Do you pay attention to the power rating of a resistor?
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When a current runs through a resistor, it turns electrical power into heat. The maximum allowed dissipation for a given resistor is the maximum power rating of that resistor. For many circuits, small power ratings (for example 0.25 W) are sufficient. An example would be resistors used in signal conditioning. The vendors generally declare the power rating for a temperature not surpassing 70 degrees C. Sometimes, resistors need to work in a setting where the temperature exceeds that limit. The maximum power rating of the resistor should be reduced by a certain amount in cases where it might work in temperatures surpassing the limit.
A guideline that's frequently used by designers is to halve the dissipation rating for each additional 30 degrees C above the 70 degrees Celsius mark. This is an unwritten rule since a large number of manufacturers are hesitant to release precise information regarding the degree of derating needed for their specific product. Of course, this rule is limited in the sense that you can't continue raising the temperature while lowering the power and expect the resistor to work - it will at some point burn if you choose this route.
Very high temperatures are easily reached in the presence of some components. Processor chips, high speed memories or even mechanical devices within the same case as our resistor may raise the temperature significantly. Hard disk drives, for example, often operate at temperatures above 70 degrees C and the resistors being used there have to be wisely selected and manufactured.
It is usually valuable to figure out approximately what temperature increase will likely take place in the resistor when it is used at its highest allowed (or any other) power rating. We have no simple and easy way of calculating these temperature increases, because they depend so much on the way that the resistor has been put together, and also on factors like the surface finish. The figure below demonstrates a "Resistor Derating Chart", based on a typical resistor. It shows the temperature rise that may be found in practice with dissipation for resistors rated at 0.5 W and 1 W respectively. The temperature could reach as high as 140 degrees C.
A guideline that's frequently used by designers is to halve the dissipation rating for each additional 30 degrees C above the 70 degrees Celsius mark. This is an unwritten rule since a large number of manufacturers are hesitant to release precise information regarding the degree of derating needed for their specific product. Of course, this rule is limited in the sense that you can't continue raising the temperature while lowering the power and expect the resistor to work - it will at some point burn if you choose this route.
Very high temperatures are easily reached in the presence of some components. Processor chips, high speed memories or even mechanical devices within the same case as our resistor may raise the temperature significantly. Hard disk drives, for example, often operate at temperatures above 70 degrees C and the resistors being used there have to be wisely selected and manufactured.
It is usually valuable to figure out approximately what temperature increase will likely take place in the resistor when it is used at its highest allowed (or any other) power rating. We have no simple and easy way of calculating these temperature increases, because they depend so much on the way that the resistor has been put together, and also on factors like the surface finish. The figure below demonstrates a "Resistor Derating Chart", based on a typical resistor. It shows the temperature rise that may be found in practice with dissipation for resistors rated at 0.5 W and 1 W respectively. The temperature could reach as high as 140 degrees C.
High power resistors are usually wire-wound resistors. The bigger wire-wound resistors are made of corrosion resistant wire wrapped on to a porcelain or ceramic type former. They are often utilized to dissipate high inrush currents such as the ones produced in motor-control, electromagnet or elevator control and electric motor braking circuits. Wire-wound resistors are available in a wide variety of power ratings, commonly up to a kilowatt. Nevertheless greater power ratings are also possible.Independent of the resistor material, all fixed resistors comply with Ohm's law which needs to be used when the required power dissipation of the resistor has to be calculated. If you are not able to obtain a resistor with a high enough power rating, consider utilizing multiple resistors in series or parallel. In this manner the power will be distributed over the several resistors.
More information about resistors
More information about resistors