RESISTORS DEFINITIONS AND USE TYPES; MATERIALS, SIZE LABELING AND COLOR CODES VARIABLE RESISTORS LIMITATIONS AND PROBLEMS IN USE Definition • An electrical component that opposes the flow of either direct or alternating current • Employed to protect, operate, or control the circuit • To limit the current • Used in electrical circuits to maintain a constant relation between current flow and voltage: V = IxR 2 http://bkaragoz.kau.edu.sa Example in Current Limiting • In the following circuit enough current must flow to make the LED light up, but not so much that the LED is damaged. The LED turns on at 1.7 V and the maximum voltage it can have is 2 V. Calculate the resistance necessary to have 10 mA through the LED. 3 http://bkaragoz.kau.edu.sa Types of Resistors • Fixed valued – – – – – – Wire wound Carbon composition Carbon film Metal film Metal oxide Semiconductor • Variable – Potentiometer and rheostat – Sensors • Strain gage • Thermistor • Light dependant resistor (LDR) 4 http://bkaragoz.kau.edu.sa Examples of Resistors Potentiometer Rheostat 5 http://bkaragoz.kau.edu.sa Construction of Resistors Carbon resistor Carbon film resistor 6 http://bkaragoz.kau.edu.sa IC Diffused Resistors 7 http://bkaragoz.kau.edu.sa Cutaway View of a Resistor 8 http://bkaragoz.kau.edu.sa 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. 9 http://bkaragoz.kau.edu.sa 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… 10 http://bkaragoz.kau.edu.sa Carbon Composition and Metal Film Resistors 11 http://bkaragoz.kau.edu.sa The Resistor Color Code Color Number Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Grey 8 White 9 Silver 10% Gold 5% Red 2% Brown 1% 12 Labeling of Resistors using Color Codes http://bkaragoz.kau.edu.sa Preferred Values in E 12 Series 13 http://bkaragoz.kau.edu.sa Tolerance of resistors (fourth band of color code) • The tolerance of a resistor is shown by the fourth band of the color code. • Tolerance is the precision of the resistor and it is given as a percentage. • For example a 390 resistor with a tolerance of ±10% will have a value within 10% of 390 , between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390 ). 14 http://bkaragoz.kau.edu.sa Precision Resistors • Precision resistors will have a tolerance of 2%, 1%, 0.5%, 0.25% or 0.1% • Some precision resistor values are identified using the color code with 5 color bands, – the first 3 bands indicate the rated resistance value – the 4th band is the multiplier – the 5th band indicates the tolerance • Other precision resistors use an alphanumeric coding • The cost of the device is directly related to the precision – higher precision = higher cost 15 http://bkaragoz.kau.edu.sa Color Coding of Precision Resistors 16 http://bkaragoz.kau.edu.sa 17 http://bkaragoz.kau.edu.sa Alphanumeric Labeling • Two or three digits, and one of the letters R, K, or M are used to identify a resistance value • The letter is used to indicate the multiplier, and its position is used to indicate decimal point position 18 http://bkaragoz.kau.edu.sa Fixed IC Resistors • Often it is convenient to use multiple resistors mounted in IC packages in many of today's modern circuits 19 http://bkaragoz.kau.edu.sa 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 20 http://bkaragoz.kau.edu.sa Variable Resistors • Variable resistors are designed so that their resistance values can be changed • A potentiometer is a three terminal variable resistor used to divide voltage • A rheostat is a variable resistor used to control current 21 http://bkaragoz.kau.edu.sa Examples of Variable Resistors 22 http://bkaragoz.kau.edu.sa Linear vs Logarithmic Taper Potentiometers 23 http://bkaragoz.kau.edu.sa Wattage vs. Size • The wattage increases with the size of a Carbon resistors • • • • • 24 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 http://bkaragoz.kau.edu.sa Power Resistors • Used when the device must accommodate high currents • Physically large to surface can dissipate heat into the environment • Heats the air around the device – may be very hot 25 http://bkaragoz.kau.edu.sa Resistors in Different Sizes 26 http://bkaragoz.kau.edu.sa Resistors on Board 27 http://bkaragoz.kau.edu.sa Surface Mount 28 http://bkaragoz.kau.edu.sa Zero Ohm Resistors 29 http://bkaragoz.kau.edu.sa Resistors at High Frequencies 30 http://bkaragoz.kau.edu.sa Noise Produced by Resistors Pure signal used for evaluation; input voltage (y axis) versus time (x axis) 31 Effect of commercial components on signal; output voltage (y axis) versus time (x axis) http://bkaragoz.kau.edu.sa Noise Produced by Resistors Pure signal used for evaluation; input voltage (y axis) versus time (x axis) 32 Signal when foil-based resistors are used; output voltage (y axis) versus time (x axis) http://bkaragoz.kau.edu.sa Criteria for Selection of Resistors • • • • • • • • 33 Value of resistance Power rating Frequency of operation Temperature dependence Size Type: axial lead or surface mount Tolerance Price http://bkaragoz.kau.edu.sa For next lecture (September 26, 2011) • Study the behavior of passive components (resistor, capacitor and inductor) in DC and AC circuits • Define the impedance • Study the characteristics of series R-C and R-L in DC and AC circuits • Pretest (from the above EE 250 topics), you may use EE 250 textbook for preparation to the test. 34 http://bkaragoz.kau.edu.sa