IC vs NTC thermistor

[Urahara]

Hi

I hv managed to practice with a LM35DZ with my 18F4550 and it was quite straightforward.

I have a broken home thermometer, one of those that has a LCD panel and an external probe that allows you to measure both inside and outside temperature. Out of curiosity, I cracked up the plastic covering the external probe and found it to be a bead-like component.

From what I read, this is a NTC thermistor. And from what I can google, it seems that a lot of commercially made probes are NTC based. In fact, I was not able to find one that uses ICs-based like the LM35 or DS18B20, except DIY-ones.

I am just curious abt what is being experimented at home and what's being produced outside. Will one get out of touch as a result, at least in the temperature arena. Are there examples of C codes working with these thermistors.

Thks!

 

[richard.c]

Hi,

The humble thermistor is a good way to explore temperature measurement and control.

The cheap and small bead type are the better ones to use for diy.
Just connect up to your adc input with a resistor as a simple potential divider - as easy as that - and it can give accurate results down to 0.1deg C, over a narrow range of 10-20deg C, if you use an averaging routine on the result.


See also Nigels ADC Pic tutorials for obtaining very stable and accurate results.

Don't forget other devices like JK thermocouples for very high temps, simple diodes or the hi tech ds1820 ics.

 

[BeeBop]

From what I read, this is a NTC thermistor. And from what I can google, it seems that a lot of commercially made probes are NTC based. In fact, I was not able to find one that uses ICs-based like the LM35 or DS18B20, except DIY-ones.

Thks!

Yes, NTC thermistors are very fast, and very cheap, so that is what you will find in most commercial applications, however they are not linear, and in that respect, much more difficult to use for a one off project, etc. I suspect most of these types will not mind spending a few dollars for this ease of use. The Dallas parts have as a strength, the ability to place many parts on the same one wire bus, which is an easy way to save pins.... on the downside, they are quite slow, and very expensive, when compared with thermistors.

I'm sure, if you've been googling, you've come across the Steinhart-Hart equation:
http://en.wikipedia.org/wiki/Steinhart-Hart_equation. There are myriad scheme and solutions to getting these things more linear, as well. The main idea here is that if you look closely at a curve, it becomes a straight line. If the section of the curve you are interested in is short, then your line can be straighter.

So the difficult part is not writing source code, but conditioning the signal in a way which favors your application.

Microchip has some application note for using an NTC with their variable gain op-amp:
https://www.electro-tech-online.com/custompdfs/2008/08/00867a.pdf
https://www.electro-tech-online.com/custompdfs/2008/08/00897c.pdf
https://www.electro-tech-online.com/custompdfs/2008/08/00897b.pdf

https://www.electro-tech-online.com/custompdfs/2008/08/00929a_cn.pdf

https://www.electro-tech-online.com/custompdfs/2008/08/00685b.pdf

There are a million sites with info on how to set up an NTC thermistor:
**broken link removed**
http://www.facstaff.bucknell.edu/mastascu/elessonsHTML/Sensors/TempR.html
**broken link removed**
**broken link removed**
**broken link removed**
**broken link removed**
**broken link removed**
http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=6338

but the link I'm looking for, I can't seem to find at the moment.

 

[Urahara]

Thks for the info, esp to BeeBop for the various links!

From an uC implementation perspective, it sure seems the LM35 is easier than the thermistor. I actually had a good read of Nigel's tutorial before my first posting (must say it is a very well-written article for beginners like myself!), and to hv a good measurement, I would need to hv a good ref voltage.

Does anyone know where I can find probes bases on LM35/34s? I am curious to see how widely available they are.

Thks!

 

[BeeBop]

Thks for the info, esp to BeeBop for the various links!

From an uC implementation perspective, it sure seems the LM35 is easier than the thermistor. I actually had a good read of Nigel's tutorial before my first posting (must say it is a very well-written article for beginners like myself!), and to hv a good measurement, I would need to hv a good ref voltage.

Does anyone know where I can find probes bases on LM35/34s? I am curious to see how widely available they are.

Thks!

Yes, I agree on Nigel's site.

hv = have? Please spell it out.

Where in the world are you? You should fill in your location, so you can get better info. Those parts are a good choice, and very widely available. They will be in all the big-name places, but I kind of like this one:
https://www.futurlec.com/ICLinearOthers.shtml scroll down a ways. They have precision voltage references and temperature sensors on the same page.

Edit - they seem to have data sheets for most of what they have on site. You'll find links on the page.

 

[Urahara]

Hi BeeBop

Yes, hv=have. I am based in Asia (been having little time on the internet with all the catching up on the Beijing Olympics ).

Think you misunderstand me. Am not asking abt the LM35 IC per se, but abt whether there are temperature sensor "PROBES" out there that use the LM35 as the sensor.

 

[BeeBop]

Ha ha, sorry, I did misunderstand. If I come across any, I'll post them here.

I think what you want is a survey of the market, right? Rather than looking to buy a probe?

The best I've seen on hobby sites it the LM34 embedded in a piece of epoxy coated heat shrink...

 

[sky]

what are the differences between LM35 & thermistors
which is best to use?

 

[BeeBop]

what are the differences between LM35 & thermistors
which is best to use?

The term 'best' is subjective, at best. so what is best is all up to you and what you wish to do.

An LM35 puts out a voltage ~ 10 mV per degree temperature change. This is linear, and quite easy to use.
A thermistor changes its resistance with a change in temperature. This change is not linear, however, and each brand of thermistor will have different characteristics.

The advantage of the LM35 is ease of use. The advantages of the thermistor are low price and they are very fast.