A thermistor short for thermic resistor is a two legged temperature dependent resistor.

There are NTC and PTC variants, with a Negative or Positive Temperature Coefficient respectively.

## NTC

In an NTC, the resistance decreases with increasing temperature. NTCs are very robust and cheap, therefore they are widely used as temperature sensor. They can measure a wide range of temperatures and are quite sensitive around room temperatures which makes them very useful for example in thermometers and thermostats.

Common characteristics of a NTC are:

• $R_{25}$, its resistance value at 25 degrees Celsius (typically in the kilo ohm range)
• The change of resistance as function of temperature usually given in the form of a table with values for $\frac{R_T}{R_{25}}$ where $R_T$ is the resistance value at temperature T.

## PTC

In a PTC, the resistance increases with increasing temperature.

The PTC is more commonly known as the poly switch fuse. As the current through this resistor increases, it’s temperature increases. That again increases the resistance (PTC). In the end the increasing resistance limits the current. That lowers the temperature. And the current will get smaller automatically. So we have an automatic fuse, one that we don’t have to reactivate. In practical situations you might choose a PTC based on certain currents it can handle, rather than its resistance. The power supply circuitry of an Arduino Uno and a Raspberry Pi both make use of a PTC or Polyfuse to limit the current to safe values.

Polyfuse on Arduino Uno

Fun fact: the common light bulb is also a PTC. When the filament is still cold, the resistance is lower than when the lamp is turned on. That is why most current flows when the lamp is just turned on, and most lamp failures occur when you turn it on. A short flash is the result, as the filament burns out.