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Resistors Types of resistors Types of Resistors Constant Value resistors • Carbon Resistors – Higher Values; Low Power – Made of conductive Carbon powder spread on a non-conductive rod (Ceramic). – Two conductive wires are attached to the ceramic rod; one wire at each side. • To facilitate the soldering of the Resistor to another components such as Capacitors, Transistors and etc… Types of Resistors Constant Value resistors • Wound-wire Resistors – Higher Power; low values – Made of Nickel-Chromium alloy wire. – Few turns of the wire is wound around the resistor’s body • The turns have some space between themselves. Types of Resistors Variable Resistors • The value of the resistor is varied by means of either one of the following methods: – Mechanically --- a sliding metallic contact – Optically sensitive --- LDR • The resistance decreases when the Light intensity increases – Thermally sensitive --- The resistance value varies with the temperature changes • PTC – Positive Temperature Coefficient – Resistance increases with temperature increase • NTC – Negative Temperature Coefficient – Resistance decreases with temperature increase Resistance of Conductors OHM’S LOW • The Value of the resistance of a conductor Resistor – equal the Voltage across the resistor divided by the Current passing through it • R = V / I (OHMS) Ω I V R Resistance of Conductors • Resistance increases with L • Resistance decreases with A • Resistance depends on the conductor’s specific resistance • R= L x ρ / A …Ω A L Resistance of Conductors Specific Resistance of metals (ρ) • • • • COPPER … ALLUMINIUM … STEEL … COSTANTAN ALLOY – ρ = R x A / L ( Ω x m x m / m) – ρ …( Ω x m) 0.0178 Ωm 0.0290 Ωm 0.1300 Ωm 0.5000 Ωm Resistance of Conductors WIRE RESISTANCE EXAMPLE • Diameter • Length = .5 mm = 1000 mm • Constantan alloy…ρ = 0.5 Ω.m = 0.0005 Ω.mm – Calculate A = π x d² / 4 … A= 0.20 mm² • R = ρ . L / A = 0.0005 x 1000/ 0.2 • R CONSTANTAN = 2.5 Ω … Similarly, for copper wire • R Copper ( ρ = 0.0178)= 0.09 Ω. (AT 20 °C) Resistance of Conductors Variations with Temperature • Resistance of metal conductors increases with the temperature increase. – Rh = Rc + ΔR – Rh = Rc ( 1 + a ΔT) • ΔT IS THE TEMERATURE INCREASE IN DEGREES • a IS THE Resistance coefficient (Ω / ° C) – Equals the change in the resistance a conductor having a 1 OHM resistance » When the temperature change equals 1 degree centigrade Resistance of Conductors Temperature Coefficients • • • • • • COPPER +0.0038 Ω/°c ALLUMINIUM + 0.004 STEEL - 0.0045 CONSTANTAN alloy -0.000005 GRAPHITE -0.0004 TANGESTIN +0.0041 Resistance of Conductors Temperature Effect Example • In the last wire resistance example R was calculated to be =0.09 Ω for a Copper wire – Let Rc= .09 Ω at 20 degrees C. – At 55 degrees C.; R becomes • With a= 0.0038 for copper – – – – Rh = Rc ( 1 + a ΔT) Rh = .09 ( 1 + .0038 ( 55 – 20)) Rh = .09 ( 1 + .0038 x 35) Rh = .09 ( 1.133) = 0.10 Ω … » nearly 13.3% increase for 175% increase in temperature. » Similarly, for Constantan ALLOY the increase would be equal to 0.02% only. Carbon Resistors Wattage vs. Size • The wattage increases with the size of a Carbon resistors • • • • • 1/8 watt 1/4 watt 1/2 watt 1.0 watt 2.0 watt 0.145 inch long 0.250 inch long 0.375 inch long 0.562 inch long 0.688 inch long