EET 110 - Electronics Survey

Chapter 10 - Resistors

Resistors

Resistor Wire Nichrome wire used for heating - I2 R high resistance results in a voltage drop

Resistors made from a variety of materials The lead pencil resistor

Resistors

Ratings of resistors• RESISTANCE

– determined by markings

• TOLERANCE– determined by marking

• WATTAGE RATING– determined by size or marked

Types of resistors

Wire-wound resistors usually larger, power resistors

Carbon Composition resistors a cylindrical shaped block a carbon compound

Film type resistors a layer of resistive material on an insulated rod

Both Carbon Composition and film type resistors are embedded in a cylindrical package.

Types of resistors

Resistor networks a number of resistors packaged together as

group of identical resistors.

Precision resistors usually +/- 1%

Adjustable resistors changeable after the circuit is constructed -

trimmer or variable types

Rheostats & Potentiometers

Potentiometers a POT variable resistor with three terminals

• each end of a resistive path and a wiper Rotary or slide pot

Trimmer screw variable

Taper may be linear or non-linear (logarithmic)

Resistor Color Code

consists of 4 bands of color1st & 2nd band give value3rd gives multiplier4th (if present) gives tolerance

Resistor Color Code

• Black 0 1

• Brown 1 10

• Red 2 100

• Orange 3 1000

• Yellow 4 10000

• Green 5 100000

• Blue 6 1000000

• Violet 7 10000000

• Gray 8

• White 9

Resistance is read by taking the color code

And applying the color code here RED = 2, VIOLET = 7, BROWN = x10 gives 270 ohms GOLD = +/- 5%

Color Code Mnemonic

Bugs Bunny Raids Our Young Garden, But Violets Grow Wildly

Black Brown Red, Orange, Yellow, Green, Blue, Violet, Gray, White

Precision Resistors

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

Size gives wattage1/8 watt

1/4 watt

1/2 watt

1 watt

2 watt

Series connection of resistors

Resistors connected in series result in added resistance

100 ohm

1000 ohms

500 ohm

Rtotal = 100 + 500 + 1000 ------ 1600 ohms

Series resistors

Voltage is distributed across the resistors

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

1k

1k

+9V

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’

VT = R i/RT x VT (page 108)

see example 10-2

Parallel connection

In Parallel resistors, the INVERSE of the total is equal to the total of the inverses of each resistor in parallel.

150 150 150

1/Rt = 1/R1 + 1/R2 + 1/R3

1/Rt = 3/150 = 1/50

Rt = 50 ohms

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

21

21

RR

xRRRT

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

Parallel Resistors

To find the total resistance of resistors in parallel

32

1111

1 RRR

RT

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

Two resistors in parallel, in series with a third

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

These two resistances in series gives RT = 40 + 20 = 60 ohms total

Homework

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