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I'm relatively new to electronics, and I'm trying to make a thermometer using a LM335 temperature sensor. By reading the datasheet I thought it was simple, connect up 5V to a resister, then to the V+, connect up V- to GND, and I should get somewhere just below 3V when reading the V+ pin.

However, when I do this, I'm actually getting 4.08. I've tried two different sensors, so I'm reasonably sure that it's not just a DOA.

Any ideas about what I could be doing wrong? What I can do to try to work it out?

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2 Answers 2

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Are you sure you have it connected properly? If you are getting 4.83V at V+ and 4.88V at your supply it sounds like you have an open circuit.

Check the pinout against the datasheet. On page two they are showing the bottom views of the package.

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    \$\begingroup\$ That does it! I had it in backwards. Now I'm getting 3.02v, but when I hold onto the sensor to warm it up, it gets higher, and when I let go, it cools down. \$\endgroup\$
    – gorilla
    May 12, 2010 at 0:45
  • \$\begingroup\$ Nice deductive work there, jluciani! \$\endgroup\$
    – pingswept
    May 12, 2010 at 1:31
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What value resistor are you using?

From the datasheet, LM335 needs about 400uA to 5mA of current. Assuming the diode will drop 3V, 2V is dropped across the resistor. To give between 400uA to 5mA, your resistor value should be between 400ohm to 5kohms.

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  • \$\begingroup\$ I was originally using a 10K ohm. I've just switched it to a 440 ohm, and now I'm seeing 4.83V at the V+ pin, while my 5v is actually 4.88v (measured at the Vcc side of the resister. \$\endgroup\$
    – gorilla
    May 12, 2010 at 0:15

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