# Calculating Temperature Rise of a Resistor

##### Member
85C/Full Power
Based on
From Yageo resistors:
At 155C the resistor has a 0 watts rating. So that is to say that this resistor should not get above 155C.
I think at 70C air temp and 100% power the body of the resistor is at 155C.
So (155-70=85) My guess is that 100% power causes a temp rise if 85C.
I see your logic, but don't agree this can be inferred from the graph; that at 70degC the body temperature will rise 85DegC at 100% power. There is not a reason presented to provide that inference is true. The same inference cannot be made of transistors, so why here?

#### languer

##### Active Member
You may be able to use the info from this Vishay document, http://www.vishay.com/docs/20103/geninfo.pdf), and the Pmax plot to solve for Rth. Without going into solving it, I think it may end up with a similar answer to what Ron provided, but the document does provide a bit of background on how the plot is generated.

##### Member
You may be able to use the info from this Vishay document, http://www.vishay.com/docs/20103/geninfo.pdf), and the Pmax plot to solve for Rth. Without going into solving it, I think it may end up with a similar answer to what Ron provided, but the document does provide a bit of background on how the plot is generated.
I read through the doc, especially the section on the temp rise.

Turns out to be one equation with two unknowns.

Rth=temp rise / power

Pmax = (film temp - ambient temp) / Rth = [(film temp - ambient temp)*P) / temp rise

We can use the chart from Ron to state that Pmax =100, P=V*I (a known or calculatable number), ambient we can say is 25 degC. But film temp and temp rise are unknown and not in any of the charts in the document or posted here.

#### ronsimpson

##### Well-Known Member
Rth=temp rise / power
LOL It took me tooooo long to understand Rth is thermal resistance.
Pmax = (film temp - ambient temp) / Rth = [(film temp - ambient temp)*P) / temp rise
Something funny here. You must know "Max film temp" to know "Pmax".
from PDF said:
Thus the maximum permissible power rating Pmax. is
dependent on the maximum permissible film temperature,
the ambient temperature ϑu, and the thermal resistance.
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I have a non contact temperature meter, a power supply, a resistor and 10 minutes. This should be easy to solve.