Download File - St Mary`s Technology

2/17/2011
Electronics and Electricity
Electronic and Electricity
Overview/ Introduction Video
– Sources of Energy
– The terminology and basic principles of Electricity
– Ohm’s Law
– Series and Parallel circuits
– The functions/ limitations /symbols of electrical
components
– Design/Experimenting/ Building and
Troubleshooting Electrical circuits
Main Points from the Video
• Electricity - is the movement of charged
atomic particles called electrons
• The Battery – Creates the force that
moves the electrons
• Electrons – have a negative (-) charge and
for this reason they travel from the
negative terminal to the positive terminal
of the battery
Main Points from the Video
• A Conductor – provides the path for the
electrons to travels on
• Flowing Electrons = Current (measured
in Amperes)
• The force moving electrons = Voltage
(Potential Difference)
• A circuit is the path that current flows in
• A Light bulb will reduce the current in
the circuit
Energy
• Energy is the ability to do work
• Electrical energy is one of many types of
energy
• Others include:
•Wind
•Wave
•Solar
•Mechanical
•Nuclear
•Heat
•Sound
•Chemical
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Sources of Energy
Sources of Energy
• Energy sources are divided up into two
types:
– Renewable
– Non Renewable.
• Non renewable sources or fossil fuels as
they are often known include peat, coal and
oil will run out as there is only a limited
supply.
• Renewable sources like hydro electric
power, wind and wave power will not run
out.
Principle of the Conservation of
Energy
• Energy cannot be created or destroyed
but can only be transferred from one
form to another.
Current
• Current – is the
movement of
electrons through a
conductor
• Current - is measured
in Amps (I)
• An Ampere or Amp –
is the measure of the
amount of electric
charge(electrons)
passing a point in a
circuit
Conductors and Insulators
• Conductor – a material that conducts
electricity. Examples include copper, steel
and water.
• Insulator – a material that does not or is
a poor conductor of electricity. Examples
include plastic, rubber or wood.
Voltage or Potential Difference
Current will only
flow when the
circuit is
complete
• Voltage or Potential Difference – is the
power to drive electrons around a circuit,
and it is measured in volts.
• Voltage is created by the battery
• Electrons leave the negative side and
they are then attracted to the positive
side
• The higher the voltage the more
electrons that will flow (higher current)
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Resistance
• Resistance = opposition to the flow of
current in a circuit
– Wayne Rooney (current) vs. Richard Dunne
(resistance)
• All electrical components have a certain
amount of resistance
• High Resistance = Low Current
• Low Resistance = High Current
• Resistance is measure in Ohm’s Ω
Ohm’s Law
• V = voltage (volts)
• I = Current (amps)
• R = Resistance (Ω)
V=IxR
I = V/R
R = V/I
Cover they quantity
in which you want to
calculate
Series and Parallel Circuits
Main Points from the Video
• When the light bulbs are in series the
current must flow through all of the
light bulbs
• When connected in series the bulbs
barely glow as the voltage is shared
across the four light bulbs
• In series if one bulb is disconnected it
will break the circuit
Main Points from the Video
• When the four bulbs are connected in
parallel all light bulbs shine brightly
• If one bulb is disconnected the others
will shine brightly
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Series and Parallel Circuits
Series and Parallel Circuits
• In a Series circuit, components are
connected one after the other and
there is only one path for the current
to flow around the circuit.
• In a Parallel circuit the components
are connected side by side and this
forces the current to take more than
one path.
Series Circuits
• The voltage in a Series circuit is shared
by each of the components and how it is
shared depends on the resistance of
each of the component.
• If two components have equal
resistance the voltage will be shared
evenly.
Parallel Circuits
Parallel Circuits
• In a parallel circuit the current is shared
between the different paths.
• The amount of current in each path is
dependent on the total resistance of the
path.
• The current in each path will add up to
the total current of the circuit.
• It is important to remember that there
will be a greater flow of current in the
path of least resistance.
Parallel Circuits
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Parallel Circuits
Parallel Circuits
• Whilst current is divided up in each
path on the circuit voltage is not, the
voltage in each path will remain the
equal to the voltage that is applied to
the circuit (i.e. the battery voltage).
• The number of components or the
amount of resistance will make no
difference to the voltage
Resistors
Resistors
• Although all compounds have some
resistance to the flow of current, resistors
are electrical components designed
specifically to reduce the flow of current in
a circuit.
• Resistors are used of a number of reasons:
– To protect components from being damaged
by too much current
– To create a potential divider circuit i.e. to direct
current normal in to a transistor.
– To create a time delay with capacitors
Fixed Resistors
Resistor - Potential Divider Circuit
• The most common type of resistor is the
fixed resistor.
• Fixed Resistors have coloured band printed
on to them to show the resistance
• Each colour represents a number
• When calculating the resistance the
coloured bands are read from left to right
with the tolerance band on the right hand
side.
Fixed Resistor Symbol
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Fixed Resistors
Fixed Resistors
Fixed Resistor -Tolerance
Fixed Resistor –Tolerance Example
• Resistors are not one hundred percent
accurate.
• The tolerance band indicates the
percentage accuracy of the resistor.
Variable Resistor
• A variable resistor can change the
resistance between two points in a circuit
by rotating a spindle.
• By rotating the spindle the resistance can
be increased and decreased in the circuit.
• A variable resistor has three pins,
however only two pins must be
connected, the middle pin and either of
the outer pins.
• If a 330Ω resistor has a gold tolerance
band. The operating value of the
resistor will be anywhere between 330
Ω ± 5%. The maximum value will be 330
+ 5% and the minimum value will be
330 – 5%.
• 5% of 330 =_____________________
• Maximum value of resistor =
• Minimum value of resistor =
Light Dependent Resistor (LDR)
• The resistance of an LDR varies depend of the
level of light falling on it.
• Darkness = High Resistance
• Light = Low Resistance
• LDR are used for creating light and dark sensor
circuits.
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Thermistor
Thermistor
• The resistance of a thermistor increases and
decreases with temperature change.
• A wide range of themistor are avaiable but
the most common is the NTC (Negative
temperature coefficient) in which the
resistance decreases with temperature
increases, which make them suitable for fire
alarms.
• Thermistors are generally used in hot and
cold sensor circuits.
Diode
Diodes and LED’s
LED – Light Emitting Diode
• LED’s give off light
when current flows
through them
• LED’s must always
be connected in
series with a resistor
as they are damaged
by high voltage
• The anode(+) is
recognisable by the
longer leg
• A diode allows
electricity to flow in
one direction only
and blocks the flow
in the opposite
direction
• Silicon Diode are the
most common type
and they have a
band marking the
positive terminal
+
-
Bulb
• Give off light
when current
flows through it
• Bulbs have no
polarity thus they
can be connected
anyway around
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Motor
• Motors translate
current into circular
motion
• Motors can be
connect either way
around, but the way
they are connected
will determine if the
rotate clockwise or
anti clockwise
Buzzer
• A buzzer gives off
continuous sound
when current flows
through it
• They have polarity
which is indicated by
the colour of the
wires
+
-
Capacitors and Capacitance
Capacitors and Capacitance
Capacitors
• The amount of time that a capacitor takes
to charge up and discharge can be used for
timing circuits
• Capacitors are also used to smoothen out
current
• Capacitors are used to store an electrical charge.
• When power is supplied to a circuit that includes
a capacitor, the capacitor charges up.
• When power is turned off the capacitor
discharges its electrical charge slowly.
• Capacitance refers to how much charge a
capacitor can hold, and it is measured in Farads
(F).
• One farad is a very large amount of capacitance
therefore capacitors are found in µF (µ = 10^-6).
Capacitors and Capacitance
There are two types of capacitors
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Switches
Switches
Switches
Switches
• There are many different types and
classifications of switch available
• Switches are used to control the flow of
electricity to a circuit
• A switch allows you to turn a circuit/s on
or off
Common Switch Symbols
Poles and Throws
• A Pole is the moving contact in a switch
• A Throw is the stationary contact in a switch
Pole
Throw
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Single Pole Single Throw Switch
Double Pole Single Throw
• An SPST switch has one single moving contact
and one stationary contact
Double
Pole
1 Pole
SPST Symbol
1 Throw
1 Contact Position
= 1 Throw
Single Pole Double Throw Switch
SPDT
DPST
• A DPST switch is used to turn on/off two
different circuits at the same time
• Example: The Hair Drier
1 Pole
2 Throws
– A hair drier has two circuit
• The Heating Element
• The Fan
– Both circuits must work together
DPST symbol
Single Pole Double Throw Switch
SPDT
• This type of switch can be used to control
two different circuits. When one circuit is
on the other is off.
• The two circuits can never be on or off at
the same time.
SPDT Applications
• High and Low beam Head lights in a car
• When the high beams are on the low beams
are off and when visa versa
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Double Pole Double Throw (DPDT)
x
• A DPDT switch consists of two SPDT
switches connected together
mechanically.
• It could be used to control four devices,
turning two circuits on and off at the
same time.
DPDT Motor Circuit
Double Pole Double Throw (DPDT)
• A much more common use for the DPDT
switch is to control the forward and reverse
motion of a motor
DPDT Motor Circuit
DPDT
Slide Switch this
direction for
clockwise
motion
Slide Switch
this direction
for Anticlockwise
motion
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Limit or Micro Switches
• This type of switch is turned on and off by small
amounts of movement.
• These are generally used to control motor circuits.
• They have 3 terminals or pins:
– COM –this terminal is always connected to the
terminal
– NO – this is connected when you want the switch to be
a push to make switch, in which the switch is off but
when pressed it goes on.
– NC- this is connected when you want the switch to act
as a push to break switch, in which the circuit is always
connected to the power but when the switch is
pressed it is disconnected from the power.`
The Transistor
• The transistor has two
functions:
1. To act as an electronic
switch
2. To amplify current
• A transistor has 3 pins:
– Base – which activates the
transistor
– Collector – which is the
positive lead
– Emitter – Which is the
negative lead
Potential Dividers
Potential Dividers
• Potential Dividers are used to split the
voltage of a circuit.
• They are widely used in technology to get
the right voltage at the base of a transistor
Potential Dividers
How a Potential Divider Works
• A voltage divider consists of two resistors in
series so that the applied voltage is divided
between the two resistors to produce the
required base voltage.
• A Potential Divider
works on the principle
that current will
always follow the path
of least resistance:
High resistance =
Higher voltage
between the base
and emitter
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Circuits
Circuits and Designing Circuits
Designing Circuits
1. Task
2.Input (sensor)
3.Processes
4.Output
Task
Input/
Sensor
Process
Output
Heat
Sensor with a
Fan
Potential Divider
(1 variable
resistor and a
Thermistor)
Transistor
Relay and Motor
Dark
Sensor with a
light
Potential Divider
(1 variable
resistor and a
LDR)
Transistor
LED / Light Bulb
Moisture
Sensor with a
Heating element
Potential Divider Transistor
(1 variable
resistor and
moisture probes)
• Circuits are explained using circuit diagram
• Each component has a unique symbol that is
used all round the world
Designing Circuit
• Task – this is what you want your circuit to do
or achieve
• Input or Sensor - responds to a change in the
surrounding (light, motion, temperature etc)
• Processes – reacts to a change and triggers an
output
• Output Devices – signal or react to a change
that has been picked up in the input stage
(light/LED, buzzer, motor etc...)
Automatic Light Sensing Circuit
• Using an LDR as part of the voltage divider will
allow the output to be controlled by changing
light levels.
• Remember the resistance of an LDR varies
from 400Ω in light to about 10MΩ in darkness.
Heating Element
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Automatic Light Sensing Circuit
Input
Potential Divider
(LDR and Variable
Resistor)
Output
LED
Process
Transistor
Automatic Light Sensing Circuit
Automatic Dark Sensing Circuit
• The LDR has low resistance
when bright which allows
current to flow
• The other resistor is a variable
resistor (set to high resistance)
• Current follows the path of
least resistance i.e.. through
the transistor
• In darkness no current will flow
in the potential divider due to
high resistance, this means
there is no voltage between
the base and the collector and
no output
Automatic Dark Sensing Circuit
• By switching the LDR and
the Variable resistor you
will create a Dark Sensing
Circuit
• When Bright , LDR has
low resistance
• When dark, the LDR has
high resistance
• This means current will
flow through the
transistor when it is dark
only as this will be the
path of least resistance
Variable Resistor V.s Fixed Resistor
• A variable resistor is
used as opposed to the
a fixed resistor as this
allow the sensitivity of
the light/ dark sensor to
be controlled
• In other words it allows
you to set what level of
brightness or darkness
the out put will be
triggered
Hot/ Cold & Wet Dry Sensor Circuit
• The hot / cold or wet/dry sensor circuit are
almost identical in appearance and operation
to the light / dark sensor, the only difference
is the LDR is replaced thermistor a set of
moisture probes.
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Automatic Temperature Sensing
Circuit
Low Temperature Sensing Circuit
• Replacing the LDR with a thermistor will
give a temperature sensing circuit.
• The resistance of NTC thermistors
decreases as the temperature rises.
• The circuit below sound an alarm at low
temperatures(output).
Thermistor
High Temperature Sensing Circuit
Changing the position of the variable resistor
and thermistor will sound an alarm at high
temperatures.
Automatic Moisture/ Dry Sensing
Circuit
Moisture
Probes
Automatic Moisture/Dry Sensing
Circuit
• Replacing the LDR with a set of moisture
probes will create a moisture/dry sensing
circuit.
• Moisture probes use the liquid as a
conductor
• This mean when liquid is present there is
low resistance and when there is no liquid
present their is high resistance between
the probes
Darlington Pair
• A Darlington pair consists of two
transistor connected together.
• In this configuration the emitter of the
first transistor is connected to the base
of the second transistor.
Changing the position of the moisture
probes and the variable resistor will
change it from a moisture to a dry sensor
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Darlington Pair
• This configuration give a much greater gain
then a single transistor and it id used when
the base voltage is very small.
• In addition due to the amplification this
configuration is very sensitive to change and
even the smallest change in light levels for
example will give an output.
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