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!