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LM34C Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part No. LM34C
Description  Precision Fahrenheit Temperature Sensors
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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Typical Applications
The LM34 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 002 F 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 high-
er or lower than the surface temperature the actual temper-
ature of the LM34 die would be at an intermediate tempera-
ture between the surface temperature and the air tempera-
ture 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 clos-
er to the air temperature than to the surface temperature
To minimize this problem be sure that the wiring to the
LM34 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 LM34 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
Vb terminal of the circuit will be grounded to that metal
Alternatively the LM34 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 LM34 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 var-
nishes such as Humiseal and epoxy paints or dips are often
used to insure that moisture cannot corrode the LM34 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
Capacitive Loads
Like most micropower circuits the LM34 has a limited ability
to drive heavy capacitive loads The LM34 by itself is able to
drive 50 pF without special precautions If heavier loads are
anticipated it is easy to isolate or decouple the load with a
resistor see
Figure 3 Or you can improve the tolerance of
capacitance with a series R-C damper from output to
ground see
Figure 4 When the LM34 is applied with a
499X load resistor (as shown) it is relatively immune to
wiring capacitance because the capacitance forms a bypass
from ground to input not on the output However as with
any linear circuit connected to wires in a hostile environ-
ment its performance can be affected adversely by intense
electromagnetic sources such as relays radio transmitters
motors with arcing brushes SCR’s transients etc
as its
wiring can act as a receiving antenna and its internal junc-
tions can act as rectifiers For best results in such cases a
bypass capacitor from VIN to ground and a series R-C
damper such as 75X in series with 02 or 1 mF from output
to ground are often useful These are shown in the following
circuits
Temperature Sensor
Single Supply b50 to a300 F
TLH6685 – 6
TLH6685 – 7
FIGURE 3 LM34 with Decoupling from Capacitive Load
TLH6685 – 8
FIGURE 4 LM34 with R-C Damper
Temperature Rise of LM34 Due to Self-Heating (Thermal Resistance)
Conditions
TO-46
TO-46
TO-92
TO-92
SO-8
SO-8
No Heat Sink
Small Heat Fin
No Heat Sink
Small Heat Fin
No Heat Sink
Small Heat Fin
Still air
720 FW
180 FW
324 FW
252 FW
400 FW
200 FW
Moving air
180 FW
72 FW
162 FW
126 FW
190 FW
160 FW
Still oil
180 FW
72 FW
162 FW
126 FW
Stirred oil
90 FW
54 FW
81 FW
72 FW
(Clamped to metal
(43 FW)
(95 FW)
infinite heat sink)
Wakefield type 201 or 1
disc of 0020
sheet brass soldered to case or similar
TO-92 and SO-8 packages glued and leads soldered to 1
square of
printed circuit board with 2 oz copper foil or similar
5




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