# Thermistor Circuit

Discussion in 'The Projects Forum' started by magikal, Nov 23, 2008.

1. ### magikal Thread Starter Active Member

Nov 23, 2008
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I am working on a project that outputs What is the simplest way to get a thermistor circuit output of the temperature (or 1/10 of the temperature) in voltage? Is using voltage dividers the easiest way? Also, can anybody tell me a specific thermistor that operates in the 15C - 35C range?

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
I dont suggest you voltage divider because you will read inaccurate values due to loading effects. I recommend you to use a non-inverting op amp configuration to read the voltage produced by the thermistor.
Search in google for one, there are many out there.

3. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
MIK3, I think you are confusing a thermister with a thermocouple. Thermocouples produce a voltage, a thermistor varies it's resistance to temperature.

I'd use a simple constant current source, with a voltmeter across the thermistor myself. The problem is calibrating the range.

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Bill,
I know the difference between a thermocouple and a thermistor. He will make a voltage divider with the thermistor and a fixed resistor and drive the input of the amplifier (or voltage follower if he don't want to amplify it).

5. ### magikal Thread Starter Active Member

Nov 23, 2008
31
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Thanks for the help. The range needed is not very high, but accuracy is important. What sort of thermistor (or maybe some other device) does this nicely?

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
The 15 - 35 C range is good for any thermistor. How about you tell us what resistance you want at 25 C? All thermistors have a stated resistance measured at 25 C. Then they have a response curve, which is non-linear.

Perhaps you might be interested in the LM35?

By the way, what does your project wish to output?

7. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Sorry, but where is the voltage from the thermistor supposed to come from? It's either a voltage divider circuit or a current source.

8. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The power for the thermistor-fixed resistor voltage divider will come from the op amp's power supply. Its just an easy solution, not very complicated for a newbie

9. ### steveb Senior Member

Jul 3, 2008
2,433
469
If accuracy is important, then you will need a voltage reference with any type of thermister circuit. However, the nonlinearity is really a pain. You can make a linearizing resistor network which makes the response more linear over a narrow temp range, but still not straightforward. I think someone else mentioned a chip solution which is simpler since it provides a linear response to temperature.

Last edited: Nov 23, 2008
10. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Not only that, but the output is scaled in engineering units.

11. ### magikal Thread Starter Active Member

Nov 23, 2008
31
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Sorry, I am not very experienced with circuits. I want the output to be a voltage that can be displayed on an ADC. What chips allows for a linear voltage output?

12. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
What you wish is a voltage that can be converted with some accuracy by an ADC. What is the number of bits in your ADC?

Get a data sheet for an LM35.

13. ### Andrew Leigh Well-Known Member

Sep 8, 2008
92
2
Hi,

you say you need accuracy but have not indicated what tolerance. To my mind a Pt100 sensor is the way to go, you can achieve ~ +- 0,35°C accuaracy in the lower temperature range. Have alook at the following article, it may be of assistance.

http://beta.octopart.com/Microchip__TMPSNS-RTD1__0.pdf

Cheers
Andrew

14. ### magikal Thread Starter Active Member

Nov 23, 2008
31
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Thanks for the help, but I cannot use micro-controllers for the project. An output by the ADC of 2-3 significant digits is enough. Also, are there any simple chips containing the summer circuit or a multiplication circuit that works with voltage?

15. ### magikal Thread Starter Active Member

Nov 23, 2008
31
0
BTW, is the LM35 a micro-controller?

16. ### steveb Senior Member

Jul 3, 2008
2,433
469
No, it's a temperature sensor that is very simple to use. You get a voltage output that in linearly related to temp in degrees C. I think 10 mV per degree.

Do a search for LM35 datasheet and you should find the PDF for it.

17. ### magikal Thread Starter Active Member

Nov 23, 2008
31
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How do i compare the difference, or find the product of two temperatures produced by a LM35?

18. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
You convert to a digital value for each and work with the numbers.

19. ### magikal Thread Starter Active Member

Nov 23, 2008
31
0
Conversion can be done with an ADC right? Can you inform me of the chips that can do some mathematical operations with these numbers after converted?

20. ### steveb Senior Member

Jul 3, 2008
2,433
469
If you want, you can take the difference between two temperatures with a differential amplifier made with an OPAMP. See Vol III in the E-books here.