Resistor power rating When a current runs through a resistor, it turns electrical power into heat. The maximum allowed dissipation for a certain resistor is the power rating of that resistor. In some applications we may safely use whatever resistors we have available, no matter what their power dissipation rating is. However, in real-life circuits, there are numerous applications for resistors dissipating plenty of heat, and the maximum power dissipation in datasheets is generally declared under the condition that the environment temperature is 70°C. If we need to use a resistor above 70°C, we must reduce the maximum power rating of the resistor. If the datasheet doesn't advise differently, it's usually a good idea to reduce the maximum power rating of the resistor by 50% for each 30°C rise above 70°C. Common sense should be used, however, since there are other temperature limits. For example, increasing the temperature too high could make the solder melt and render the circuit unusable, regardless of the derating.
Very high temperatures may be measured in the presence of some components. Processors and high speed memories sharing the same case as our resistor may raise the temperature significantly. Hard disk drives, for example, often operate at temperatures over 70°C and the resistors being used there have to be wisely chosen and manufactured. Power dissipation will create a rise in the resistor temperature. The temperature increase is approximately 60°C above ambient when the resistor is dissipating maximum power permitted. Experience is sometimes key to an optimal design, as it's often extremely difficult to determine the effect of the dissipation on the resistor temperature. There are so many parameters that have an impact on the cooling of a component on-board, which makes calculations complicated. At times it's needed to cool the air inside the case to avoid damage to the components. For instance, if the air inside an enclosure reaches 70°C, then a resistor could quite easily reach 130°C.
High power fixed resistors are usually wire-wound resistors. The bigger wire wound power-resistors are made from corrosion resistant wire wrapped on to a porcelain or ceramic type holder. Popular uses of high power resistors include motor braking resistors, electric powered heating or safety grounding resistors. These types of resistors possess power ratings as much as 500 Watt and are also linked with each other to make resistor banks. No matter what is the resistor material, all fixed resistors obey Ohms law which should be utilized when the necessary power dissipation of a resistor has to be calculated. It's also important to note that when numerous resistors will be linked in parallel or series then their overall power rating is increased because the current and therefore power is distributed across the several resistors. More information can be found at: http://www.resistorguide.com/power-rating/