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LM335 question

derick petrey
Prodigy 20 points
Other Parts Discussed in Thread: LM335, LM334

Hello, recently ive taken on a project involving temperature, an ic i found laying around was LM335 and according to datasheet it seems easy enough to interface to using a microcontroller. Using the equation from the datasheet  Vout_T = Vout_To * (T/To) and then converting to degrees F by use of 9/5*C + 32, the conversion works just fine but im using the TO-46 package to measure the ambient temp in my living room and it seems like the sensor is not sensitive enough in free standing air? it seems it responds great when i touch the sensor to something cold or hot.

So i guess my real question is how can i make the LM335 measure temperature accurately in say my living room a big open space with rather still air?

  • 0
    TI__Expert 4905 points

    Hi Derick,

    The big open space with rather still air will probably change temperature very slowly. So perhaps it is measuring the right temperature? You need to make sure it's not touching a wall or something similar as that will influence the temperature sensor greatly.

    So how do you know the temperature is not accurate?

    Take care,

    Emmy
    Precision Systems Applications
    Texas Instruments

  • 0 in reply to Emmy Denton
    Prodigy 20 points

    I have a digital thermometer and a mercury thermometer that both agree, and the fact that its really cold in here wen i open say a window and the voltage changes but only by a small amount. So the temp sensor seems to think its 76 degrees when its actually like in the 60's.

    When i calibrate the sensor at 77degrees F or 25 C do i leave the pot hooked up for good, or do i calibrate it and then un-hook the pot? I leave it hooked up after i calibrate for 2.982V @ 25 C

    It seems to me the case is not changing temp in the free-air, but it does react nicely if its set next to a "cold" source or a "hot" source

    Thank you look forward to a reply

  • 0 in reply to derick petrey
    TI__Expert 4905 points

    You will need to leave the pot connected. Also the LM334 may have a slower thermal response time than the thermometer you are using and probably "filtering" the small temperature changes. You could try to lower it's thermal mass. Mount it on the end of some very long fine wires and see what you get. I assume that it's now mounted on a PCB. Leave the POT on the PCB, just remove only the LM334.

    Take care,
    Emmy
    Precision Systems Applications
    Texas Instruments SVA