Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Purchased some thermistors (NTC 50,100,250 ohms)

  1. Feb 24, 2005 #1
    I have just purchased some thermistors
    (NTC 50,100,250 ohms). I want to wire it with a led.
    so it will controll the leds light output with changing temp. I have already tried both series and parrallel configurations. the series confguration does not work at all. In parrallel configuration the thermsistor gets
    very hot and the led dimms till it goes out. Am I doing somthing wrong? Is their a special way to wire
    thermistors? thanks

    power supply 9v 300ma
     
    Last edited: Feb 24, 2005
  2. jcsd
  3. Feb 25, 2005 #2
    those are sensors , you could to measure their resistance with an ohmmeter , but the way its usually done is to use a small current of 1 mAmp or less and measure the voltage across the thermistor..
    the way you do it is set up a voltage divider ..

    Page 9 has what i think you are looking for..
    http://cache.national.com/ds/LM/LM35.pdf
    check out their whole typical applications section ..
    hope this helps..
     
  4. Feb 25, 2005 #3

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Matt, you're putting way too much current into your thermistor. As willib suggested, 1mA should be considered an upper bound for most NTCs (typically a few hundred microamps). Anything more than this renders your set-up meaningless, as the self-heating effects will drown out ambient temperature changes.
     
  5. Feb 25, 2005 #4

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Just to note -- the LM35 is NOT a thermistor; it's an entire IC designed for ease-of-use. It is, by the way, a nice little part.

    (For the record, I used to work at National, right beside the guys who did the applications work for the temp sensors. :smile:)

    - Warren
     
  6. Feb 26, 2005 #5
    thanks for the help. Still I have hooked these things up just about every way I can think and they dont respond. Im begining to think they do not work at all. Im still going to keep trying to get them to work though.
     
  7. Feb 26, 2005 #6

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Mat377d,
    Evidently you have not understood the previous posts. The basic message is, What you want to do cannot be done the way you are trying to do it. It simply does not work that way.

    One way do make it work would be to dirve a transisor amplifer with your thermistor, use the out put of the transistor to power your led. Do do this you will have to understand the meaning of terms like 1ma.
     
  8. Feb 26, 2005 #7

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think I've used these, (the fahrenheit version : LM34). I must chime in with my word of appreciation. :approve: These critters are neat !!

    Edit : sorry for the sidetrack : :uhh:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Purchased some thermistors (NTC 50,100,250 ohms)
  1. Ntc thermistor (Replies: 2)

Loading...