PTC Thermistors
(55)Thermistor – what is it and how does it work?
Thermistors are resistive components whose resistance decreases or increases as the temperature changes. It is natural for resistance to increase when the temperature rises. As the molecules of the medium vibrate more and more, they increase its flow resistance. When the temperature increases, so does the resistance, the magnitude of which can be measured with an ohmmeter. Thermistors belong to a family of resistors with a very high sensitivity to temperature change. Thanks to this feature, these low-cost and easy-to-use electronic components allow for a very accurate evaluation of the temperature of the measured component or the environment. An important advantage of thermistors is their wide measuring range and high mechanical resistance.
As the temperature increases, however, the resistance of the thermistor does not necessarily increase. In order to find out what the differences between the various types of thermal resistors are and their significance in terms of use, it is necessary to delve into their most important characteristic features.
Design and types of thermistors – PTC and NTC thermistors
Thermistors are divided depending on their response to a change in temperature. There are two most common types of thermistors, whose behaviour varies dramatically after the change in temperature:
- PTC thermistors – thermistors with a positive temperature coefficient;
- NTC thermistors – thermistors with a negative temperature coefficient.
The former is a traditional example where the resistance value increases as the temperature increases. In contrast, the exact opposite effect is obtained with an NTC thermistor. An increase in temperature causes a decrease in resistance. The dependence of the change in resistance of a component on a change in temperature is described in the thermometric characteristic, which is described in the next section of the article.
Thermistors owe their properties to the use of suitable materials. The manufacturing process of a thermistor involves embedding the conductors in a mixture of powdered metal oxides and suitable chemical compounds, and bonding them using an appropriate bonding material. The thermistors created in this way undergo a sintering process to dry and harden the bond, and the bond itself is protected with a special glass coating.
For the manufacture of PTC thermistors, vanadium and titanium oxides doped with selected chemical compounds are most commonly used, while NTC sensors are usually made from a mixture of chemical compounds such as chromium, manganese, cobalt, iron, copper, lithium or nickel. The materials used in the construction of thermistors give them specific properties that determine their intended use.
In the wide range of products available on the market, we can find thermistors in various shapes. They may come in the form of balls, compressed discs resembling a tablet, or in the shape of a miniature cylinder. In each case, they feature two leads in the form of wires. Thanks to their simple design and compact size, thermistors are exceptionally low-cost components for electronic circuits.
The mechanical durability of thermal resistors allows them to measure temperatures ranging from -150°C to as high as 800°C, and their high measurement sensitivity makes them suitable for a number of demanding applications.
What limits thermistors is their maximum rated voltage, i.e. the voltage that is suitable for continuous operation of the component, as well as the maximum operating voltage, which is the condition for correct and safe operation. When selecting the right product, the limits recommended by the manufacturer must be taken into account, as exceeding the rated voltage may result in damage to the component.
Thermometric characteristic
The basic equation describing the change in resistance of a sensor as a function of the change in its temperature is the thermometric characteristic.
where:
RT – resistance of a thermistor at a specific temperature [Ω];
T – temperature of the thermistor [K];
T0 – reference temperature [K];
RT0 – resistance of the thermistor at the reference temperature [Ω];
B – material constant of the thermistor ranging from 2000 to 6000 [K]. The magnitude of this parameter strictly depends on the materials used and determines the sensitivity of the device. The higher the value, the higher the sensitivity of the thermistor.
Application of PTC thermistors
PTC thermistors (a.k.a. posistors) are used as overload and overtemperature protections. As the current increases, the temperature of the thermal resistor rises and the resistance increases as a logarithmic function. As a result of the increasing resistance, the current in the circuit decreases and stabilises at the required level. Thermistors can be found, among other things, in electric motors, common household appliances (e.g. microwaves or ovens), automation systems or in the automotive industry.
A special type of PTC thermistors are polymeric fuses. Their design is based on the thermistor principle – if the maximum current is exceeded, the temperature of the component rises and the resistance spikes, causing the circuit to break (however, a low current will still flow through the circuit).
Thermistors also have an important early warning function, being the most important part of fire protection systems. As soon as the temperature rises above the set point, the alarm system sends out a signal to warn of the possibility of a fire. Extremely sensitive and reliable measurement is very important in this case, which is guaranteed by the thermoresistive components discussed in this article.
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