What is a temperature dependent resistor?

A temperature-dependent resistor, also called a thermistor, is an electronic component whose electrical resistance changes depending on the ambient temperature. They are fundamental components in many electronic devices and systems because they allow temperature changes to be precisely measured and monitored.

The physical principles underlying these functions are closely related to the properties of semiconductor materials from which the thermistors are made. As temperature increases, the number of electrons that can move freely increases, resulting in a change in electrical resistance. In the case of NTC thermistors, this leads to a decreasing resistance, while in the case of PTC thermistors, an increase in resistance is observed.

1. Basic types

Types of temperature dependent resistors

There are two main types of temperature dependent resistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). Both types of thermistors have specific characteristics that make them suitable for different applications.

NTC thermistors are the most commonly used types of temperature dependent resistors. They are designed so that their resistance value decreases with increasing temperature. This means that when the ambient temperature increases, the resistance of the NTC thermistor decreases. This effect is directly proportional, meaning that a given increase in temperature will result in a given decrease in resistance. NTC thermistors are commonly used in applications such as temperature sensors, thermal switches, and overload protection devices.

PTC thermistors, on the contrary, show an increase in electrical resistance with increasing temperature. This means that the resistance of a PTC thermistor increases as the ambient temperature increases. This type of temperature-dependent resistor is often used in situations where overcurrent protection is required. As the temperature rises (which is often a sign of an overcurrent condition), the resistance of the PTC thermistor increases, reducing the current flow through the circuit and protecting the device from damage.

The PT100 and PT1000 sensors have become indispensable in the world of PTC elements. These temperature sensors are extremely precise and offer a high degree of accuracy when measuring temperatures. They are used in numerous applications in both industry and the private sector, for example in control technology, refrigeration technology and heating technology. The PT100 and PT1000 sensors have the advantage that they are insensitive to electromagnetic interference fields and therefore guarantee high measurement accuracy. These properties make them a reliable tool for temperature measurements.

Both types of temperature-sensitive resistors play an important role in a variety of applications. NTC thermistors are ideal for applications where accurate temperature measurement is required, while PTC thermistors are particularly useful in situations where overcurrent protection is needed. Careful selection of the correct type of temperature-dependent resistor can significantly improve the performance and reliability of electronic devices and systems.

How does a temperature-dependent resistor work?

The change in resistance with temperature in an NTC thermistor can be explained by the behavior of semiconductors. As the temperature increases, more electron energy is added, allowing them to jump from the valence band to the conduction band. This increases the number of free charge carriers and decreases the resistance.

On the other hand, increasing the temperature of PTC thermistors leads to an increase in resistance because the increased temperature disturbs the crystal lattice structure of the material and thus impedes the flow of electrons.
The exact relationship between temperature and resistance is described by the Steinhart-Hart equation, which reads:

1/T = A + Bln(R) + C(ln(R))^3

Where T is the absolute temperature in Kelvin, R is the resistance of the thermistor, and A, B, and C are constants specific to the material of the thermistor. ln denotes the natural logarithm.

This equation allows the temperature to be determined very accurately based on the measured resistance of the thermistor. It is important to note that the constants A, B, and C must be determined by calibration, which is usually done by measuring the resistance at three known temperatures.

Applications of the temperature dependent resistor

Temperature dependent resistors are used in a wide variety of applications because they provide accurate and reliable temperature measurements. Here are some examples of their application:

Electronics: In electronics, thermistors are commonly used for temperature monitoring and control in devices such as computers, batteries, and heating or cooling systems. They help maintain the optimum operating temperature and protect against overheating.

Automotive: In the automotive sector, thermistors are used to monitor and control engine temperature, coolant temperature and air temperature in the intake tract. They help optimize fuel consumption and reduce emissions.

Medical devices: Thermistors are used in medical devices to accurately monitor patient body temperature. For example, they are used in digital thermometers and medical sensors.

Home appliances: Thermistors are used in household appliances such as refrigerators, ovens and air conditioners to control temperature. They help improve the efficiency of these appliances and reduce energy consumption.

Industrial process control: in industrial applications, thermistors are used to monitor and control process conditions. For example, they can be used in the food and beverage industry to monitor temperature during cooking or pasteurization.

Advantages and disadvantages of the temperature-dependent resistor

Temperature dependent resistors have a number of advantages and disadvantages that affect their use in various applications.

Advantages of temperature dependent resistors:

  • Wide temperature range: temperature-dependent resistors can cover a wide temperature range. For example, Pt resistance sensors can measure temperatures from -240°C to 1000°C.
  • High accuracy: Thermistors are known for their high accuracy and sensitivity, especially NTC thermistors.
  • Long-term stability: Pt resistance sensors are known for their high long-term stability, which means they can provide accurate measurements over long periods of time.
  • Ease of use: Temperature-dependent resistors are relatively simple to use and can be easily integrated into electronic circuits.

Disadvantages or challenges of using temperature-dependent resistors include

Non-linear behavior: NTC thermistors in particular exhibit non-linear behavior, meaning that the resistance does not change in proportion to the temperature. This can make interpretation of results difficult and often requires calibration or complex mathematics to accurately determine temperature.

  • Self-heating: thermistors can heat up due to current flow, which affects the measurement. This is especially relevant when measuring in low temperature or low heat capacity environments.
  • Limited temperature range: although some thermistors (such as Pt resistance sensors) can cover a wide temperature range, other types have a more limited range. For example, silicon-based PTC thermistors have a parabolic relationship between temperature and resistance and are suitable for specific temperature ranges.

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