The wattage rating is a very important parameter in resistors.It indicates the maximum amount
of power a resistor can dissipate as heat before drastically changing value or burning up.That
is,if power is the time rate of doing work,and energy can be changed from one form to the other(law of conservation of energy),therefore,wattage rating represents the highest level
of mechanical work convertible to heat energy per unit time in a resistor.
In commercial resistors,wattage rating is one of the three specifying values to watch out for.
The other values include the resistance in ohms,and tolerance.Again,while the ohmic value and
tolerance are given either as a printed value or a value represented by multiple color bands,
the wattage rating is identified by the size of the resistor.So,a small resistor has a small
wattage while a large resistor has a big wattage.
Typical wattage ratings for color-coded carbon resistors are 1/8,1/4,1/2,1,and 2 watts,while
the values are 1/10,1/8,and 1/6 watts for chip or wirewound resistors.The unit “watt” could be traced to James Watt,a Scottish instrument maker,whose improvement on the steam engine helped to advance the industrial revolution.According to him,1 hp(horsepower)equals 746
watts.As an example,the wattage rating of the color-coded carbon resistor given by 1/8 watt is equivalent to 1/5968 or 0.000167560322 hp.
Moreover,the wattage rating or power dissipated as heat energy when current flows in a resistor is an important value in design.The resistor must be big enough to withstand the heat energy dissipated or it will burn up.If the current through the resistor or the voltage across it is known,in addition to the resistance value,then the power dissipation can be calculated as follows:
Case #1
The wattage rating(P)of a resistor varies directly as the resistance(R),or P = I*I*R,where the constant of proportionality is the square of the current(or I*I).For example,what wattage resistor should be used in an application requiring 14 ohms and 0.3 amperes ?Using the formula above,the power is given by
P = I*I*R
P = 0.3 x 0.3 x 14
P = 1.26 watts
To handle the power with some safety factor,a 2.00 watt resistor would be required.As a
precaution,a one watt resistor should not be used as it would burn up.
Case#2
The wattage rating of a resistor also varies inversely as the the resistance or P =(V^2)/R,
where the constant of proportionality is the square of the voltage.So,if the voltage
across a 65k ohm resistor is 130 volts,then,the power dissipated is given by
P = (V^2)/R
P = 130.130/ 65,000
P = 16,900/65,000
P = 0.260 watt
With this wattage rating,the most reliable resistor to use to complete the circuit is the 1/2 watt resistor.A 1/2 will burn because the wattage rating is lower than the calculated 0.260 watt.
In a nutshell,wattage rating on a resistor is the power value of the resistor in question.And for the resistor to function effectively,there should be a match between the wattage rating and the job for which it is to be used.In both electrical and electronic circuits,this condition must be satisfied at the design stage of any work.Paying close attention to the wattage rating of a resistor is the only way to avoid burn up.Never play with a resistor if you are not technically knowledgeable in its application.
