0195136047.pdf

18 CIRCUIT CONCEPTS

TABLE 1.2.2Standard Color-Coded Bands for Evaluating Resistance and Their Interpretation

Color bands 1–4 b 1 b 2 b 3 b 4

Color of Band Digit of Band Multiplier % Tolerance in Actual Value

Black 0 100 —

Brown 1 101 —

Red 2 102 —

Orange 3 103 —

Yellow 4 104 —

Green 5 105 —

Blue 6 106 —

Violet 7 107 —

Grey 8 108 —

White 9 — —

Gold — 10 −^1 ±5%

Silver — 10 −^2 ±10%

Black or no color — — ±20%

Resistance value=( 10 b 1 +b 2 )× 10 b^3 .

of 10 ranging from 10to about 22× 106 . For example, 8.2,82, 820,... , 820 k

are standard available values.

The maximum allowable power dissipation orpower ratingis typically specified for com-

mercial resistors. A common power rating for resistors used in electronic circuits is^1 
 4 W; other

ratings such as^1 
 8 ,^1 
 2 , 1, and 2 W are available with composition-type resistors, whereas larger

ratings are also available with other types. Variable resistors, known aspotentiometers, with a

movable contact are commonly found in rotary or linear form. Wire-wound potentiometers may

have higher power ratings up to 1000 W.

The advent of integrated circuits has given rise topackaged resistance arraysfabricated by

using film technology. These packages are better suited for automated manufacturing and are

usually less costly than discrete resistors in large production runs.

An important property of the resistor is its ability to convert energy from electrical form into

heat. The manufacturer generally states the maximum power dissipation of the resistor in watts.

If more power than this is converted to heat by the resistor, the resistor will be damaged due to

overheating. The instantaneous power absorbed by the resistor is given by

p(t)=v(t)i(t)=i^2 R=v^2 /R=v^2 G (1.2.4)

wherevis the voltage drop across the resistance andiis the current through the resistance. It can

be shown (see Problem 1.2.13) that the average value of Equation (1.2.4) is given by

Pav=VrmsIrms=Irms^2 R=Vrms^2 /R=Vrms^2 G (1.2.5)

for periodically varying current and voltage as a function of time. Equation (1.2.5) gives the

expression for the power converted to heat by the resistor.