ResistanceCompiled and Presented by: Mr. D. Nedrick

What is a resistor?A resistor is a electronic device designed to limit the current that flows in a circuit.Symbol Types of resistors:Carbon composition resistorMetal oxide or metal glaze resistorWire-wound resistorLight Dependent Resistor (LDR)Thermal resistor (thermister)

Carbon resistorThese resistors generally have resistance values from 0.1 to 22M. Its structure consists of a small cylinder of carbon which is mixed with a non-conductor. A connecting wire is fixed into each end, and the resistor is given a coat of paint to protect it from moisture which might alter the resistance.Made in range of stock from 1/8 W to 3W

Metal GlazeLooks rather like the carbon resistor from the outside, but the internal structure is differentCan be made to closer tolerances than carbon resistors, andChange their resistance less with changes in temperatureMade in range of stock from 1/8 W to 3W

Wire-wound ResistorUsed in higher power applicationsThe resistance element of a wire-wound resistor is usually nickel-chromium wire. This wire is wound around a ceramic core. The whole assembly is usually coated with ceramic material or a special enamel High resistance values use wire of low conductivity, requiring many turns of fine gauge wire as well. Have resistance values from 1 to 100k.Power rating range from 1 to 50W in stock sizes. There is no limit in practice

Wirewound Resistor

Kinds of ResistorsResistors value can either be fixed or variable.A fixed resistor has a single value of resistance which remains the same under normal conditions. Variable Resistors: have an adjustable amount of resistance in a circuit. The most common variable resistors are called the potentiometer and the rheostat.

PotentiometerGenerally have carbon composition resistance elements and are commonly used to vary the value of voltage applied to a circuit. Potentiometers are commonly used as control devices in amplifiers, radios, television sets and different kinds of meters. Typical uses include volume, balance, brightness and zeroing adjustments.

RheostatThe rheostat is generally made of resistance wire. They are commonly used to control higher currents, such as those in motor and lamp circuits.

Variable resistors include the potentiometer and rheostat. A potentiometer can be connected as a rheostat.The center terminal is connected to the wiperPreset type

Precision resistors

Some wire-wound and film resistors have actual values that are nearly equal to their rated values. These are called precision resistors. They are use in special circuits such as in test instruments and critical communication devices. Precision resistors usually have a tolerance between 1 and 5%.

Light Dependent Resistor (LDR)LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically.

ThermistorThermistors also called thermal resistorsAre semiconductor devices whose use as transducers is due to the fact that their resistance changes when their temperature increases. its symbol is Thermistors are used for the measurement and control of temperature, being heated either externally or internally by the current they carry.Used as a means of sensing temperature change. Commonly found embedded in motor windings to detect overheating.

Types of thermal resistors

Resistor Colour CodeCarbon resistors used in electronic circuits are too small physically to have their resistance value printed on the body of the resistor. Instead, the resistance can be indicated with the use of colored bands or stripes. The color stripes represents numerical values.

Resistor Color Code

Color ValueColorValueBlack0Green 5Brown1Blue6Red2Violet 7Orange3Grey8Yellow4White9

ExampleResistor Value = 820 10%Tolerance 10% x 820 = 82Upper limit = 820 + 82 = 902Lower limit = 820 82 = 732Tolerance range = 732 to 902

Resistors in Series CircuitA series circuit is a circuit in which the current has only one path. In a series circuit, all of the current passes through each of the components in the circuit.

Total ResistanceRT = R1 + R2 + R3 . . .where RT is the total resistance in the circuit, and R1 through R3 . . . are the resistance ratings of the individual resistors in the circuit.The voltage across each resistor is found by using ohms law V1 = IT x R1; V2 = IT x R2 VT = V1 + V2 + V3 Kirchoffs Voltage law (KVL)

ExampleCalculate RT, IT, VR1, VR2, VR3RT = R1 + R2 + R3 = 4 + 3 + 5 = 12

SolutionIT = VT/RT = 12V/ 12 = 1A

VR1 = IT x R1 VR2 = IT x R2VR3 = IT x R3 = 1A x 4 = 1A x 3 = 1A x 5 = 4V = 3V = 5V

ExerciseCalculate: RT, IT, V1, V2, V3, PT, P1, P2 and P3

Parallel CircuitA parallel circuit is a circuit in which components are arranged so that the path for the current is divided. The voltage across all the components are the same hence:VT = V1 = V2 = V3

Parallel Circuit ContdThe current through each resistor is found using ohms law: I1 = VT/R1; I2 = VT/R2; The sum of the current through each resistor equals the supply current, hence:IT = I1 + I2 + I3Since VT/RT = VT/R1 + VT/R2 + VT/R3Divide both sides by VT we receive

Total ResistanceThe total resistance of the circuit can be calculated using the following formula:Formula: 1/RT = 1/R1 + 1/R2 + 1/R3 or RT = 1 (1/R1 + 1/R2 + 1/R3)Placing resistors in parallel always decreases the total resistance of the circuit.

ExampleFind: RT, IT, I1, I2, I31/RT = 1/6 + 1/18 + 1/9 = 3 + 1 + 2 181/RT = 6/18Hence RT = 18/6 or 3

SolutionIR1 = VT/R1IR2 = VT/ R2 IR3 = VT/R3 = 12V/6 = 12V/18 = 12V/9 = 2A = 0.67A = 1.33A

ExerciseCalculate: RT, IT, I1, I2, I3, PT, P1, P2 and P3

Resistors in Series Parallel CircuitA combination of resistors connected in series and parallel.Follow the current paths to determine if the resistors are connected in series or parallel.Resistors connected in series are connected one after the other; and parallel resistors are connected together at both ends.Series or parallel can be simplified first depending on which is furthest from the source and the circuit itself

Solving Series Parallel Circuits

Simplify the Circuit BelowStart Simplifying from the furthest point from the source

Solving Series Parallel Circuits Contd1/RP = 1/R2 + 1/R31/Rp = 1/6 +1/3 1 + 2 61/RP = 3/6RP = 6/3 = 2

Calculating Total Resistance

RT = R1 + RP = 4 + 2 = 6

Calculating Total Resistance

IT = VT/RT = 12V/6 = 2A

Calculating Voltage DropsVR1 = IT x R1 = 2A x 4 = 8V

VP = IT x RP = 2A x 2 = 4V

Calculating CurrentsI2 = VP /R2 = 4V/3 = 1.33A

I3 = VP /R3 = 4V /6 = 0.67A

Power DissipatedPT = IT2 x RT = 2A2 x 6 = 24W

P1 = V1 x IT = 8V x 2A = 16WP2 = VP2/ R2 = 4V2 /3 = 5.33W

ExerciseCalculate: RT, IT, V2, V6, PTCalculate RT and IT in the event that the 3 resistor be short circuited

ExerciseCalculate: RT, IT, PT, V6 and P2