Re: LM335Z Thermal Resistance ?


Phil Hobbs wrote:
Steve Kavanagh wrote:
I'm pondering self-heating errors in an LM335Z temperature sensor. The
National data *** gives the junction-to-ambient (still air) thermal
resistance as 202 degC/W and the junction-to-case as 170 degC/W. A
graph of the junction-to-air thermal resistance vs air velocity is also
given which starts out at 202 degC/W for still air and drops down to
about 70 degC/W at high velocities.

Can someone explain to me how the junction-to-air thermal resistance
can be less than the junction-to-case value ?


Almost all the heat goes out the leads. Copper has a thermal
conductivity of ~400 W/m/K, vs. more like 0.1 for plastic.

That makes a lot more sense than my speculations. It doesn't explain
why the TO-92 package does better than the TO-46 at high air velocities
- presumably the rather wider thermal path through the plastic
compensates (to some extent) for the high specific conductivity of
copper, and since both of them are a great deal more thermal conductive
than air (even a thin boundary layer, and the boundary layer at 1000
feet per minute - 11.4mph - is still a couple of millimetre thick) the
differences in their thermal conductivities probably won't matter as
much as all that.

For boundary layer thicknesses this paper looks as if it might be
useful

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=541526

IC temperature sensors are just as crappy as can be for air temperature
sensing, and not much better for anything else except possibly board
temperature.

Everything is pretty crappy for air temperature measurement - the one
time I did it seriously, I wound 8 metres of very thin platinum wire (
60 micron diameter) around hexagonal squirrel cage, giving me a
resistance of 270R. The wire wasn't entirely stress-free, so it
wouldn't have done for NBS, but it worked pretty well.

The LM35 is actually a pretty good temperature sensor - admittedly ten
times noiseir than a platinum or thermistor resistive sensor, but it
doesn't dissipate much power and it is very easy to use.

--
Bill Sloman, Nijmegen

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