Electrical Circuits / Electronics

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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.

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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 (?).
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Batteries Voltage Supplies
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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.
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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
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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.
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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

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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 through
it. LEDs are used as indicators to tell when a
circuit (or part of 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.
LEDs use less energy than bulbs, hence the
reason we see their use in torches now.
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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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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)

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Switch Contacts
Types of switch contacts SPST (Single Pole
Single Throw) SPDT (Single Pole Double Throw)
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Switch Contact Use
DPDT
SPST
SPDT
DPST
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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 simulate the
circuit using. Add voltmeters / Ammeters and
measure the voltage drop over each component.
How would you write up a test plan and results
for this circuit?
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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
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
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Pupil Activity (Voltage Drops)
Task Measure the voltage drop over the 2 bulbs
and resistor. Enter your findings into a table.
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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?
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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..
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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
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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.
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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
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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.
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Worked Example

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

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Worked Example
a)
b)
c)
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Pupil Problems

• For the circuit shown below calculate
• The total resistance of the circuit
• The circuit current
• The voltage drops over the resistors

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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).

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Pupil Problems

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

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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
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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.
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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 branches
and circuit current.
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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 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).
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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.

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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).

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Pupil Problems

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

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Pupil Problems

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

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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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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
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Pupil Problems
Calculate Vo in the following exercises
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Pupil Problems
Calculate Vo in the following exercises
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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)
• OR V2/R

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Data Charts
You must be able to extract data from a graph.
There are 2 types you will meet, Light Dependant
Resistor and a Thermistor. Your teacher will
work through the use of the chart.
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Pupil Activity

1. Copy the circuit shown below into your note book.
2. Using the Yenka software, construct the voltage
   divider circuit.
3. Using a multimeter measure Vo.
4. Warm the thermistor up with the slide 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.

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Pupil Activity

• Copy the circuit shown below
• into your note book.
• 2) Using the Yenka, construct the voltage divider
  circuit.
• 3) Using a voltmeter measure Vo.
• Change the LDR with the slide 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.

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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.
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Potentiometers
Some more examples of potentiometers.
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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 Problems
Calculate the voltages that would appear across
each of the resistors marked X in the circuits
below.
6v
0v
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Pupil Problems
In each of the following voltage divider
circuits determine the unknown quantity.
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Pupil Problems
What would happen to the voltage across the
thermistor as the temperature
increased? What would happen to the voltage
across the resistor in the circuit as the
temperature increased?
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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.
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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.
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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
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Transistors Activity
5V (B)
5V (A)

• Build the following transistor circuit using
  Yenka.
• 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
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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.
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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.
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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.
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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

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Motor Reversal Circuit
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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.
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Pupil Activity
9V

• Simulate 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