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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 1 2/17/2011 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) 2 2/17/2011 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 3 2/17/2011 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 4 2/17/2011 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 5 2/17/2011 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. 6 2/17/2011 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 7 2/17/2011 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 8 2/17/2011 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 9 2/17/2011 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 10 2/17/2011 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 11 2/17/2011 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 12 2/17/2011 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 13 2/17/2011 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. 14 2/17/2011 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 15 2/17/2011 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. 16