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ELIZABETHTOWN COMMUNITY AND TECHNICAL COLLEGE BEX 100 BASIC ELECTRICITY FOR NON MAJORS LECTURE GUIDE – UNIT 5 Instructor: Jerry Brown Reference: Delmar’s Standard Textbook of Electricity, Third Edition RESISTORS Objectives At the completion of the lesson, the student should be able to: 1. List the major types of fixed resistors 2. Determine the resistance of a resistor using the color code 3. Determine if a resistor is operating within its power rating 4. Connect a variable resistor for use as a potentiometer INTRODUCTION Resistors are one of the most common components in an electrical circuit Unit of resistance (R) is the ohm, named for German Scientist George S. Ohm, represented by the Greek letter, Omega (Ω) Uses of Resistors 1. Limit current flow 2. Act as a voltage divider Fixed Resistors - Have only one ohmic value which cannot be changed - Types include: Composition carbon • Resistor Power ½ Watt – 1 Watt – 2 Watt – • Metal Film • Carbon Film • Metal Glaze • Wire wound Ratings – (Composition Carbon) Determined by size 1/8” dia x 5/8” long ¼” dia x 7/16” long 5/16” dia x 11/16” long Composition Carbon Resistors Most Common Type Inexpensive Tolerance Ranges Constructed of carbon graphite/resin with nonconductive material case Change value with age or when overheated Metal Film Resistors More accurate – tolerances 0.1 – 2% Don’t change value with age More expensive than Composition Carbon Constructed by applying a film of metal to a ceramic rod Carbon Film Resistors Use film of carbon over ceramic rod similar to metal film but less expensive Higher Tolerance rating than composition carbon Metal Glaze Resistors Similar to metal film except combines metal with glass Tolerances range around 1 – 2% Wire Wound Resistors Made by winding resistive wire around ceramic core Used when high power rating needed With a hollow core, air flow cools resistor (mount vertically for chimney effect) Tend to be expensive Can induce magnetic field in high frequency AC circuits COLOR CODE First Three Bands Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Grey 8 White 9 2 Multiplier Band (Third or Fourth Band) Black 1 Brown 10 Red 100 Orange 1000 Yellow 10000 Green 100000 Blue 1000000 Silver 0.01 Gold 0.1 Tolerance Band (Last Band) Silver ±10% Gold ±5% Brown ±1% VARIABLE RESISTORS A resistor whose value can be changed or varied over a range Available in different case styles and Power ratings Types include rheostats, potentiometers, thermistors and photo-resistors Rheostats Two wire variable resistor Has moveable lever that varies resistance EXAMPLES: Dash lights dimmer Wall mounted ceiling fan speed control Potentiometers Three wire variable resistor Used as a position indicator (input to a logic device) EXAMPLES: Throttle position sensor HVAC door position sensor Thermistors Two Wire Sensors Resistance varies with temperature Can be “PTC” or “NTC” Used as input to a logic device 3 EXAMPLES Coolant temperature Duct air temperature Room temperature sensor Photo-Resistors Two wire variable resistors Resistance varies with different levels of light input EXAMPLES: Automatic headlight sensors Automatic exterior lighting sensors Sun load sensors for HVAC systems REVIEW 1. Name two functions of resistors. 2. What is a fixed resistor? 3. Name three types of fixed resistors. 4. What effects a resistor’s power rating? 5. Name an advantage of a wire wound resistor. 6. What are the color bands on a resistor? 7. What is a variable resistor? 8. Name three types of variable resistors. 4
