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LM35A Folha de dados(PDF) 5 Page - National Semiconductor (TI) |
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LM35A Folha de dados(HTML) 5 Page - National Semiconductor (TI) |
5 / 12 page 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 001 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 inter- mediate 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 tempera- ture might be closer to the air temperature than to the sur- face 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 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 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 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 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 Temperature Rise of LM35 Due To Self-heating (Thermal Resistance) TO-46 TO-46 TO-92 TO-92 SO-8 SO-8 TO-202 TO-202 no heat sink small heat fin no heat sink small heat fin no heat sink small heat fin no heat sink small heat fin Still air 400 CW 100 CW 180 CW 140 CW 220 CW 110 CW 85 CW 60 CW Moving air 100 CW 40 CW 90 CW 70 CW 105 CW 90 CW 25 CW 40 CW Still oil 100 CW 40 CW 90 CW 70 CW Stirred oil 50 CW 30 CW 45 CW 40 CW (Clamped to metal Infinite heat sink) (24 CW) (55 CW) (23 CW) 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 foil or similar Typical Applications (Continued) TLH5516 – 19 FIGURE 3 LM35 with Decoupling from Capacitive Load TLH5516 – 20 FIGURE 4 LM35 with R-C Damper CAPACITIVE LOADS Like most micropower circuits the LM35 has a limited ability to drive heavy capacitive loads The LM35 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 LM35 is applied with a 200X load resistor as shown in Figure 5 6 or 8 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 electro- magnetic sources such as relays radio transmitters motors with arcing brushes SCR 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 capac- itor 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 Figures 13 14 and 16 5 |
Nº de peça semelhante - LM35A |
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Descrição semelhante - LM35A |
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