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The answer to this question is probably staring me in the face, but I just can't see it. I'm attempting to interface with an LM335AZ temperature sensor (datasheet) at 3.3V. The datasheet clearly states that the output voltage is 10mV/K, and that at 25°C and 1mV it should output between 2.92V and 3.04V. What I do not see specified is the minimum voltage it will operate at, and the sensor I have in hand doesn't appear to work correctly at 3.3V.

With a 3.3KΩ resistor the current to the LM335 should be 1mA at 3.3V. If I directly measure the output voltage when supplying 3.3V, I get 2.46V, which correlates to −27°C; if I supply 5V I get 2.93V, or about 20°C. Suffice it to say that I'm not performing these tests outside at the North Pole. :-)

Does the LM335AZ only work on 5V, or 5V and up?

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3 Answers 3

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With 3.3K you are not getting 1mA. You are getting around 100uA.

At 25DegC the LM335 is at 3V. With a Vin of 3.3V you getting 0.3V across the resistor. 0.3V / 3.3K is apx 100uA. If you use a 300Ohm resistor you should be at about 1mA.

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  • \$\begingroup\$ Stupid decimal point. :-) Thanks, John! I'd missed that I need to be looking at the voltage drop, not Vss. \$\endgroup\$
    – blalor
    Jan 3, 2010 at 3:26
  • \$\begingroup\$ @blalor - HTML doesn't work in comments, you need to use mini-markdown: [link](http://example.com) _italic_ **bold** `code` \$\endgroup\$
    – Kevin Vermeer
    Jun 27, 2011 at 16:28
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The LM335 works a bit like a variable zener diode, as the symbol in the datasheet shows.

enter image description here

That means that it will have a voltage over it when a current flows through it. So you don't supply a voltage, you supply a current. That's why you can't find the voltage in the datasheet.
The datasheet specifies that the current must be between 400\$\mu\$A and 5mA. You have to calculate the value of R1 depending on V+ and the maximum and minimum temperatures you want to measure.

In this answer I provide a detailed calculation of the series resistor R1.

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I know this is an old thread, but just wanted to supply my findings for using LM335 at 3.3v using Arduino Pro Mini.

This article here suggests using a 2kΩ resistor at 5v, so using the R = V/I formula we can change to 1.3kΩ resistor. I had a 1.2kΩ one handy and this worked.

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