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EET 110 - Electronics Survey Chapter 10 - Resistors

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Resistors §Resistor Wire l Nichrome wire used for heating - I 2 R l high resistance results in a voltage drop §Resistors l made from a variety of materials l The lead pencil resistor

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Resistors l Ratings of resistors RESISTANCE –determined by markings TOLERANCE –determined by marking WATTAGE RATING –determined by size or marked

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Types of resistors §Wire-wound resistors l usually larger, power resistors §Carbon Composition resistors l a cylindrical shaped block a carbon compound §Film type resistors l a layer of resistive material on an insulated rod §Both Carbon Composition and film type resistors are embedded in a cylindrical package.

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Types of resistors §Resistor networks l a number of resistors packaged together as group of identical resistors. §Precision resistors l usually +/- 1% §Adjustable resistors l changeable after the circuit is constructed - trimmer or variable types

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Rheostats & Potentiometers §Potentiometers l a POT l variable resistor with three terminals each end of a resistive path and a wiper l Rotary or slide pot §Trimmer l screw variable §Taper l may be linear or non-linear (logarithmic)

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Resistor Color Code §consists of 4 bands of color §1st & 2nd band give value §3rd gives multiplier §4th (if present) gives tolerance

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Resistor Color Code Black01 Brown110 Red2100 Orange31000 Yellow 410000 Green5100000 Blue61000000 Violet710000000 Gray8 White9

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§Resistance is read by taking the color code §And applying the color code l here RED = 2, VIOLET = 7, BROWN = x10 l gives 270 ohms l GOLD = +/- 5%

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Color Code Mnemonic §Bugs Bunny Raids Our Young Garden, But Violets Grow Wildly §Black Brown Red, Orange, Yellow, Green, Blue, Violet, Gray, White

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Precision Resistors §1 % and 2% tolerance resistors must have THREE significant digits of value l Brown, Black, Black, Black, Brown l 100 x 1 w/ Brown = +/- 1% tolerance

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§Size gives wattage 1/8 watt 1/4 watt 1/2 watt 1 watt 2 watt

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Series connection of resistors §Resistors connected in series result in added resistance 100 ohm 1000 ohms 500 ohm R total = 100 + 500 + 1000 ------ 1600 ohms

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Series resistors §Voltage is distributed across the resistors

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§The voltage across each resistor in series is a proportion of the resistance to the total Here each resistor is 1/3 of the total, so the 9 volts is distributed across the three resistors evenly - 3 volts each 1k +9V

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Series Resistors §Resistors in Series have an equivalent total resistance equal to the algebraic sum of each resistance. §The voltage across any one resistor is series is given by the ‘Voltage Divider Rule’ V T = R i /R T x V T (page 108) §see example 10-2

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Parallel connection §In Parallel resistors, the INVERSE of the total is equal to the total of the inverses of each resistor in parallel. 150 1/Rt = 1/R1 + 1/R2 + 1/R3 1/Rt = 3/150 = 1/50 Rt = 50 ohms

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§To find the total resistance of two unequal resistors in parallel - use the formula

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§For example if a 60 ohm and 40 ohm resistor are connected in parallel, the total resistance is given as RT = (60 x 40)/(60 + 40) = 2400/100 = 24 ohms

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Parallel Resistors §To find the total resistance of resistors in parallel

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Series - Parallel Connection §If combinations of resistors are connected in series and parallel, a total can be derived, by alternately simplifying with the series / parallel formulas §See example 10-7

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Two resistors in parallel, in series with a third

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§First solve for the parallel combination l R T (parallel) = (30 x 60)/(30 + 60) = 20 ohms l This leaves an equivalent

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§These two resistances in series gives l R T = 40 + 20 = 60 ohms total

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Homework §Do Problems – Ch9: 1&2, Ch10: 1-6 §Prepare for lab (ohm readings) §Read Chapters 11 & 12

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