Electrical Circuits / Electronics

1
Electrical Circuits / Electronics

• Electricity is one of the most important forms of
  energy available to man. It affects everyones
  lives in many ways. If you take time to think
  about your everyday life you will realise that
  our lives are full of devices that depend upon
  electricity.
• Some important terms
• Electric current
• Electric current is the name given to the flow
  of negatively charged particles called
  electrons.
• Current is measured in amperes, usually
  referred to as amps (A). Current is the rate
  of flow of electrical charges (called electrons)
  through a circuit.

2
Electrical Circuits
Voltage In most circuits a battery or voltage
supply is used to drive the electrons through the
components. Voltage is measured in volts (V).
Resistance All materials conduct electricity. The
materials that conduct electricity well are
called conductors and those that are poor
conductors are called insulators. Metals are good
conductors while rubber and glass are good
insulators. Resistance is therefore a measure of
how much voltage is required to let a current
flow. Resistance is measured in ohms (?).
3
Batteries Voltage Supplies
4
Components - Resistors
Fixed Resistor Symbol
Resistors are basic components in electrical and
electronic circuits. They limit the amount of
current flowing in circuits or parts of circuits.
Resistors are roughly cylindrical and have
coloured stripes. They also have connection wires
sticking out of each end. The stripes indicate
the value of the resistors. The colours represent
numerical values according to a special code.
Although the symbol for ohms is ? it is often
shown as a capital R that is, 270 ohms can be
expressed as either 270 ? or 270 R.
5
Resistor Colour Code
First and second colour band Digit Multiplier
Black 0 x 1
Brown 1 x 10
Red 2 x 100
Orange 3 x 1000 or 1 K
Yellow 4 x 10 000 or 10 K
Green 5 x 100 000 or 100 K
Blue 6 x 1 000 000 or 1 M
Violet 7 Silver means divide by 100
Grey 8 Gold means divide by 10
White 9 Tolerances brown ? 1 red ? 2 gold ? 5 silver ? 10 none ? 20
6
Resistor Value Calculation
If the colours on the resistor are 1st band ?
red 2nd band ? violet 3rd band ? brown 4th band ?
gold
Then its value is 2(red) 7(Violet) x 10(Brown)
with a 5 tolerance (Gold) i.e. 270ohms 5
tolerance.
7
Pupil Assignment

• Calculate the value of the following resistors
• blue violet brown silver
• orange white brown gold
• brown black red gold
• brown black green brown
• What colours would the following resistors have?
• 270 R
• 1K5
• 33 K

8
Diodes
Diodes are devices that allow current to flow in
one direction only.
Current will flow through the diode only when the
anode (positive side) is connected to the
positive side of the circuit and the cathode
(negative side) is connected to the negative side
of the circuit.
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Light Emitting Diode
A light-emitting diode is a special diode that
gives out light when current is flowing though
it. LEDs are used as indicators to tell when a
circuit (or part or a circuit) is working. You
can tell the cathode of an LED as it is the short
leg and there is a flat on the plastic casing.
10
Switches
Switches are useful input devices (or
transducers) that have metal contacts inside them
to allow current to pass when then they are
touching. There are several ways in which the
contacts in mechanical switches can be operated.
The main types are ? push-button, toggle, key,
slide, magnetic (reed) and tilt. These switches
are digital input devices as they can only be
on or off.
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Switch Contacts
DPDT
SPST
SPDT
DPST
12
Pupil Activity
We have now seen a number of common electronic
components. Lets now try and combine some of
these into a working circuit.
Copy the circuit into your workbook Build the
circuit using the modular boards Your teacher
will demonstrate how to connect the boards
13
Series Circuits
When components are connected end to end, as in
the last activity, we say they are connected in
series. This leads to an important law,
Kirchoffs 2nd Law The sum of voltages dropped
across each component (V1, V2 ?) is equal to the
total voltage supply in the circuit.
VT V1 V2 V3
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Measuring Voltage Drops

• To measure d.c. voltage
• Connect the black lead to the COM
• socket
• Connect the red lead to the V? socket
• Make sure that d.c. is selected
• Place the lead probes on the points
• where the voltage is to be measured

Digital Multimeter
15
Measuring Voltage Drops
V
Note how voltage is measured over the
components Make sure you take a note of the
symbol for VOLTMETER
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Pupil Activity (Voltage Drops)
Task Measure the voltage drop over the 2 bulbs.
Enter your findings into a table.
Bulb No. Voltage (v)
1
2
9V
17
Pupil Activity (Voltage Drops)
Task Measure the voltage drop over the 2 bulbs
and resistor. Enter your findings into a table.
18
Prototype Board
Prototype Board is used to test circuits prior to
manufacturing the circuit in large numbers.
Build a series circuit using 2 resistors of
different values as shown by your teacher. Using
the multimeter, check the voltage drop over each
resistor. Do the results confirm Kirchoffs law?
19
Circuit Simulation
As in Pneumatics, it is possible to simulate
electrical circuits. In this case we will use a
program called Crocodile Technology. Your teacher
will demonstrate the use of Croc Clips to
simulate the circuit shown below..
20
Measuring Current
Current is measured through components or parts
of circuits, as shown in the circuit diagram
opposite. Note that it is necessary to break
the circuit and connect the meter in series with
the components. Take a note of the symbol for an
Ammeter
21
Current measurement
Using circuit simulation, measure the current
flowing through all three components in the LED
circuit.
In a series circuit the current flowing through
all components is the same. Try placing the meter
at different parts of the circuit to prove this.
In parallel circuits the same current does not
always flow through each component ? you will
find out about this later.
22
Measuring Resistance
Connect two resistors in series on a prototype
circuit board and measure the overall resistance.
You should find that Rtotal R1 R2
And the general rule for finding the sum of any
amount of resistors in series is Rtotal R1
R2 R3 Rn
23
OHMS LAW
Ohms law can be used to calculate theoretical
Voltage drops, Current and Resistance in circuits.
Using the triangle shown opposite, we can
rearrange the formula to obtain V or I.
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Ohms Law in Practice
The task is to calculate the resistance of the
lamp.
25
Worked Example

• For the series circuit shown, calculate
• The total resistance (RT)
• The circuit current (IC)
• The potential difference across both resistors
  (V1 and V2)

26
Worked Example
a)
b)
c)
27
Pupil Problems

• For the circuit shown below calculate
• The total resistance of the circuit
• The circuit current

28
Pupil Problems

• For the circuit shown below calculate
• The total resistance
• The circuit current
• The voltage drop across each resistor.
• Use Kirchoffs second law to verify your answers
  to (c).

29
Pupil Problems

• For the circuit shown below calculate
• The total resistance of the circuit
• The circuit current.

30
Pupil Problems
A circuit has three resistors in series. Their
values are 15 R, 24 R and 60 R. Calculate the
total resistance of the circuit.
Two resistors are connected in series. Their
values are 25 R and 75 R. If the voltage drop
across the 25 R resistor is 4 volts, determine
the circuit current and the supply voltage
31
Series Circuits
One of the problems with series circuits is if a
component fails, then the whole circuit fails.
Consider a set of bulbs connected in series.
If one of these bulbs fail, then current cannot
flow through the circuit, hence the remaining
bulbs will fail to light also.
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Parallel Circuits
Parallel circuits are circuits where there is
more than one path for electricity to flow along
or that have more than one branch. Each branch
receives the supply voltage, which means that you
can run a number of devices from one supply
voltage. A good example of a simple parallel
circuit is a set of Christmas-tree lights where
all the bulbs require a 230 volt supply.
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Parallel Circuits Activity
Parallel circuits can be arranged in many ways,
but are normally set out so that you can easily
see the parallel branches. A simple parallel
car-alarm circuit is shown below with the
switches wired up in parallel. Simulate the
circuit shown below, then describe its operation
in your note book.
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Resistors in Parallel
Connect two resistors in parallel on a prototype
circuit board and measure the overall resistance
The formula to calculate the theoretical value of
resistors in parallel is shown below.
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Worked Example
Calculate the resistance of the parallel branch
and the total circuit resistance.
The resistance values are R1 270 R, R2 100 R
and for the buzzer 240 R.
36
Pupil Activity (Parallel Circuits)
Task Build the circuit, Measure the voltage over
each of the bulbs. Enter your findings into a
table.
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Current in Parallel Circuits

• There are two important points to remember about
  resistors in parallel.
• The voltage drop across each resistor is the
  same.
• The sum of the currents through each resistor is
  equal
• to the current flowing from the voltage
  source.

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Worked Example
The resistance values are R1 270 R, R2 100 R
and for the buzzer 240 R.
Your teacher will work through this problem on
the white board.
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Pupil Problems
For the circuit shown below calculate (a) The
total resistance of the circuit (b) The circuit
current.
40
Pupil Problems
For the circuit shown below calculate (a) the
total resistance of the circuit (b) the circuit
current (c) the current flowing though R1 (10
R) (d) the current flowing through R2 (24 R).
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Pupil Problems
For the circuit shown below calculate (a) the
total resistance of the circuit (b) the circuit
current (c) the current flowing through R1 (660
R). (d) the current flowing through R2 (470 R).
42
Pupil Problems
A 6 R resistor and a 75 R resistor are connected
in parallel across a voltage supply of 12 V.
Calculate the circuit current.
A 440 R resistor is connected in parallel with a
330 R resistor. The current through the 440 R
resistor is 300 mA. Find the current through the
330 R resistor
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Combined Series Parallel
Consider the combined series and parallel circuit
shown in the figure below. You can see that R2
and R3 are connected in parallel and that R1 is
connected in series with the parallel
combination.
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Combined Series Parallel

• Some points to remember when you are dealing with
  combined series and parallel circuits are
• The voltage drop across R2 is the same as the
  voltage drop across
• R3
• The current through R2 added to the current
  through R3 is the
• same as the current through R1
• The voltage drop across R1 added to the voltage
  drop across R2
• (which is the same as across R3) would equal
  the supply voltage
• Vs.

45
Worked Example 2

• For the combined series and parallel circuit
  shown, calculate
• The total circuit resistance (RT)
• The circuit current (IC)
• The voltage drop across resistor R1 (VR1)
• The current through resistor R2 (I2).

46
Pupil Problems

• For the circuit shown calculate
• The resistance of the parallel combination
• The total circuit resistance.
• The branch currents

47
Pupil Problems

• For the circuit shown calculate
• The total resistance
• The circuit current
• The branch current
• The voltage drop across each resistor.

48
Pupil Problems

• For the circuit shown calculate
• The total resistance of the circuit
• The circuit current
• The current through each resistor
• The voltage drop across each resistor.

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Power in Circuits

• Electrical power is measured in watts (W).
• Electrical power can be converted into other
  forms of
• power using electric circuits. For example
  the power used
• in overcoming electrical resistance can be
  converted into
• heat this is the principle of an electric
  fire.
• The power in an electric circuit depends both on
  the
• amount of current (I) flowing and the voltage
  (V) applied.
• The formula for power in electric circuits is
• Power Voltage x Current (watts)
• P V x I (W)

50
Worked Example
An electric household lamp consumes 60 watts
from a 240 volt supply. Calculate the current
drawn by the lamp and the resistance of the lamp.
51
Pupil Problems

• In the following simplified circuit for a vacuum
  cleaner motor, calculate
• The power consumption of the motor
• The voltage of the lamp
• The total power drawn from the power supply.

52
Pupil Problems

• The torch circuit below is supplied with two 4.5
  volt batteries connected in series, with the
  current being 20 mA.
• Determine
• The resistance of the bulb
• The voltage across the bulb
• The total power drawn from the
• supply
• The power drawn by the bulb.

53
Pupil Problems

• An electric iron rated at 800 W is connected to
  a 230 V supply.
• Calculate the maximum current drawn by the
  iron. What is the
• power used by the iron at half- heat setting?
• A kettle and a toaster use the same double
  socket. If the
• kettle draws a current of 10 A and the
  toaster 3 A, find the
• power used by each of the appliances. The two
  sockets are
• wired in parallel to a 230 V supply.
• An electric drill draws a current of 1.5 amps
  from a 110 volt
• supply. Calculate the power rating of the
  drill.

54
Pupil Problems

• An emergency power generator has to drive 80
  lamps. Each
• lamp takes 60 W at 230 V. Calculate the
  current through each
• bulb if
• They are connected in series.
• They are connected in parallel.
• How many 150 W lamps can be connected in
  parallel to a 250 V supply through a 5 A fuse?

55
Pupil Problems
The rear screen heater in a car is connected to
the 12 V system and draws a current of 2 A. Find
the resistance of the circuit. In reality the 12
V, 0.5 A interior light is on the same circuit.
State whether this is a parallel or series
circuit and calculate the power and current when
both lamp and heater are on.
56
Voltage Dividers
Input transducers are devices that convert a
change in physical conditions (for example,
temperature) into a change in resistance and/or
voltage. This can then be processed in an
electrical network based on a voltage divider
circuit.
57
Voltage Dividers Activity
Build a voltage divider circuit using any 2
values of resistor. Using the multimeter measure
the voltage drop over R2. This voltage is known
as Vo or the output voltage from the divider.
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Voltage Dividers Activity
Measure the resistance of the 2 resistors from
the last activity. Enter the values into the
formula below and calculate Vo. Simulate the
circuit using croc clips and measure
Vo. Hopefully! The value of Vo should be the
same in all three cases, (within reason).
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Worked Example
60
Pupil Problems
Calculate Vo in the following exercises
61
Pupil Problems
Calculate Vo in the following exercises
62
Switches

• Switches are useful input devices
  (transducers).
• There are several ways in which the contacts in
  mechanical switches can be operated. Such as
  push button, key, slide, toggle, magnetic (reed)
  and tilt.
• These switches are digital input devices as
  they can only be on or off.
• The contacts on a switch can be NO or NC
  (normally open, normally closed)

63
Switch Contacts
Types of switch contacts SPST (Single Pole
Single Throw) SPDT (Single Pole Double Throw)
64
Pupil Activity
Copy the circuit into your note book. Simulate
the circuit using Croc Clips, then describe in
your own words how the circuit operates.
65
Analogue Transducers

• A thermistor is a device whose resistance varies
  with
• temperature. It is a temperature-dependent
  resistor. There
• are two main types.
• Negative temperature coefficient (?t or NTC)
  where
• resistance decreases as temperature
  increases.
• Positive temperature coefficient (t or PTC)
  where
• resistance increases as temperature increases.
• The circuit symbols for and typical
  characteristics of the two types of resistor are
  shown on the next slide.

66
Thermistor
NTC is the most common thermistor
67
Data Charts
In your Data book you should find a graph which
describes how the resistance of a thermistor
changes with temperature. Your teacher will work
through the use of the chart.
68
Strain Gauges
Strain gauges are really load sensors. They
consist of a length of resistance wire and when
stretched their resistance changes. Strain gauges
are attached to structural members (beams, etc.)
and as they are loaded, a reading on a voltmeter
can be obtained.
69
Light Dependent Resistor
The LDR (sometimes called a photoresistor) is a
component whose resistance depends on the amount
of light falling on it. Its resistance changes
with light level. In bright light its resistance
is low (usually around 1 K). In darkness its
resistance is high (usually around 1 M).
70
Pupil Activity

• Use your Data Book to find the resistance of an
  ORP12 LDR for the following light conditions
• 10 Lux
• 40 Lux
• 100 Lux

71
Pupil Activity

1. Copy the circuit shown below into your note book.
2. Using the Electrical Modular Boards, construct
   the voltage divider circuit.
3. Using a multimeter measure Vo.
4. Warm the thermistor up with your fingers and re
   measure Vo.
5. Describe the operation of the voltage divider.
6. Reverse the position of the thermistor and
   resistor. Repeat 3,4 5.

72
Pupil Activity

• Copy the circuit shown below
• into your note book.
• 2) Using the Electrical Modular
• Boards, construct the voltage
• divider circuit.
• 3) Using a multimeter measure Vo.
• Cover the LDR up with your
• hand and re measure Vo.
• 4) Describe the operation of the
• voltage divider.
• 5) Reverse the position of the
• LDR and resistor. Repeat 3,4
• 5. Describe what is happening.

73
Pupil Activity
A potentiometer configured as a variable resistor
can be used in a circuit as a voltage or current
control device. They are often used in voltage
divider circuits to adjust the sensitivity of the
input.
Build a voltage divider using a potentiometer.
Check its operation by measuring Vo from the
voltage divider.
74
Potentiometers
Some more examples of potentiometers.
75
Voltage Divider Sensitivity
With an analogue sensor it is normally desirable
to adjust the sensitivity of the circuit. Rather
than using a fixed resistor we can replace it
with a variable resistor (or potentiometer). This
allows us to fine tune the sensitivity of the
voltage divider.
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Pupil Activity
To save money and inconvenience the residents
want the outside light to come on when it gets
dark. They also want to be able to adjust the
sensitivity from summer to winter nights. Build
the following circuit using modular circuit
boards.
Adjust the variable resistor so as Vo goes higher
when your hand is about 100mm above the LDR
77
Pupil Problems
Calculate the voltages that would appear across
each of the resistors marked X in the circuits
below.
6v
0v
78
Pupil Problems
In each of the following voltage divider
circuits determine the unknown quantity.
79
Pupil Problems
In each of the following voltage divider circuits
determine the unknown quantity.
80
Pupil Problems
An NTC (negative temperature coefficient)
thermistor is used in a voltage divider circuit
as shown below. Using information from the graph
shown, determine the resistance of the thermistor
and hence calculate the voltage that would appear
across it when it is at a temperature of 80?C
OR 20?C.
81
Pupil Problems
What would happen to the voltage across the
thermistor in the circuit shown previously as
the temperature increased? What would happen
to the voltage across the resistor in the
circuit shown previously as the temperature
increased?
82
Pupil Problems
A thermistor (type 5) is used in a voltage
divider circuit as shown below. The
characteristics of the thermistor are shown in
your Data Book. If the voltage V2 is to be 4.5 V
at 100 ?C, determine a suitable value for R1.
State whether V2 will increase or decrease as the
temperature drops. Explain your answer
83
Voltage Dividers
We have seen that Voltage Dividers, divide the
voltage depending on the value of resistors used.
In addition, if we include a variable resistor,
we can alter the sensitivity of the voltage
divider. If we include a thermistor, we can
measure changes in temperature. If we include a
LDR, we can measure changes in light levels. If
we include a potentiometer, we can measure
changes in position.
84
Transistors
The transistor is a semiconductor device. This
means that it is sometimes a good conductor of
electricity and sometimes a poor one. A
transistor is made up of three layers of
semiconductor materials that are either n type
or p type. There are two types of bipolar
transistor available pnp or npn.
Transistors come in many variations and sizes.
However, they all are reliable, efficient, small
and relatively cheap.
85
Transistors

• A transistor is an electronic switch
• Transistors amplify current which enables
  them to drive heavy loads such as motors
• A voltage of 0.7V will switch on a NPN
  transistor

Collector
Base
Emitter
NPN Bipolar Transistor
86
Transistors Activity
Simulate the circuit using Croc Clips. Fill in
the table shown below.
Base resistor value (K) Base/Emitter Voltage (mV) Base current (?A) Lamp on/off
2200
1000
470
220
100
47
33
22
10
1
87
Transistors

• Your teacher will work through a number of
  problems which will look at calculating
• Base current
• Voltage drop over R
• Base Emitter voltage

M
R
Vin
88
Transistors Activity
5V (B)
5V (A)

• Build the following transistor circuit using
  modular boards.
• Adjust the voltage reaching the transistor base
  by altering the value the potentiometer.
• At what voltage does the transistor switch on?
• Measure the current flowing to the base.
• Now measure the current flowing in the collector
  leg.
• What is the transistor doing?

10K
Buzzer
1k
89
Relays
Although relays are often considered to be output
devices, they are really output switches from
electric or electronic circuits.
When an electric current flows into the relay
coil, the coil becomes an electromagnet. This
electromagnet attracts the armature and moves the
contacts. This movement provides the switching,
just as the contacts in any other switch do.
90
Relays
The relay is a very useful device because it is
the vital link between microelectronics and
high-energy systems that require substantial
amounts of current. The relay is perhaps the most
commonly used switch for driving devices that
demand large currents.
91
Relays Protection Diode
As seen earlier, relays have a coil that is
energised and de-energised as the relay switches
on and off. During this process of switching, the
coil can generate a large reverse voltage (called
a back e.m.f.). This reverse voltage can cause
considerable damage to components, especially
transistors. The transistors and other sensitive
components can be protected by the inclusion of a
diode that provides a path for the current caused
by the reverse voltage to escape.
92
Solenoid
A solenoid is another output transducer that has
a coil inside. Circuits containing a solenoid
require a protective diode as well.
93
Pupil Activity - Relay

• Build the following circuit using modular
  boards
• Change the resistance of the thermistor by
  heating it up with your hand
• Listen carefully, you should hear the relay
  contacts opening and closing

Now add a bulb and 5V power supply to the output
side of the relay and heat the thermistor up
again, (your teacher will demonstrate the correct
connections). The bulb should light when the
thermistor is hot. It might be necessary to
replace the 10K with a potentiometer.
94
DPDT Relay
As electric motors normally draw larger
currents, relays are ideal devices for such
circuits. By using DTDP switching, relays can
control the direction of rotation of motors.

• Simulate a sensing circuit using an LDR in a
  voltage divider
• Add a transistor driving circuit and a DPDT
  relay
• Connect the relay up so as the motor drives
  clockwise and anticlockwise
  depending on the amount of light hitting the LDR

95
Motor Reversal Circuit
96
Capacitors
Capacitors are electronic components that store
electricity for short periods of time within
electronic circuits or networks.
Electrolytic capacitors are polarity conscious.
This means that they must be connected the right
way round. The negative lead must be connected
to zero volts with the positive terminal towards
the higher voltage side of the circuit.
97
Capacitors
Polyester capacitors are for small-value uses
and can be connected without regard to
polarity. Capacitance in measured in farads, but
because this is a very large measurement most
capacitors are rated in ?F (microfarads) or in nF
(nanofarads).
98
Pupil Activity
9V

• Construct the following circuit
• Allow the capacitor to charge up
• Connect the end of the LED to 0V
• The LED should light up for a short period of
  time

10K
100uF
0V
99
555 Integrated Circuit
An integrated circuit (or IC) is simply an
electronic package that contains a number of
components on a silicon chip. The 555-timer IC
that you are going to use is a very versatile
chip that has many applications.
100
Pupil Activity - Monostable
Simulate the circuit shown, press the switch and
observe the circuit operation. Monostable means
one stable state. The light comes on but always
goes back to its original state.
101
ASTABLE CIRCUIT
102
Pupil Activity - Astable
Build the following circuit on Breadboard,
describe its operation.