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.

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.

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.

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.

Temperature Rise of LM35 Due To Self-heating (Thermal Resistance,

θ

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

180˚C/W

140˚C/W

220˚C/W

110˚C/W

90˚C/W

Moving air

100˚C/W

40˚C/W

90˚C/W

70˚C/W

105˚C/W

90˚C/W

26˚C/W

Still oil

100˚C/W

40˚C/W

90˚C/W

70˚C/W

Stirred oil

50˚C/W

30˚C/W

45˚C/W

40˚C/W

(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.

Noise Voltage

DS005516-34

Start-Up Response

DS005516-35

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