NTC and PTC

Just like you can build light resistive sensors, it is possible to build temperature resistive sensors. There are many types of temperature transducers, in this post I will talk just about NTC and PTC, also called thermistors. Others sensors will stay to others posts.

How it works?

The NTC decrease the resistance as the temperature rises and PTC increase resistance with the increase of temperature. These components are specified by the resistance in the environment temperature (usually 25°C). This graphic shows the typical behavior of thermistors, the scale vary depending on the manufacturer’s specifications.

The NTC offers an exponential response on the resistance variation with the temperature, in some cases it is necessary have a linear response. For that it is necessary to put a resistor in parallel with a NTC, also known as linearization resistor.

To calculate the linearization resistance:

$R_{lin}=R_{Tc}\frac{B-2T_{c}}{B+2T_{c}}$

$R_{lin}$ is the value of the linearization resistance;
$R_{Tc}$ is the thermistor’s resistance in the middle of the used band;
$T_{c}$ is the temperature in the middle of the used band;
$B$ is the sensor exponential coefficient.

B

is calculated in this form:

$B=\frac{T_{1}T_{2}}{T_{2}-T_{1}}ln\frac{R_{T1}}{R_{T2}}$

$T_{1}$ and $T_{2}$ are arbitrary temperatures;
$R_{T1}$ and $R_{T2}$ are the NTC resistances in $T_{1}$ and $T_{2}$ temperatures respectively.

Linearization can cause a reduction in sensitivity.

While the PTC, there are the silicon PTC which already has a linear behavior, also known as silistor, and the ceramic PTC. The behavior of both is showed in the graphic below.

The ceramic PTC have the Curie temperature, which is the temperature where is the lowest resistance, stays in the minimum point on the purple curve in the graphic above.

Materials

The most used materials to NTC are manganese, nickel and copper. To PTC the most used material is the polycrystalline ceramic in a barium titanate ( $BaTiO_{3}$ ) substrate.

Copper and manganese NTCs. Nickel plated copper NTC.

PVC NTC with epoxy resin encapsulation on the left and with ABS tube on the right.

Various others types of NTC.

Silicon PTC.

Polycrystalline ceramic PTC.

Application

PTCs are more used to protection circuits against short-circuit and over-current, heat regulation and temperature protection and the NTC is more used to temperature measure, compensation and control. You can use thermistors in the circuits in the same way the LDR is used, instead controlled by light it is controlled by temperature. Here is thermistor’s symbols used in schematics.