I am interested in knowing the actual power dissipation of an SMD resistor at a 70C ambient temperature. I am looking at a 10kOhm 2512 1Watt resitor RMCF2512JT10K0.

from the electrical specifications section of the datasheet, the resistor is rated 1W at 70C:


now from the derating curve, it seems that this power rating is referring to a 155C operational temperature limit:

derating curve

we can conclude that 1W of power would rise the resistor's temperature by 155-70=85C and that the thermal resistance between the resistor and ambient is 85C/W. This can be used to estimate the temperature rise of the resistor while dissipating a specific power.

So far so good until we compare the above calculations with the temperature rise curve: temperature rise 1 temperature rise 2

My question is

  1. How can we explain the last curve (and table) where 100% power dissipated by RMCF2512 will cause 150C temperature rise!
  2. The description of the test done is also ambiguous.

The resistor surface generated temperature variation after applied rated voltage

Did they apply the rated voltage to the resistor (200V to 75K 1Watt resistor => only 0.53Watts)?

  1. What is the meaning of that last table (surface temperature)!?

Probably the 100% power table illustrates the maximum point surface temperature rise of the resistor, likely the center of the top surface, such as you would measure with an IR camera. This is the "hot spot" temperature and is of interest because it could cause damage to wire insulation etc.

Panasonic (for some parts) specifies measuring the temperature of the hottest of the two terminals, and derating based on that. See this:

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