Positive Temperature Coefficient Of Resistance

The temperature coefficient of resistance is commonly described as even the change in material’s electrical resistance as temperature changes by one degree.

So, when we look at the electrical resistance of conductors like gold, aluminium, silver, and copper, it all comes down to the process of electron collision within the substance. When the temperature rises, the process of electron collision accelerates. As a result, as the conductor’s temperature increases, so will its resistance.

Temperature Coefficient of Resistance

There are two basic reasons why material resistance varies with temperature.

One effect is caused by the number of collisions between charged particles and ions in the substance. Because the frequency of collisions rises with temperature, it is reasonable to expect a small boost in resistance with temp.

This may not always be the case because a few materials have a negative resistance value. This may be explained by the fact that more charge carriers are discharged when the temperature rises, resulting in a drop in resistivity with temperature. This phenomenon is common in semiconductor materials, as one might assume.

When examining the temperature dependence of resistance, it is commonly believed that the temperature coefficient of resistance maintains a linear law. This is true at room temperature and for metals and several other things. However, it has been observed that the resistant effects caused by the number of encounters are not necessarily constant, especially for these materials at extremely low temperatures.

It has been demonstrated that resistivity is inversely related to the mean free route between collisions, resulting in rising resistance with rising temperature. For temperatures above about 15°K, this would be limited by mechanical vibrations of the atoms, resulting in the characteristic linear area. Impurities restrict resistivity below this temperature.

The resistance of a conductor at any given temperature may be determined using the temp, its temperature correlation of resistance, its resistance at a reference temperature, and the temperature of operation. In general, the resistance temperature dependency formula is as follows:

R = R'(1+a(T-T’))

where,

R = resistance at T

R’= resistance at T’

T = material temperature

T’= reference temperature

a = temperature coefficient for resistance 

Positive Temperature Coefficient

A Positive Temperature Coefficient is a substance that increases in electrical resistance as its temperature rises. The materials with a higher coefficient increase quickly with temperature. A Positive Temperature Coefficient material is intended to attain the highest temperature possible for quite a given i/p potential because electrical resistance increases as temperature rises to a certain threshold. Unlike Negative Temperature Coefficient materials or linear resistance heating, the positive temperature coefficient of resistance elements is self-limiting. Some materials, such as Positive Temperature Coefficient rubber, have an exponentially increasing temperature coefficient.

Positive Temperature Coefficient Thermistors

Positive temperature coefficient thermistors are resistors having a positive temperature coefficient, which means that resistance increases as temperature rises.

Positive temperature coefficient thermistors are classified into two types depending on the materials used, the structure of the device, and the production method. The original positive temperature coefficient thermistors are silistors, which employ silicon as the semiconducting material. Because of their linear feature, they are employed as positive temperature coefficient temperature sensors.

The swapping type positive temperature coefficient thermistor is the second category. The resistance-temperature curve of a swapping type positive temperature coefficient thermistor is very nonlinear. When a swapping type positive temperature coefficient thermistor is warmed, the resistance initially decreases until a critical temperature is achieved. The resistance increases significantly when the temperature climbs over the critical threshold. This kind of positive temperature coefficient thermistor is commonly used in positive temperature coefficient heaters, sensors, and other applications. This second group includes polymer positive temperature coefficient thermistors, composed of a specific material and frequently employed as resettable fuses.

Uses of Positive Temperature Coefficient Thermistors

Heaters that Regulate Themselves

If a current is sent via a switching PTC thermistor, it will self-stabilise at a certain temperature. It implies that when the temperature drops, so does the resistance, allowing more current to pass and therefore heating the device. Similarly, when the temperature rises, so does the resistance, restricting the current travelling through the device and cooling it. The power consumption by the PTC thermistor has then become nearly independent of voltage over a rather large voltage range. Positive Temperature Coefficient thermistors are frequently built of ceramics in various forms and sizes, and PTC ceramic heaters are an excellent alternative for producing regulated electrical heat due to their design versatility.

Starting the Motor

In such instances, the self-heating function of a PTC thermistor in series with such a winding can be exploited. The PTC transistor has a low impedance when the circuit is activated, enabling current to flow through the starting winding. As the motor begins, the PTC thermistor warms up and changes to a high resistance state at one point. The time necessary for this is determined using the required motor startup time. When the PTC thermistor is heated, the current across it becomes insignificant, and the starting winding current is turned off.

Conclusion

A positive coefficient for a material indicates that its resistance increases as temperature rises. Pure metals often have thermal coefficients of resistance that are positive. Certain metals can be alloyed to produce coefficients close to zero. This was all about the positive thermal coefficients of resistance. I hope this helps you!