Power rating

 

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:

First law of Joule: P = I * V

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 rated for a temperature of 70°C, above this resistor starts to derate. This means that above this temperature the resistor can only utilize a reduced power level. This is illustrated by a derating curve.

Resistor Derating Curve: in general up to 70 degrees celsius, the resistors full power rating is possible, at higher temperatures a linear derating occurs.

Resistor Derating Chart – The horizontal axis represents the percentage of nominal rating for a given ambient temperature. In this case, the resistor’s full power rating is given until 70°C.

Next to the influence of the ambient temperature, there are several other factors impacting the derating. The most important factors are detailed below.

  • Enclosures
    The rate of heat loss is limited by installing the resistor in an enclosure. The enclosure limits air flow and therefore the removal of heat by convection. Radiated heat will removed at a lower rate, because the walls of the enclosure act as a thermal barrier. The effect of the enclosure on the heat loss rate is strongly dependent on the size, shape, orientation, material and wall thickness. It is difficult to indicate how these parameters affect the temperature rise.
  • Forced cooling
    Increasing the heat transfer by forced convection allows for a higher watt dissipation than for normal natural convection. This can be achieved by creating an air flow, or even liquid cooling. Some resistors are designed with conducting air fins, to create a bigger surface for heat dissipation.
  • Component grouping
    On a circuit board resistors are often positioned close to each other. The heat radiation of one resistor will be received by the next resistor and therefore have an extra increase in temperature for a given power consumption.
  • Altitude
    Resistors loose heat by convection and radiation. When the air density decreases, the convection will also decrease. Above 30 km convection has dropped so low that only heat dissipation by radiation exists.

Power resistors

For most electronic circuits the power rating is not a key parameter, since those resistors dissipate low amounts of energy of one watt or less. In power electronics however, the power rating is an important characteristic. Generally speaking hereto is referred when power ratings are one watt or higher. Typical applications include power supplies, dynamic brakes, power conversion circuits, power amplifiers and heaters.