Electronics > Projects, Designs, and Technical Stuff
Resistor power ratings
wraper:
Again, temperature of backplate, not internal resistor.
OM222O:
The temperature of back plate and "internal resistor" are not two separate things in a different universe. depending on the power dissipated by the "internal resistor" the temperature rise to the tab will be that power times 3.3! adding that to ambient temperature will be the final tab temp. it's honestly simple math, not rocket science. unless since last time I checked the definition of thermal resistance and power have changed ... Again I'm not sure where the hostility comes from. some input from other users won't be bad either. please let them respond rather than shouting the same thing over and over again :-+
wraper:
--- Quote from: OM222O on May 27, 2019, 04:14:06 am ---The temperature of back plate and "internal resistor" are not two separate things in a different universe. depending on the power dissipated by the "internal resistor" the temperature rise to the tab will be that power times 3.3! adding that to ambient temperature will be the final tab temp. it's honestly simple math, not rocket science. unless since last time I checked the definition of thermal resistance and power have changed ... Again I'm not sure where the hostility comes from. some input from other users won't be bad either. please let them respond rather than shouting the same thing over and over again :-+
--- End quote ---
Oh, my.
Tab temperature = (heatsink temperature) + (thermal resistance of thermal interface, say thermal paste) * (power dissipated).
3.3 * power is resistor temperature above tab temperature. NOT TAB TEMPERATURE!
P.S.
Heatsink temperature = (ambient temperature) + (heatsink thermal resistance) * (power dissipated). If heatsink has zero thermal resistance, it stays at ambient temperature regardless of power dissipated. Thus in ideal case tab temperature is only determined by thermal resistance of thermal interface (thermal paste) and dissipated power. Specs in this datasheet other than tab size have nothing to do with either of those. But often you can find Case-to-heatsink thermal resistance spec which is basically a ballpark of what you could expect. But it really depends on thermal interface you will use.
OM222O:
ah yes, you are right. I got that backwards :P the temperature of resistor will be higher not the tab. :palm:
disregarding the de rating, the max temperature of the resistor is uncomfortably close to the operating point, even with an ideal heat sink at 35 watts. as tim mentioned, those figures are probably measured under a liquid freon bath.
wraper:
--- Quote from: OM222O on May 27, 2019, 04:41:24 am ---ah yes, you are right. I got that backwards :P the temperature of resistor will be higher not the tab. :palm:
disregarding the de rating, the max temperature of the resistor is uncomfortably close to the operating point, even with an ideal heat sink at 35 watts. as tim mentioned, those figures are probably measured under a liquid freon bath.
--- End quote ---
If you mount it on a modern air cooled CPU heatsink with heat pipes, it will stay below 50oC easily. Again, max spec is max spec, not something necessarily practical to do. It shows limits of the component (which BTW is not something extreme in this case). Not that you should push it to this limit. Usually it's more practical to put several parts in parallel and put them on usual heatsink rather than put one part and cool it with liquid nitrogen.
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