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LM335Z Thermal Resistance ?

S

Steve Kavanagh

Jan 1, 1970
0
I'm pondering self-heating errors in an LM335Z temperature sensor. The
National data sheet 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 ?

Steve
 
Steve said:
I'm pondering self-heating errors in an LM335Z temperature sensor. The
National data sheet 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 ?

It is certainly curious. However, note that the junction to case
thermal reisitance is what you have to add onto the thermal resistance
of a heat-sink to calculate the final junction temperature.

TO-92 packages are notoriously difficult to couple to TO-92 clip-on
heat sinks - the shape of the plastic package isn't well-defined, and
if you squeeze your metal heat-sink hard enough to force close contact
over an extended area of the package, you break the package - so I
guess that 170 degC/W includes a lot of case-to-heat-sink thermal
resistance.

Fast-moving turbulent air might conceivably do better. The fact that
the physically bigger and solid TO-92 packages does better than the
empty metal TO-46 can at high air-velocities is persuasive.

An experiment might be in order - try sticking an LM335Z into a 6.35mm
(0.25") hole drilled into a decent sized chunk of aluminium, after
initially filling the hole with zinc-oxide loaded silicone grease, and
see what sort of thermal resistance you get there,
 
P

Phil Hobbs

Jan 1, 1970
0
Steve said:
I'm pondering self-heating errors in an LM335Z temperature sensor. The
National data sheet 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.

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.

Cheers,

Phil Hobbs
 
Phil said:
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.
 
J

John Larkin

Jan 1, 1970
0
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.

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.

There is an LM35 in a TO-220 package, which can be mighty handy.

John
 
P

Phil Hobbs

Jan 1, 1970
0
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.

The TO46 has Kovar leads, which are much lower in thermal conductivity.

Cheers,

Phil Hobbs
 
Phil said:
The TO46 has Kovar leads, which are much lower in thermal conductivity.

The TO-92 package actually does worse than the TO-46 at high air
velocities - I wasn't reading the data sheet carefully enough.
Curiously, the TO-92 package always has the shortest time constant,
even though the thermal resistances cross over at about 800 feet per
minute, which imples that the effective thermal mass of the TO-92
package declines with increasing air-flow rate, and at a faster rate
than that of the TO-46 package.

Should we invoke the thermal mass of the quasi-static layer of air in
the laminar part of the boundary layer? I'd be surprised if there is
enough mass there to do any good ... Or do we have to figue that there
is a signnificant thermal gradient inside the LM35 package at high air
flows, thus decreaisning the thermal mass in a way that you wouldn't
see with a hermitcally sealed TO-46 metal can package.
 
S

Steve Kavanagh

Jan 1, 1970
0
Some interesting thoughts. Thanks.

I guess I could believe that the J-C measurement invoved something
along the lines of the thermal-grease filled hole Bill suggested while
the J-A values consist primarily of leads-to-air effects.

I was thinking of gluing or clamping the case to the object I want to
measure the temperature of...maybe if I solder the leads to a tiny PC
board and bond it down too it might work better.

Steve
 
Steve said:
Some interesting thoughts. Thanks.

I guess I could believe that the J-C measurement invoved something
along the lines of the thermal-grease filled hole Bill suggested while
the J-A values consist primarily of leads-to-air effects.

I was thinking of gluing or clamping the case to the object I want to
measure the temperature of...maybe if I solder the leads to a tiny PC
board and bond it down too it might work better.

Farnell has a bunch of thermally conductive glues and encapsulants.

The cheapest - and it isn't cheap - is a conductive rubber jointing
compound (order code 130-485) for which they claim 2W/(m.K). I've used
something similar and it worked well.

More expensive is the self-shimming thermal conductive adhesive (order
code 537-020) which only offers 0.815 W/(m.K), in a layer not thinner
than 0.15mm.

Even more expensive is the thermally conductive slicone potting
compound, which offers
0.6 W/(mK). the rder code is 422-7852 for 2kgm.
 
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