Types of Thermistor

Thermistor Types

There are two types of thermistors:

· Negative Temperature Coefficient (NTC) Thermistor

· Positive Temperature Coefficient (PTC) Thermistor

NTC Thermistor

In an NTC thermistor, when the temperature increases, resistance decreases. And when temperature decreases, resistance increases. Hence in an NTC thermistor temperature and resistance are inversely proportional. These are the most common type of themistor.

The relationship between resistance and temperature in an NTC thermistor is governed by the following expression:

Where:

· RT is the resistance at temperature T (K)

· R0 is the resistance at temperature T0 (K)

· T0 is the reference temperature (normally 25oC)

· β is a constant, its value is dependant on the characteristics of the material. The nominal value is taken as 4000.

If the value of β is high, then the resistor–temperature relationship will be very good. A higher value of β means a higher variation in resistance for the same rise in temperature – hence you have increased the sensitivity (and hence accuracy) of the thermistor.

From the expression (1), we can obtain the resistance temperature co-efficient. This is nothing but the expression for the sensitivity of the thermistor.

Above we can clearly see that the αT has a negative sign. This negative sign indicates the negative resistance-temperature characteristics of the NTC thermistor.

If β = 4000 K and T = 298 K, then the αT = –0.0045/oK. This is much higher than the sensitivity of platinum RTD. This would be able to measure the very small changes in the temperature.

However, alternative forms of heavily doped thermistors are now available (at high cost) that have a positive temperature co-efficient. The expression (1) is such that it is not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistors is very definitely a non-linear sensor.

PTC Thermistor

A PTC thermistor has the reverse relationship between temperature and resistance. When temperature increases, the resistance increases. And when temperature decreases, resistance decreases. Hence in a PTC thermistor temperature and resistance are inversely proportional.

Although PTC thermistors are not as common as NTC thermistors, they are frequently used as a form of circuit protection. Similar to the function of fuses, PTC thermistors can act as current-limiting device.

When current passes through a device it will cause a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings then the device heats up. In a PTC thermistor, this heating up will also cause its resistance will increase. This creates a self-reinforcing effect that drives the resistance upwards, therefore limiting the current. In this way, it acts as a current limiting device – protecting the circuit.

Thermistor Characteristics

The relationship governing the characteristics of a thermistor is given below as:

Where:

R1 = resistance of the thermistor at absolute temperature T1[oK]

R2 = resistance of the thermistor at temperature T2 [oK]

β = constant depending upon the material of the transducer

We can see in the equation above that the relationship between temperature and resistance is highly nonlinear. A standard NTC thermistor usually exhibits a negative thermal resistance temperature coefficient of about 0.05/oC.