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Resistor question from an electronics newbie

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jimanny

New Member
Hello all,

I have a 10K thermistor that is about 10.5K ohms at 75 degrees F and about 7 ohms at 100 degrees F. Is there a way to combine resistors or other components so that the total resistance of the circuit goes higher as the thermistor resistance drops?

Thanks in advance.

- jimanny
 

Dean Huster

Well-Known Member
You have what's called an NTC or Negative Temperature Coefficient thermistor: the resistance goes down as the temp goes up. A PTC (Positive Temperature Coefficient) thermistor would be your best answer. But there some circuits that could be designed that controlled a FET to do what you want ... but getting a PTC version would be a lot cheaper, simpler, take up less space, be more reliable, etc.

By the way, 10.5K down to 7 ohms for a 25° change is a heckuva thermistor. I'm dubious. I'd believe 10.5K down to 7K ohms maybe!

Dean
 

jimanny

New Member
Dean,

Thanks very much for your reply. You are right it was 7K not 7 ohms!

The thermistor I have is from Radio Shack. I've tried other ones from Mouser and Jameco, but they didn't work as nice as this one (slow reaction time and not as pretty) so I was hoping I could somehow build the circuit around it. Do you or anyone know where to find good PTC thermistors? I would also be interested in those flat ones as well.

Thanks - jimanny
 

Noggin

Member
www.digikey.com seems to have nearly everything I've ever looked for. Just make sure you order a lot of stuff at the same time from them to take advantage of the flat $5.00 handling fee, and they ship orders that come in before 8:00 PM CST on the same day!

I placed an order two days ago at 6:30 and by 7:30 I had a shipping confirmation email :)
 

Roff

Well-Known Member
jimanny said:
Hello all,

I have a 10K thermistor that is about 10.5K ohms at 75 degrees F and about 7 ohms at 100 degrees F. Is there a way to combine resistors or other components so that the total resistance of the circuit goes higher as the thermistor resistance drops?

Thanks in advance.

- jimanny
What are you trying to do? There may be a way of doing what you want with your NTC thermistor.
 

jimanny

New Member
Ok, thanks for the info. I'll look more closely through the digikey website.

In the end what I'm trying to do is drive the adjustment pin of an LM317 with this NTC thermistor to control a fan. The datasheet has an example with one 240 ohm resistor connect across the out and adjust pins and one POT connected from adjust to ground. When the POT is changed to a higher resistance, the voltage out is also higher. What I was looking to do is somehow replace the POT with this NTC thermistor so when the ambient temp goes higher (resistance goes lower) I still get a higher resistance in place of the POT, and also a higher voltage finally compensating for a rise in temp. I've seen several circuits on the web to, control fan speed, though none with an LM317 and a thermistor.

Thanks if you have any thoughts on this.

- jimanny
 

Dean Huster

Well-Known Member
What our good buddy Ron was alluding to was the fact that if the thermistor was part of a simple 2-resistor (the thermistor and a resistor) voltage divider to control the input to a comparator, switching the fixed resistor with the thermistor would pretty much reverse the action of the thermistor with respect to the output voltage.

Dean
 

jimanny

New Member
Yes, I was looking for a replacement sub-circuit that would replace that lower fixed resistor and provide a higher resistance as the thermister resistance dropped. I tried many different series-parallel resistor combinations only to discover that no matter what, the drop in resistance from the thermistor would always drop the overall resistance. I did think up one circuit which worked with a pot where pin 1 and the center pin where each connected to ground through equal resistors, and the resistance was measured from the 3rd pin to ground. When the resistance dropped across the parallel resistors, the resistance at pin 3 increased. Now if I could only figure out how to make the same thing happen with a thermistor instead of a pot.

hhhhmmmm...
 

Phasor

Member
To clarify Ron/Dean's suggestion:
 

Attachments

  • NTCthermistor.jpg
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Roff

Well-Known Member
Phasor put the thermistor in the wrong line. The circuits below should work. I simulated them, but I haven't built them.
Solving for Rset in the circuit on the left,

Rset=Rtherm*((Vout/1.25)-1)
For example, if you want Vout=12v when Rtherm=7k,
Rset=60.2k

Solving for Rset in the circuit on the right,

Rset=(Vout-1.25)/((1.25/Rt)*Ib)
If Vout=12v when Rt=7k, and Ib=50uA, then
Rset=47k

However, if Ib=60uA and Rset=47k, then Vout=12.5v.

Probably not a problem.
 

Attachments

  • LM317_therm.gif
    LM317_therm.gif
    5.7 KB · Views: 482

jimanny

New Member
Thanks guys for all the replies to my question. I really need it since I am an electronics goober, but slowly learning. Ron, your circuit looks like it may be just what I need. Thanks! I'll try it out and post back what happens.

- jimanny
 
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