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!