The thermal resistance Rth [K/W] of a resistor indicates by how much the temperature rises when subjected to a load of 1 W. A resistor with Rth = 1,5 K/W is subjected to 200 W and heats by 300 K, i.e. at an ambient temperature of 35 °C it will assume a temperature of 335 °C. Resistors with indirect cooling have several Rth connected in series. The total is indicated by Rth_total and can also be used to calculate the temperature of a resistor conductor at a given surface temperature. The Rth is dependent on the design and type of cooling. It changes when the cooling condition – this unit is used in power electronics where it is a parameter for heat sinks and for the contact resistance of semiconductors – of load resistors with the exception of thermal resistors is of little practical use because this value varies considerably as a function of the actual load and of the installation and cooling conditions.
