What is the power rating of a resistor? The power rating of a resistor defines the maximum energy a resistor can (safely) dissipate. As is stated by Joule’s first law, the generated electrical power is related to the voltage and current: When the electrical power equals the dissipated heat (by radiation, convection and conduction), the temperature of the resistor will stabilize. The temperature is not equal across the resistor. The resistor body is slightly hotter than the terminals, with the highest temperature at the center of the body. The higher the rate of heat dissipation to the environment, the lower the temperature rise will be. Larger resistors with a bigger surface area can generally dissipate heat at a higher rate. If the (average) power dissipation is larger than the power rating, the resistor may be damaged. This can have several consequences. The resistance value can shift permanently, the lifetime can significantly be reduced or the component is completely damaged resulting in an open circuit. In extreme cases the excessive power can even cause a fire. Special flameproof resistors are available, that cause a circuit brake before the temperature reaches a dangerous state. Power rating definition The power rating of a resistor defines the maximum energy a resistor can (safely) dissipate. Resistor derating The nominal power rating is defined for a certain ambient temperature in free air. Note that the amount of energy that a resistor in practise can dissipate without causing damage, is strongly dependent on the operating conditions and therefore not equal to the nominal power rating. For example, a higher ambient temperature can significantly reduce the power rating. This effect is referred to as derating. It should be taking into account by the designer. Often the power rating is chosen largely above the electric power. Typically resistors are...
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