1. Resistors Types of resistors

2. 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…

3. 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.

4. 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

5. 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 R V I

6. 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

7. Resistance of Conductors Specific Resistance of metals (ρ) COPPER … 0.0178 Ωm ALLUMINIUM …0.0290 Ωm STEEL …0.1300 Ωm COSTANTAN ALLOY0.5000 Ωm –ρ = R x A / L ( Ω x m x m / m) –ρ …( Ω x m)

8. Resistance of Conductors WIRE RESISTANCE EXAMPLE Diameter =.5 mm Length= 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)

9. Resistance of Conductors Variations with Temperature Resistance of metal conductors increases with the temperature increase. –R h = R c + ΔR –R h = R c ( 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

10. 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

11. 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.

12. Carbon Resistors Wattage vs. Size The wattage increases with the size of a Carbon resistors 1/8 watt 0.145 inch long 1/4 watt 0.250 inch long 1/2 watt 0.375 inch long 1.0 watt 0.562 inch long 2.0 watt 0.688 inch long