Typical Performance Characteristics (Continued)
Noise Voltage
Start-Up Response
DS005516-34
DS005516-35
The TO-46 metal package can also be soldered to a metal
surface or pipe without damage. Of course, in that case the
V− terminal of the circuit will be grounded to that metal.
Alternatively, the LM35 can be mounted inside a sealed-end
metal tube, and can then be dipped into a bath or screwed
into a threaded hole in a tank. As with any IC, the LM35 and
accompanying wiring and circuits must be kept insulated and
dry, to avoid leakage and corrosion. This is especially true if
the circuit may operate at cold temperatures where conden-
sation can occur. Printed-circuit coatings and varnishes such
as Humiseal and epoxy paints or dips are often used to
insure that moisture cannot corrode the LM35 or its connec-
tions.
Applications
The LM35 can be applied easily in the same way as other
integrated-circuit temperature sensors. It can be glued or
cemented to a surface and its temperature will be within
about 0.01˚C of the surface temperature.
This presumes that the ambient air temperature is almost the
same as the surface temperature; if the air temperature were
much higher or lower than the surface temperature, the
actual temperature of the LM35 die would be at an interme-
diate temperature between the surface temperature and the
air temperature. This is expecially true for the TO-92 plastic
package, where the copper leads are the principal thermal
path to carry heat into the device, so its temperature might
be closer to the air temperature than to the surface tempera-
ture.
These devices are sometimes soldered to
a small
light-weight heat fin, to decrease the thermal time constant
and speed up the response in slowly-moving air. On the
other hand, a small thermal mass may be added to the
sensor, to give the steadiest reading despite small deviations
in the air temperature.
To minimize this problem, be sure that the wiring to the
LM35, as it leaves the device, is held at the same tempera-
ture as the surface of interest. The easiest way to do this is
to cover up these wires with a bead of epoxy which will
insure that the leads and wires are all at the same tempera-
ture as the surface, and that the LM35 die’s temperature will
not be affected by the air temperature.
Temperature Rise of LM35 Due To Self-heating (Thermal Resistance,θJA)
TO-46,
TO-46*,
TO-92,
TO-92**,
SO-8
SO-8**
TO-220
no heat
sink
small heat fin
no heat
sink
small heat fin
no heat
sink
small heat fin
no heat
sink
Still air
400˚C/W
100˚C/W
100˚C/W
50˚C/W
100˚C/W
40˚C/W
40˚C/W
30˚C/W
180˚C/W
90˚C/W
90˚C/W
45˚C/W
140˚C/W
70˚C/W
70˚C/W
40˚C/W
220˚C/W
105˚C/W
110˚C/W
90˚C/W
90˚C/W
26˚C/W
Moving air
Still oil
Stirred oil
(Clamped to metal,
Infinite heat sink)
(24˚C/W)
(55˚C/W)
*Wakefield type 201, or 1" disc of 0.020" sheet brass, soldered to case, or similar.
**TO-92 and SO-8 packages glued and leads soldered to 1" square of 1/16" printed circuit board with 2 oz. foil or similar.
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