LM34 Temperature Sensor

The LM34 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Fahrenheit temperature. The LM34 thus has an advantage over linear temperature sensors calibrated in degrees Kelvin, as the user is not required to subtract a large con-stant voltage from its output to obtain. Low cost is assured by trimming and calibration at the wafer level. The LM34's low output impedance, linear output, and precise inherent calibration make interfacing to readout or control circuitry especially easy. It can be used with single power supplies or with plus and minus supplies. As it draws only 75 µA from its supply, it has very low self-heating, less than 0.2ÊF in still air. The LM34 is rated to operate over a −50Ê to +300ÊF temperature range, while the LM34C is rated for a −40Ê to +230ÊF range (0ÊF with im-proved accuracy).

The LM34 series is available packaged in hermetic TO-46 transistor packages, while the LM34C, LM34CA and LM34D are also available in the plastic TO-92 transistor package. The LM34D is also available in an 8-lead surface mount small outline package. The LM34 is a comple-ment to the LM35 (Centigrade) temperature sensor.


  •  Calibrated directly in degrees Fahrenheit
  •  Linear +10.0 mV/˚F scale factor
  •  1.0˚F accuracy guaranteed (at +77˚F)
  •  Rated for full −50˚ to +300˚F range
  •  Suitable for remote applications
  •  Low cost due to wafer-level trimming
  •  Operates from 5 to 30 volts
  •  Less than 90 µA current drain
  •  Low self-heating, 0.18˚F in still air
  •  Nonlinearity only ±0.5˚F typical
  •  Low-impedance output, 0.4Ω for 1 mA load

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 0.02§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 higher or lower than the surface temperature, the actual temperature of the LM34 die would be at an intermediate 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 temperature. To minimize this problem, be sure that the wiring to the LM34, 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 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 varnishes 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, lightweight 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 1 . Or you can improve the tolerance of capacitance with a series R-C damper from output to ground; see Figure 2 . 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 environment, 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 junctions 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 0.2 or 1 mF from output to ground are often useful. These are shown in the following circuits.

To find more application/usage of LM34 in two-wire remote temperature sensor, temperature to digital converter, expanded scale thermometer, voltage to frequency converter then please refer to the datasheet.


For datasheet click here



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