A thermistor is a temperature-sensitive resistor.
In fact, the word thermistor is a portmanteau combining the words thermal and resistor.
In this lesson we will be looking at how thermistors work, and briefly examining how they are used.
There are two general types of thermistor. Those with positive temperature coefficients (PTCs) and those with negative temperature coefficients (NTCs).
Positive temperature coefficient thermistors increase their resistance as temperature increases.
And negative temperature coefficient thermistors, on the other hand, decrease their resistance as temperature increases.
To explain how this works we're going to have to briefly go over some theory on how semiconductors work.
As you may know, electrons don't really orbit a nucleus, like planets orbit the sun.
Instead the electrons randomly zip around the nucleus in mathematically defined positions we call electron clouds, where the exact position isn't known at any one time.
The key feature for us is that even though the electrons all exist in the same cloud at the same time, they have different energy levels.
But electrons can only exist within certain energy levels.
We can represent these available energy levels as energy bands.
And between the energy bands are the forbidden bands.
These are energy levels that electrons are not able to remain in.
In order to move up a band an electron will have to gain enough energy to jump over the forbidden zone.
Losing the energy increases the chance that the electron will be recaptured by the lower energy band.
Different elements each have different energy bands which can contain a different number of electrons. It all gets very complicated and is way beyond the scope of this lesson.
The only bands we need to concern ourselves with are the last two.
The bottom one is the valence band. This is the highest energy level available for electrons that will have a strong attachment to the atom. There is a strong draw on electrons to fill this band.
But there is another band above this one. This is the conduction band.
Electrons which have sufficient energy to reach this band will only have a loose attachment to the atom.
Conduction band electrons are easily drawn away from the atom and are available to flow in an electric current.
But, how does this all relate to thermistors?
Thermistors are made of semiconductors, which have very particular properties because of their energy bands.
As mentioned in our Diodes lesson, n-type semiconductors have extra valence electrons floating around.
It turns out that since these extra electrons come from a different element (whatever it is which was used to dope the silicon), they have their own energy band.
This is called the donor level.
Those extra electrons are also known as donor electrons.
Notice that the energy level of the donor level is only a little less than is required to boost up into the conduction band.
Negative temperature coefficient thermistors are made from these n-type semiconductors.
When the NTC thermistor is exposed to thermal energy those electrons easily gain the additional energy required to get to the conduction band.
This has the effect of increasing the conductivity of the NTC thermistor. Or, put another way, it decreases the resistance of the NTC thermistor.
Positive temperature coefficient thermistors, naturally, work a little differently.
PTCs are made from p-type semiconductor material.
In p-type semiconductors, instead of having extra electrons, the valence band has fewer. This is represented as holes in the valence band.
These holes are just waiting for the opportunity to grab unsuspecting electrons.
When thermal energy is added to a semiconductor it is not only the electrons which gain energy. The molecular structure of the material also gains energy.
This causes the molecules to vibrate more.
And the more a molecule vibrates, the greater the odds that it will interfere with a passing electron.
Interfering with an electron can mean scattering it in a new direction, which will cause it to lose energy.
This happens in n-type semiconductors as well, but it's not really an issue as there are so many extra electrons zipping around.
In the case of p-type semiconductors it also means that the electron may be captured by one of those available holes.
This has the effect of lowering the thermistor's conductivity, increasing its resistance.
It is possible to keep heating a positive temperature coefficient thermistor until it begins boosting electrons up to the conduction band once more.
But at that point you are most likely way outside of the temperature range that the equipment was intended for, and probably have bigger problems.
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