In the specifications of all Microhm's resistors, power derating curve is a must. It shows the relationship between rated power and terminal temperature. Higher ambient temperatures can significantly reduce the rated power. This effect is called derating. The rated power of a resistor means the max amount of energy it can dissipate without failing or degrading. The maximum power rating of Microhm's shunts is 36W, precision resistor MVR can reach 50W.

 
What will affect the rated power of resistors .Let's figure it out.
 
1. The Resistor Shell

The heat loss rate is limited by installing a resistor in the housing. The outer casing limits the flow of air and therefore removes heat by convection. The radiant heat will be removed at a lower rate because the walls of the outer casing act as a thermal barrier. The effect of the outer casing on the heat loss rate is highly dependent on size, shape, orientation, material and wall thickness, which will lead to temperature rise.

2. Pressure Cooling
Increasing heat transfer by forced convection allows for higher watt dissipation than normal natural convection. This can be achieved by generating a gas stream or even a liquid cooling. Some resistors are designed with conductive air fins to create a larger heat sink surface.
 
3. Component Distribution

On the PCBA, the resistors are usually placed close to each other accordingly to Microhm's customers. The thermal radiation of one resistor will be received by the next resistor, so the temperature will increase.


4. Application Environment

The resistor dissipates heat by convection and radiation. When the air density is reduced, the convection is also reduced. Above 30 kilometers, the convection is reduced so low that only radiation dissipates heat.

Microhm's engineers summarized the rated power of the resistor requires a certain environment to get the correct power value, and the value of the resistor power obtained under the change of environment and connection mode is also different.