Thermistor help

joeyd999

Joined Jun 6, 2011
4,425
A cheap, simple and accurate way is also the Dallas DS18B20 (10 bit capability), that I already mentioned.
Cheap, no: $5.39 in single quantities.

Simple, maybe: If code libraries are available for the one-wire interface, then yes. If not, then no.

Accurate: 0.5C (-10 to 85). The thermistor is still better.

One thing that bugs me about the part is the 1.5mA active current. During the active times, that current will cause the part to dissipate 7.5mW (at 5V). This will cause an self-induced temperature error dependent upon the thermal characteristics of the package. There is no data for this on the datasheet, but given a typical TO-92 thermal resistance of 80 to 310 C/W, an increase of 0.6 to 2.3C can be expected during those times.

They also seem to give no thermal time-constant data (I only quickly perused the datasheet!). This is oftentimes important, especially if you are trying to design an active controller.

Final objections: 3 wires vs. 2. Must observe polarity/pinout. Not easy making a temp probe from a TO-92 package (intended for use as PCB mount). Lead length limited by digital interface. Resolution is limited to manufactures design. I've run the thermistors to 0.01C resolution (and better!).
 

joeyd999

Joined Jun 6, 2011
4,425
You don't even need an ADC to digitize the thermistor. A common solution is to use a capacitor and measure frequency or period, depending on whether the thermistor exhibits positive or negative temperature coefficient. You can easily get 12 to 16-bit resolution using this technique. No additional components required!
Alternatively, since the LM35 looks pretty much like an ideal temperature dependent current source, I suppose you could use it to charge a cap and use the charge time to some V as linearly representative of temperature.

But then you'd need an accurate and stable cap. Or use dual slope integration (discharge the cap with another accurate, stable, and constant current source).

You could probably even discharge thorough a precision resistor, but your timing then becomes non-linear (which could be compensated through code).
 

pilko

Joined Dec 8, 2008
213
"Cheap, no: $5.39 in single quantities"-- Wrong ---I have never paid more than a doller
"self-induced temperature error"---Wrong---Active current 1.5mA standbye current only 1uA

We could go on add nauseum, but I am getting too old so I conceed to you the last word if you wish to add one.

pilko
 

Thread Starter

chimera

Joined Oct 21, 2010
122
haha!! This has been a very interesting thread! Im glad I started it :p It turned out to be an all out battle! Anyways..the thermistor seems to be a good choice but I ended up using the LM35 sensor. It is very accurate from 0 to 80 C. I could have pushed it further but it was already well above my temperature requirements.

The only advice I give is (if using a uC like I am for A-to-D coversion):
1- Check reference voltage (1.5V should be more than enough)
2- Keep in mind of the A-to-D conversion when using 10 bit or 8 or whatever bit--this is just Vref/2^A-D bit resolution---therefore V_ADC=Vin*(Vref/2^bits)
3- Take the equation (sensor function) in the datasheet and solve for temperature.
4- Plug the V_ADC function from step 2 into equation from step 3.
5-Display on LCD using uC
 

MrChips

Joined Oct 2, 2009
21,634
So the op finally shows up with the project completed with no reference to all the comments, except haha!! Should teach us something about endless arguing amongst ourselves for nothing.
 

steveb

Joined Jul 3, 2008
2,436
So the op finally shows up with the project completed with no reference to all the comments, except haha!! Should teach us something about endless arguing amongst ourselves for nothing.
Also troublesome is the fact that there is no explicit thanks for attempts to help, and not even a hint of appreciation for the time and effort given. This is one of the only uses I see for the ignore list. It provides a nice way to tag the people you don't want to make an effort for in the future.
 

Thread Starter

chimera

Joined Oct 21, 2010
122
Also troublesome is the fact that there is no explicit thanks for attempts to help, and not even a hint of appreciation for the time and effort given. This is one of the only uses I see for the ignore list. It provides a nice way to tag the people you don't want to make an effort for in the future.

Firstly, I do appreciate the help. A sincere thanks to each and every one who contributed. Please do not feel unappreciated. Rest assured, your
efforts have helped me.

I do admit, I could have been more present during the replies but with the holiday season, it was a little difficult.

Suggestion:

Being a young engineer, it helps if the seasoned veterans of the engineering world provide more concise and focused replies. For instance, if you go through each and every reply thoroughly, one might find that the thread leaned towards individualistic methods and ideology.
Due to this, I was not able to pin point a direction and start working.

I do expect future help. That is how forums work and hopefully one day I can return the efforts. Happy Holidays!
 
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