LM35
SNIS159C –AUGUST 1999–REVISED JULY 2013
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APPLICATIONS
The LM35 is applied easily in the same way as other integrated-circuit temperature sensors. Glue or cement the
device to a surface and the temperature should 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 intermediate temperature between the surface temperature and the air temperature, which is
especially 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 temperature.
To minimize this problem, ensure that the wiring to the LM35, as it leaves the device, is held at the same
temperature 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 temperature as the surface, and that the
temperature of the LM35 die is not affected by the air temperature.
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, mount the LM35 inside a sealed-
end metal tube, and then dip into a bath or screw 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 condensation 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 connections.
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.
Table 1. Temperature Rise of LM35 Due To Self-heating (Thermal Resistance, θJA
)
SOIC-8(2)
small heat
fin
,
TO, no heat
sink
TO(1), small
heat fin
TO-92, no heat TO-92(2), small
SOIC-8, no
heat sink
TO-220, no
heat sink
sink
heat fin
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)
(1) Wakefield type 201, or 1-in disc of 0.02-in sheet brass, soldered to case, or similar.
(2) TO-92 and SOIC-8 packages glued and leads soldered to 1-in square of 1/16-in printed circuit board with 2-oz foil or similar.
8
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