Chapter 5: Resistors

What is a resistor? Definition: A device used in electrical circuits to maintain a constant relation between current flow and voltage. Resistors are used to step up or lower the voltage at different points in a circuit Resistors work as they turn voltage energy into heat 2 Functions: To limit current To produce desired voltage Analogy

Resistors

Resistor’s Schematic Symbol Composed of a zig-zag line What is the value of the resistor on the right? Reference Designators 300 Ω 30kΩ

Surface Mount vs. Through Hole

Types of Fixed Resistors Carbon Compostion Metal Film Metal-Oxide Film Resistor Networks Wirewound Resistors Old-skool High accuracy Low noise and good thermal properties High accuracy at high resistance Many packaged resistors High power applications

Surface Mount Resistors (SMT) Surface Mount Technology

Resistor Properties Value in Ohms Accuracy Power rating (We’ll look at this one first)

What is Resistor Power Rating? Examples: ¼ watt, ½ Watt, 20 watts P = V*I = V2/R Heat Sinks

Determining Value and Accuracy Tolerance Color Code: Brown = 1% Red = 2% Gold = 5% Silver = 10%

Video See lecture notes first http://www.youtube.com/watch?v=kvQBhXo_tF0

More in class practice problems Blue Blue Blue Gold 66 MΩ +/- 3.3 MΩ Brown Black Red Silver 1000 Ω +/- 100 Ω Brown Black Orange Silver 10000 Ω +/- 1000 Ω Orange Violet Black Gold 37 Ω +/- 1.85 Ω Find tolerance range as well … or you can just measure it with an Ohmmeter!

Reading Resistor Value on a Surface Mount Resistor 324 320,000 Ω 670 67 Ω 103 10k Ω 100 10 Ω

Homework Do problems 1 and 2 on page 85 Include the range (if applicable)!

Variable Resistors

Pictures of Variable Resistors

Linear vs. Logarithmic “Pots”

Knob rotation (Usually 20 turns) Linear vs. Logarithmic “Pots” Knob rotation (Usually 20 turns) See page 69 for schematic of application

Resistance of a Wire The longer a wire is, the more resistive it is. The thicker a wire is, the less resistive it is. (Think parallel) (Also think of a fire hose. There is always pressure drops across a pipe, but less pressure drop for bigger hoses.) Resistance is given by the following equation Therefore as a wire gets longer, the resistance change gets linearly larger. “The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance. The longer the conductor, the more scattering events occur in each electron's path through the material, so the higher the resistance. Different materials also affect the resistance.”

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