Electrical Engineering ⇒ Topic : Power Rating of a Resistor

Power Rating of a Resistor

The ability of a resistor to dissipate power as heat without destructive temperature build-up is called power rating of the resistor.

Power rating of resistor = I²R or r V²/R [See Fig. (A)]

                                              FIGURE (A)

Suppose the power rating of a resistor is 2 W. It means that I²R or V²/R should not exceed 2 W. Suppose the quantity I²R (or V²/R) for this resistor becomes 4 W. The resistor is able to dissipate 2 W as heat and the remaining 2 W will start building up the temperature. In a matter of seconds, the resistor will burn out.The physical size of a resistor is not necessarily related to its resistance value but rather to its power rating. A large resistor is able to dissipate (throw off) more heat because of its large physical size. In general, the greater the physical size of a resistor, the greater is its power rating and vice-versa.