube to the screen. This arrangement is called an electron gun. Thetube also contains electrodes to deflect the electron beam up/down and left/right.

The electrons are called cathode rays because they are emitted by the cathode and thisgives the oscilloscope its full name ofcathode ray oscilloscope or CRO.

A dual trace oscilloscope can display two traces on the screen, allowing you to easilycompare the input and output of an amplifier for example. It is well worth paying themodest extra cost to have this facility.

Precautions

An oscilloscope should be handled gently to protect its fragile (and expensive)vacuum tube.

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Circuit symbol for an oscilloscope

Cathode Ray Oscilloscope (CRO)


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Oscilloscopes use high voltages to create the electron beam and these remainfor some time after switching off - for your own safety do not attempt to examinethe inside of an oscilloscope!

Setting up an oscilloscope

Oscilloscopes are complex instruments with many controls and they require some careto set up and use successfully. It is quite easy to 'lose' thetrace off the screen if controls are set wrongly!

There is some variation in the arrangement and labelling ofthe many controls so the following instuctions may need tobe adapted for your instrument.

1. Switch on the oscilloscope to warm up (it takes a

minute or two).2. Do not connect the input lead at this stage.3. Set the AC/GND/DC switch (by the Y INPUT) to DC.4. Set the SWP/X-Y switch to SWP (sweep).5. Set Trigger Level to AUTO.6. Set Trigger Source to INT (internal, the y input).7. Set theY AMPLIFIER to 5V/cm (a moderate value).8. Set the TIMEBASE to 10ms/cm (a moderate speed).9. Turn the timebase VARIABLE control to 1 orCAL.10.Adjust Y SHIFT (up/down) and X SHIFT (left/right) to give a trace across the

middle of the screen, like the picture.

11.Adjust INTENSITY (brightness) and FOCUS to give a bright, sharp trace.12.The oscilloscope is now ready to use!

Connecting the input lead is described in the next section.

Connecting an oscilloscope

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This is what you should seeafter setting up, when thereis no input signal connected


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The Y INPUT lead to an oscilloscope should be a co-axial lead and the diagram shows its construction.The central wire carries the signal and the screen isconnected to earth (0V) to shield the signal fromelectrical interference (usually called noise).

Most oscilloscopes have a BNC socket for the y

input and the lead is connected with a push andtwist action, to disconnect you need to twist andpull. Oscilloscopes used in schools may have redand black 4mm sockets so that ordinary,unscreened, 4mm plug leads can be used ifnecessary.

Professionals use a specially designed lead andprobes kit for best results with high frequency signalsand when testing high resistance circuits, but this isnot essential for simpler work at audio frequencies

(up to 20kHz).

An oscilloscope is connected like a Voltmeter but you mustbe aware that the screen (black) connection of the input leadis connected to mains earth at the oscilloscope! This meansit must be connected to earth or 0V on the circuit beingtested.

Obtaining a clear and stable trace

Once you have connected the oscilloscope to the circuit youwish to test you will need to adjust the controls to obtain aclear and stable trace on the screen:

The Y AMPLIFIER (VOLTS/CM) control determinesthe height of the trace. Choose a setting so the trace occupies at least half thescreen height, but does not disappear off the screen.

The TIMEBASE (TIME/CM) control determines the rate at which the dot sweepsacross the screen. Choose a setting so the trace shows at least one cycle of the

signal across the screen.Note that a steady DC input signal gives a horizontal line trace for which the timebase setting isnot critical.

The TRIGGER control is usually best left set to AUTO.

If you are using an oscilloscope for the first time it is best to start with an easy signalsuch as the output from an AC power pack set to about 4V.

Measuring voltage and time period

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Construction of a co-axial lead

Oscilloscope lead and probes kit

The trace of an AC signal

with the oscilloscope

controls correctly set


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Time period

Time is shown on the horizontal x-axis and the scale is determined by the TIMEBASE(TIME/CM) control. The time period (often just called period) is the time for one cycleof the signal. The frequency is the number of cyles persecond, frequency = 1/time period

Ensure that the variable timebase control is set to 1 or CAL (calibrated)before attempting to take a time reading.

Time = distance in cm time/cmExample: time period = 4.0cm 5ms/cm = 20ms

and frequency = 1/time period=1/20ms = 50Hz

Timebase (time/cm) and trigger controls

The oscilloscope sweeps the electron beam across the screen from left to right at asteady speed set by the TIMEBASE control. Each setting is labelled with the time thedot takes to move 1cm, effectively it is setting the scale on the x-axis. The timebasecontrol may be labelled TIME/CM.

At slow timebase settings (such as 50ms/cm) you can see a dot moving across thescreen but at faster settings (such as 1ms/cm) the dot is moving so fast that it appearsto be a line.

The VARIABLE timebase control can be turned to make a fine adjustment to the speed,

but it must be left at the position labelled 1 or CAL (calibrated) if you wish to take timereadings from the trace drawn on the screen.

The TRIGGER controls are used to maintain a steady trace on the screen. If they areset wrongly you may see a trace drifting sideways, a confusing 'scribble' on the screen,or no trace at all! The trigger maintains a steady trace by starting the dot sweepingacross the screen when the input signal reaches the same point in its cycle each time.

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Slow timebase, no inputYou can see the dot moving

Fast timebase, no inputThe dot is too fast to seeso it appears to be a line


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For straightforward use it is best to leave the trigger level set to AUTO, but if you havedifficulty obtaining a steady trace try adjusting this control to set the level manually.

Y amplifier (volts/cm) control

The oscilloscope moves the trace up and down in proportionto the voltage at the Y INPUT and the setting of the YAMPLIFIER control. This control sets the voltage representedby each centimetre (cm) on the the screen, effectively it issetting the scale on the y-axis. Positive voltages make thetrace move up, negative voltages make it move down.

The y amplifier control may be labelled Y-GAIN orVOLTS/CM.

The input voltage moving the dot up and down at the same time as the dot is swept

across the screen means that the trace on the screen is a graph ofvoltage (y-axis)against time (x-axis) for the input signal.

The AC/GND/DC switch

The normal setting for this switch is DC for all signals, including AC!

Switching to GND (ground) connects the y input to 0V and allows you to quickly checkthe position of 0V on the screen (normally halfway up). There is no need to disconnectthe input lead while you do this because it is disconnected internally.

Switching to AC inserts a capacitor in series with the input to block out any DC signalpresent and pass only AC signals. This is used to examine signals showing a smallvariation around one constant value, such as the ripple on the output of a smooth DCsupply. Reducing the VOLTS/CM to see more detail of the ripple would normally takethe trace off the screen! The AC setting removes the constant (DC) part of the signal,allowing you to view just the varying (AC) part which can now be examined more closelyby reducing the VOLTS/CM. This is shown in the diagrams below:

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Varying DC (always positive)

Switching to GND allows you

to quickly check the position

of 0V (normally halfway up).


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Displaying a ripple signal using the AC switch

Switch in normal DC position.

The ripple is difficult to see, but if VOLTS/CM is reduced

to enlarge it the trace will

disappear off the screen!

Switch moved to AC position.

The constant (DC) part of thesignal is removed, leaving

just the ripple (AC) part.

VOLTS/CM reduced to

enlarge the ripple.

The ripple can now be

examined more closely.

>>>>>Power Supplies

Types of Power Supply

There are many types of power supply. Most are designed to convert high voltage ACmains electricity to a suitable low voltage supply for electronics circuits and otherdevices. A power supply can by broken down into a series of blocks, each of whichperforms a particular function.

For example a 5V regulated supply:

Each of the blocks is described in more detail below:

Transformer- steps down high voltage AC mains to low voltage AC. Rectifier- converts AC to DC, but the DC output is varying. Smoothing - smooths the DC from varying greatly to a small ripple. Regulator- eliminates ripple by setting DC output to a fixed voltage.

Dual Supplies

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Some electronic circuits require apower supply with positive and negativeoutputs as well as zero volts (0V). Thisis called a 'dual supply' because it islike two ordinary supplies connectedtogether as shown in the diagram.

Dual supplies have three outputs, for example a 9V supply has +9V, 0V and -9Voutputs.

Transformer only

The low voltage AC output is suitable for lamps, heaters and special AC motors. It isnot suitable for electronic circuits unless they include a rectifier and a smoothingcapacitor.

Transformer + Rectifier

The varying DC output is suitable for lamps, heaters and standard motors. It is notsuitable for electronic circuits unless they include a smoothing capacitor.

Transformer + Rectifier + Smoothing

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The smooth DC output has a small ripple. It is suitable for most electronic circuits.

Transformer + Rectifier + Smoothing + Regulator

The regulated DC output is very smooth with no ripple. It is suitable for all electroniccircuits.

Transformer

Transformers convert AC electricity from one voltageto another with little loss of power. Transformers workonly with AC and this is one of the reasons whymains electricity is AC.

Step-up transformers increase voltage, step-downtransformers reduce voltage. Most power suppliesuse a step-down transformer to reduce thedangerously high mains voltage (230V in UK) to asafer low voltage.

The input coil is called the primary and the outputcoil is called the secondary. There is no electricalconnection between the two coils, instead they arelinked by an alternating magnetic field created in the

soft-iron core of the transformer. The two lines in themiddle of the circuit symbol represent the core.

Transformers waste very little power so the power outis (almost) equal to the power in. Note that as voltageis stepped down current is stepped up.

The ratio of the number of turns on each coil, calledthe turns ratio, determines the ratio of the voltages. A step-down transformer has a

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Transformer

circuit symbol

Transformer


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large number of turns on its primary (input) coil which is connected to the high voltagemains supply, and a small number of turns on its secondary (output) coil to give a lowoutput voltage.

turns ratio =Vp

=Np

andpower out = power in

Vs Ns Vs Is = Vp IpVp = primary (input) voltage

Np = number of turns on primary coilIp = primary (input) current

Vs = secondary (output) voltage

Ns = number of turns on secondary coilIs = secondary (output) current

Rectifier

There are several ways of connecting diodes to make a rectifier to convert AC to DC.The Bridge rectifieris the most important and it produces full-wave varying DC. A full-

wave rectifier can also be made from just two diodes if a centre-tap transformer isused, but this method is rarely used now that diodes are cheaper. A single diode canbe used as a rectifier but it only uses the positive (+) parts of the AC wave to producehalf-wave varying DC.

Bridge rectifier

A bridge rectifier can be made using four individual diodes, but it is also available inspecial packages containing the four diodes required. It is called a full-wave rectifierbecause it uses all the AC wave (both positive and negative sections). 1.4V is used up

in the bridge rectifier because each diode uses 0.7V when conducting and there arealways two diodes conducting, as shown in the diagram below. Bridge rectifiers arerated by the maximum current they can pass and the maximum reverse voltage theycan withstand (this must be at least three times the supply RMS voltage so the rectifiercan withstand the peak voltages).

Bridge rectifierAlternate pairs of diodes conduct, changing over

the connections so the alternating directions of

AC are converted to the one direction of DC.

Output: full-wave varying DC(using all the AC wave)

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Single diode rectifier

A single diode can be used as a rectifier but this produces half-wave varying DC whichhas gaps when the AC is negative. It is hard to smooth this sufficiently well to supplyelectronic circuits unless they require a very small current so the smoothing capacitor

does not significantly discharge during the gaps.

Single diode rectifierOutput: half-wave varying DC(using only half the AC wave)

Smoothing

Smoothing is performed by a large value Electrolytic capacitorconnected across theDC supply to act as a reservoir, supplying current to the output when the varying DCvoltage from the rectifier is falling. The diagram shows the unsmoothed varying DC(dotted line) and the smoothed DC (solid line). The capacitor charges quickly near thepeak of the varying DC, and then discharges as it supplies current to the output.

Note that smoothing significantly increases the average DC voltage to almost the peak

value (1.4 RMS value). For example 6V RMS AC is rectified to full wave DC of about4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this increases to almostthe peak value giving 1.4 4.6 = 6.4V smooth DC.

Smoothing is not perfect due to the capacitor voltage falling a little as it discharges,giving a small ripple voltage. For many circuits a ripple which is 10% of the supplyvoltage is satisfactory and the equation below gives the required value for thesmoothing capacitor. A larger capacitor will give less ripple. The capacitor value mustbe doubled when smoothing half-wave DC.

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Smoothing capacitor for 10% ripple, C =5 Io

Vs f

C = smoothing capacitance in farads (F)Io = output current from the supply in amps (A)Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC

f = frequency of the AC supply in hertz (Hz), 50Hz in the UK

Regulator

Voltage regulator ICs are available with fixed

(typically 5, 12 and 15V) or variable outputvoltages. They are also rated by themaximum current they can pass. Negativevoltage regulators are available, mainly foruse in dual supplies. Most regulators includesome automatic protection from excessivecurrent ('overload protection') andoverheating ('thermal protection').

Many of the fixed voltage regulator ICs have 3 leads and look like power transistors,such as the 7805 +5V 1A regulator shown on the right. They include a hole for attaching

a heatsink if necessary.

Choosing a zener diode and resistor:

1. The zener voltage Vz is the output voltage required2. The input voltage Vs must be a few volts greater than Vz

(this is to allow for small fluctuations in Vs due to ripple)3. The maximum current Imax is the output current required plus 10%4. The zener power Pz is determined by the maximum current: Pz > Vz Imax5. The resistor resistance: R = (Vs - Vz) / Imax6. The resistor power rating: P > (Vs - Vz) Imax

Example:output voltage required is 5V, output current required is 60mA.

1. Vz = 4.7V (nearest value available)2. Vs = 8V (it must be a few volts greater than Vz)3. Imax = 66mA (output current plus 10%)4. Pz > 4.7V 66mA = 310mW, choose Pz = 400mW5. R = (8V - 4.7V) / 66mA = 0.05k = 50 , choose R = 47 6. Resistor power rating P > (8V - 4.7V) 66mA = 218mW, choose P = 0.5W

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Voltage regulator


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aec lab basics

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