What exactly does each part of the circuit do? Measures current Circuit control Source of Electrical Supply Circuit protection Measures electrical pressure Circuit load Conductor

What exactly does each part of the circuit do? Measures current Circuit control Source of Electrical Supply Circuit protection Measures electrical pressure Circuit load Conductor

What exactly does each part of the circuit do? Measures current Circuit control Source of Electrical Supply Circuit protection Measures electrical pressure Circuit load Conductor

What exactly does each part of the circuit do? Measures current Circuit control Source of Electrical Supply Circuit protection Measures electrical pressure Circuit load Conductor

What exactly does each part of the circuit do? Measures current Circuit control Source of Electrical Supply Circuit protection Measures electrical pressure Circuit load Conductor

What exactly does each part of the circuit do? Measures current Circuit control Source of Electrical Supply Circuit protection Measures electrical pressure Circuit load Conductor

What exactly does each part of the circuit do? Measures current Circuit control Source of Electrical Supply Circuit protection Measures electrical pressure Circuit load Conductor

What is: Open Circuit Closed Circuit Short Circuit

How to decode a resistor.

What is a resistor? All components of an electrical circuit have resistance (including the conductors). A resistor is a component that is specifically designed to oppose current flow. Resistors are generally made from alloys of metal or carbon based compounds A few are made from semi-conductor type material (like silicon)

Linear resistors – wire wound

Linear resistors – carbon compound

Power ratings of res The power rating of a resistor is the amount of heat (I2R – in watts) that a resistor can dissipate before the losses are too great or the resistor is destroyed. For large power ratings resistance wire is used whereas for low (and physically small) power ratings carbon based resistors are used.

Tolerance and preferred values of resistance To make manufacturing cheaper a series of standard values are manufactured. These vary for the three main tolerance ranges. Tolerance is the degree of accuracy that the resistor had been manufactured to. For example a 10Ω resistor with a silver tolerance band has a tolerance of ±10% and can range from 9Ω to 11Ω. A variable or “tuning” resistor can be placed in a circuit to compensate for the difference.

Non-linear resistors - Thermistors Temperature sensitive resistors 2 types Positive temperature coefficient (PTC) Negative temperature coefficient (NTC) PTC – resistance increases with an increase of temperature. Made from metallic oxide or silicon. NTC – resistance decreases with a decrease of temperature. Made from chromium or nickel with small amounts of semi-conductor material.

Non-linear resistors – Voltage dependant resistors (VDRs) Resistance varies as the voltage varies Used for lightning protection where under normal circumstances the do not conduct but when the voltage reaches a certain level the resistance drops. Uses also include over voltage protection and signal compensation.

Non-linear resistors – Light dependant resistors (LDRs) Resistance varies as the light varies Usually made from cadmium sulphide mounted on a plate and covered with a vacuum-deposited metallic grid

Resistor circuit symbols Resistor (general) Variable resistor (rheostat) Variable resistor (potentionometer) Adjustable resistor PTC thermistor NTC thermistor Voltage dependant resistor Light dependant resistor + to C - to C

Remembering the resistor colour code Black Beer Rots Our Young Guts But Vodka Goes Well Black is NO color - nothing, zero Brown, think brow-ONE. Red, think TWO red lips. Orange, think "orange tree" tree rhymes with three. Yellow, think Yell FOUR help. Green, think of a green FIVE dollar bill Blue, think "sick (six) and blue" Violet, think "violet clouds in heaven, rhymes with SEVEN. Grey, thing Gray-EIGHT (great) White, think of white wine, rhymes with NINE

How does length affect resistance? Longer conductor =

How does length affect resistance? Longer conductor = more resistance Shorter conductor =

How does length affect resistance? Longer conductor = more resistance Shorter conductor = less resistance

How does length affect resistance? Longer conductor = more resistance Shorter conductor = less resistance Resistance α Length

Conductor cross sectional area

How does c.s.a. affect resistance? Larger conductor =

How does c.s.a. affect resistance? Larger conductor = less resistance Smaller conductor =

How does c.s.a. affect resistance? Larger conductor = less resistance Smaller conductor = more resistance

How does c.s.a. affect resistance? Larger conductor = less resistance Smaller conductor = more resistance Resistance α 1 c.s.a

How does the conductor material affect resistance? The measure of how resistive a material is, is called resistivity. 1 m Ω

Sooo… If we put all of them together….. R = ρ x ℓ A Where: R = resistance of conductor in ohms ρ = resistivity of conductors in ohm meters ℓ = length of conductor in meters A = c.s.a. of conductor in meters2

How does temperature affect resistance? When current flows through a conductor it heats up The amount of change in resistance is known as the temperature coefficient of resistance If the resistance goes up – positive temperature coefficient (conductors) If the resistance goes down – negative temperature coefficient (insulators and semiconductors)

Equation for resistance and temperature Rh = Rc [1 + (α x Δt)] Where: Rh = Final (hot) resistance in ohms Rc = Initial (cold) resistance in ohms α = Temperature coefficient of resistance in omhs per ohms per degrees celcius Δt = difference in temperature in degrees celcius

Effects of temperature on a circuit Most electrical components are made with a certain amount of smoke and if they get too hot it all gets let out! Heat increases the resistance in conductors thereby increasing the voltage drop across the cable, also there is a risk of insulation meltdown – resulting in short circuit Motors, transformers, alternators and lamp filaments all start up with a lower resistance – leading to high starting currents.

But wait….. There’s more! R1 = d22 hence R2 = R1 d1 2 R2 d12 d2 R1 = A2 hence R2 = R1 A1 R2 A1 A2 R1 = ℓ1 hence R2 = R1 ℓ2 R2 ℓ2 ℓ1 R1 = d22 hence R2 = R1 d1 2 R2 d12 d2 R2 = R1A1ℓ2 A2ℓ1

Series circuits