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

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Discussion Overview

The discussion centers around the use of NTC thermistors (50, 100, 250 ohms) for controlling LED light output based on temperature changes. Participants explore wiring configurations, current limitations, and potential issues with the setup.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes attempts to wire thermistors with LEDs using both series and parallel configurations, noting issues with the series configuration and overheating in the parallel setup.
  • Another participant suggests measuring the thermistor's resistance with an ohmmeter and using a voltage divider with a small current (1 mA or less) to avoid self-heating effects.
  • A different participant emphasizes that exceeding 1 mA can render the setup ineffective due to self-heating, which obscures ambient temperature changes.
  • One participant clarifies that the LM35 mentioned is not a thermistor but an integrated circuit designed for temperature measurement, providing additional context about its use.
  • Another participant expresses frustration over the inability to get the thermistors to respond, questioning their functionality.
  • A later reply asserts that the participant's approach is fundamentally flawed and suggests using a transistor amplifier driven by the thermistor to control the LED, while also emphasizing the importance of understanding current limits.
  • One participant reiterates the distinction of the LM35 as an IC, sharing appreciation for its design and functionality, while acknowledging a sidetrack in the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of the current setup and the proper method for using thermistors. There is no consensus on the best approach to wire the thermistors or on their functionality.

Contextual Notes

Participants mention limitations regarding current levels and the potential for self-heating effects, but do not resolve the underlying issues with the thermistor setup or the specific configurations attempted.

mat337d
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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:
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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..
 
Last edited by a moderator:
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.
 
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
 
thanks for the help. Still I have hooked these things up just about every way I can think and they don't respond. I am beginning to think they do not work at all. I am still going to keep trying to get them to work though.
 
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.
 
chroot said:
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.
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 : :rolleyes:
 

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