Thermistor for measuring/controlling temperature


Hi folks!
I’m going to talk about a little electrical component
for measuring temperature, called a thermistor.
I originally needed it for measuring the water temperature
in a set of experiments, determining if cold water boils faster than
hot. But just for fun I also got it to turn on
this PC fan when the heat was too high. What’s a thermistor? A thermistor is usually a ceramic or polymer
material with two wires connected to it.
The key is that the thermistor’s resistance changes with temperature.
With some of them the resistance increases as the temperature increases.
Those are called positive temperature coeffient or PTC thermistors.
With others, the resistance decreases as the temperature increases.
Those are called negative temperature coeffient or NTC thermistors. I couldn’t find a thermistor locally but some
quick research turned up that automobile coolant temperature
sensors are just thermistors in a protective case.
Which was a good thing since that saved me having to make a case
of my own. With this one there’s only one obvious place
to connect to, this threaded part.
So where do the two thermistor wires go inside? Well, one goes to the threaded part
and the other goes to the brass case. So I wrapped one wire tightly around the case.
And after attaching a ring terminal to a wire, and finding suitable nuts,
I attached another wire to the threaded part. I then connected up a meter
and put it on the resistance scale. Remember, the resistance changes with the
temperature. I put my fingers on the probe part and it
started to heat up. Sure enough, the resistance on the meter started
to decrease as the temperature increased.
So it contains an NTC, or negative temperature coefficient thermistor. To be useful, I needed to know what resistance
matches what temperature. Unfortunately I couldn’t find a resistance
to temperature table for this particular one.
So I had to make my own. To do that I prepared a tall container,
this soda can with all the paint sanded off. I filled it with water
and put the probe part of the sensor in the water.
I also put in my kitchen thermometer. The long length of my thermometer is why I
went with a tall container. For my purposes,
I’d need to know the resistance to temperature values
for temperatures near the freezing point of water.
So I put the whole thing in my freezer until the thermometer read near 0 celsius
or 32 fahrenheit. Then I took it out,
connected my meter to measure resistance, and put it on my stove heating element.
I then started recording the resistance on the meter
and the corresponding temperature on the thermometer as the stove element heated up the water.
I stopped once it reached boiling, which is 100 celsius or 212 fahrenheit. And here’s the chart drawn using the data
from my table. As you can see, the resistance decreases as
the temperature increases. You can also see that it’s a curve. The whole reason I’d gone through this process
was to do some experiments to determine if cold water
boiled faster than hot water, as some people think it does,
and to find out why that myth might exist. That resulted in this graph of water temperature
as it was heated up over time. But that’s a whole other topic
which I cover in another video I’ll point out at later. Here’s the fun I had making a PC fan turn
on when the thermistor gets too hot. First here’s the circuit without the thermistor.
It’s very simple. The positive from my homemade power supply
goes to the positive of the fan and the negative of the fan
goes back to the power supply. When I turn on the power supply, the fan starts
spinning. Now here it is with the thermistor.
The positive from the power supply goes to one end of the thermistor.
The other end of the thermistor goes to the positive of the PC fan
and the negative of the fan goes back to the power supply. But this time when I turn on the power supply,
the fan doesn’t start spinning. That’s because the thermistor is nice and
cool at room temperature and so it has a high resistance.
The fan needs a certain amount of electrical current going through it
in order to spin. But the higher the resistance in the thermistor,
the lower the current is going through the fan.
And the resistance of the thermistor is high enough
to make that current too low. Now I use this hair dryer to heat up the thermistor.
That causes the thermistor’s resistance to decrease.
As it decreases, the current increases. And when the current’s high enough, the fan
starts spinning. Notice that the fan is blowing on the thermistor,
making it cool off more quickly. Here I’m showing the airflow direction using
this tissue paper. As the thermistor cools, it’s resistance increases.
And that causes the current going through the fan to decrease.
And after a minute or so the current is too low to keep the fan spinning,
so it stops. Well, thanks for watching! See my youtube channel, rimstarorg for more
videos like this. That includes the video containing the experiments
to test if cold water boils faster than hot water
and why people might think that. Another on how to power a compact fluorescent
light using just two AA batteries.
And one on how to make a solar panel out of transistors,
in this case to power a calculator. And don’t forget to subscribe if you like
these videos, or give a thumbs up or leave a question or
comment below. See you in a bit!

100 thoughts on “Thermistor for measuring/controlling temperature

  1. Thanks. Another video's in the works. It's just taking a little longer than usual and I don't want to just rush one out.

  2. My Whirlpool gas dryer isn't getting hot enough and the clothes never get dry, the gas flame works, it's just shutting the heat off too soon, so I bought what is called a thermistor in a sealed plastic case that screws onto the exiting chamber after the blower. This did not solve the problem.

    When I remove the thermistor as to not allow it to sense any warm temperature and left it plugged in the flame stays on endlessly. At first I assumed that even though the encased thermistor was new in the package that it might be defective straight from the manufacturer, but now that I see this video there are a few of these disc-like thermistors attached inside the dryers wiring harness, they are not seemingly positioned in any specific spot as if to sense temperature, but just part of the wiring, I always thought these were perhaps some sort of resistor, but instead they are identical to what you call a thermistor.

    When I was at a dead end on what to do next then this video helped me to decide that the next thing I should try is to replace the short harness sections that contain these thermistors. Thanks for making this video.

  3. I have a question for you , i gotta remote controlled car motor and i would like to place my custom fan for cooling which would be controlled by a NTC Thermistor. But i am a little bit confused about the temperature that i want it to turn on. Do you mind to help? I was thinking to M35 or 5D-11 would work. the heat goes on those brushless motors pretty high sometimes. I would like to find the right thermistor that it would turn on the fan around 150-175F. which model thermistor should i use do you think?

  4. Good!! Good for my study. Electrical stuff makes me confused, but simple expression like this is good for my understanding electrical works. Thanks a lot!!

  5. Hello, I want to connect a NTP to the source of a TEC (Peltier) so that it the current that the TEC gets slowly builds up. How should I choose the right NTP?

  6. You sound like just the guy I need to talk to. I am building my own carbonator for my soda pop addiction for which there is plenty of instruction on You Tube and the internet in general.  What I would like you to do, is to show us how to build a portable cooler on wheels (which uses common rechargeable batteries [AA orAAA]), which uses common and cheap space age materials, that would pass the Energy Star test with flying colors. It should incorporate the carbonator which can be represented by a black box. The cooler should come on at 33 degrees Faranheight  and turn off around 34 or 35 degrees, and maybe contain a couple of trimmer potentiometers for controlling temperatures. You can likely run a few second manufacturer add that can't be skipped to pay for your time.

  7. i need to measure the temperature of hat water inside the tube. i can make the hole for set up this thermister but m not getting how to put it. the water will be in contact with both wires. ?????

    can u answer ??

  8. how hot did your sensor get to turn on fan ??

    also is there a engine coolent temp sensor that will turn on a fan at a 98 degrees

  9. Hey Rimstar, great video.

    A very clear and practical demonstration that makesit alot easier to understand the use of thermistors.

    Thanks. 🙂

  10. I'm looking to sound an alarm when my water temperature reaches 180 degrees Fahrenheit. Can you lead me in the righy direction of how to go about doing that? Thanks

  11. I would like to use something like this to turn on and off an electric pvc sprinkler valve. On at 65 F, off at 75F do you have any suggestions?

  12. hey,

    the idea is just awesome…but upto wat temperature does this system works or upto wat temperature will it sense the temp/…..

  13. thank u so much man i was trying to hoom it up with my amp that has a built in one uf not i well buy one an hook it up thank u again i can bymuy this for $5 an they sell smd for $80 same oit come

  14. Does the thermistor have a maximum current limit to it ? How much current can it handle before it breaksdown ?.

  15. how can I use that as PTC? for me to cut the current as the temp increases on our project, serves as a safety element. Thanks in Advance! ^_^

  16. this is actually really helpful for my a level. i could easily understand thermistor in less than 5 minutes.

  17. very good! but I have a question: what temperature reach the thermistor 1 k ohm when you apply 12 volt? it is very hot (about 85 C) and isn't dangerous if you put it in a fuel tank to have a low sensor that light a bulb? sorry for my poor English. Ciao.

  18. efficent delivery of accurate info with noob tips integrated (like where is tr second wire), i can see why u have 155k subs. do u have a teaching background?

  19. I want to build a circuit to trigger a buzzer if the temperature in my fridge gets too high. I lost a bunch of groceries after an electrical snafu in my home and wanted to prevent that from happening again.
    Do you have any other videos which might help me understand how to design a battery supply which only began to feed power to something should another power source fail? I suppose I just want to learn how to design a portable back up battery. Portable as in I can easily carry it from one place to another and portable as in it could be easily adjusted for use with most DC supplied electronics.

  20. I want to connect NTC thermistor to 12 CPU fan(0.40 A) for temperature 38°C or above which value (ohm) will I should use??

  21. how do you set a threshold for example when the thermistor reaches a certain amount of temperature the fan gets turn on….thank you?

  22. Comrade thank you, I can't believe I couldn't find anything online when I typed in "thermal protection resistors" or "thermistor" even only came back with typical temp. measuring therms, not resistance regulating ones. PTC and NTC.. That's what I was missing I'm guessing.. Lol. Thanks again for the help, got yourself a like/subscrip!

  23. There was the "Boiling water freezes quicker" myth.
    Or is that boiled water.
    maybe the momentum of the temperature drop…
    Does measurement have momentum?
    There goes another year of my life.

  24. DID YOU NOTICE THAT YOUR WIRES THAT ARE CONECTED TO THE COMPONENT ARE ACTUALLY MELTING AS YOU PERFORM THIS EXCERCISE ON THE OUTTER PART OF THE POT.  Not to say that this isn't a good example of how this works but leaves the results open to inaccurate readings. " Perhaps a can of freeze and a heat gun or hair drier would have been a bit safer here "  However good effort

  25. in a real life scenario, aprox. how fast will let's say a NTC 5d-20 change from initial resistance to final resistance with 2A as final current in a current limiter configuration? Seconds, milliseconds? Minutes?

  26. as fan rotate only in one direction as usual ,but if we want to rotate it in one direction as a the temperature increases and rotates in opposite direction when the temperature decrease or vice versa .which equipment should we use tomake

    its our project can anyone help me in this matter please!!!!!

  27. Hi
    Thank you for your great help.
    I have a dishwasher thermistor on which T120K carved.

    I do not know how to find resistance vs temperature.

    Would you please help me?

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