Thermal resistance

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Underlying technologyHeat dissipationThermal resistance


The transmission of thermal energy within a substance is not immediate but with a certain delay. The speed with which the vibration energy (thermal energy) is transmitted within a substance directly depends on its properties. See cicon2 Heat transmission.


The thermal conduction properties of a substance is characterised by the thermal resistance (Rth), where Rth equals the difference in temperature at the edges / amount of heat transmitted within a certain interval.




Thermal resistance is measured in Kelvin per Watt.


The thermal resistance (Rth) of a substance depends on the length of the route of thermal conductivity, its cross section (A) and the substance's specific thermal conductivity (λ), whereby the thermal resistance increases with the length of the thermal conductance route and decreases with the cross section of the route and the substance's thermal conductivity.



l is measured in metres, λ in Frml_LambdaUnit and the cross section in m2 . The result is the thermal resistance measured in Kelvin per Watt.


Click to expand or collaps information.  Example of thermal resistance of a copper rod

The thermal energy produced by a high-brightness LED shall be transmitted via copper rod to a heat sink positioned on the outside.


The copper rod has a cross section of 0.053m2, the median heat conduction route is 0.032m. The heat conductivity of pure copper is 401Frml_RthUnit. The value for the copper rod is defined as 376 Frml_RthUnit.



The resulting thermal resistance is 0.00161 K/W. This means that the temperature at the contact to the LED is 0.00161* thermal energy higher than at the contact to the heat sink.


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