The thermistor is a temperature-sensitive resistor that is made of semiconductor materials. They are used in a wide range of applications that require precise temperature measurement and control. Thermistors are divided into two types based on their temperature coefficient of resistance (TCR): negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thermistors. This article will discuss the properties, applications, and working principles of thermistors in detail.
Properties of Thermistors:
Thermistors are made of semiconductor materials, typically oxides of nickel, manganese, or cobalt. These materials have a high resistance at room temperature, which decreases significantly with increasing temperature. The resistance of a thermistor can be modeled using the Steinhart-Hart equation, which relates resistance to temperature:
1/T = A + Bln(R) + C(ln(R))^3
where T is the temperature, R is the resistance, and A, B, and C are constants that depend on the material and the specific thermistor.
NTC thermistors have a negative TCR, meaning their resistance decreases as temperature increases. PTC thermistors have a positive TCR, meaning their resistance increases as temperature increases. The TCR of a thermistor is an important parameter that determines its sensitivity to temperature changes.
Thermistors have a high sensitivity to temperature changes, which makes them ideal for temperature measurement and control applications. They are also small in size and relatively inexpensive, which makes them suitable for use in a wide range of applications.
Working Principle of Thermistors:
The working principle of a thermistor is based on the fact that the resistance of a semiconductor material changes with temperature. When a thermistor is exposed to a change in temperature, its resistance changes, which can be used to measure the temperature.
The resistance of a thermistor can be measured using a simple circuit that includes a voltage source, a resistor, and the thermistor. When the temperature changes, the resistance of the thermistor changes, which leads to a change in the voltage across the resistor. This voltage change can be measured and used to calculate the temperature.
Types of Thermistor:
The sensitivity of a thermistor to temperature changes depends on its TCR. NTC thermistors have a higher sensitivity to temperature changes than PTC thermistors, which makes them more suitable for temperature measurement applications. Following types of thermistors are available.
Negative Temperature Coefficient (NTC) thermistors:
These thermistors have a negative temperature coefficient of resistance, which means that their resistance decreases as the temperature increases. They are commonly used as temperature sensors, inrush current limiters, and temperature-compensating circuits.
Positive Temperature Coefficient (PTC) thermistors:
These thermistors have a positive temperature coefficient of resistance, which means that their resistance increases as the temperature increases. They are commonly used in overcurrent protection circuits, self-regulating heaters, and temperature sensors.
Bead thermistors:
These are the most common type of thermistors and are used in temperature sensors and control systems. They are small, cylindrical, and have a high sensitivity to temperature changes.
Disk thermistors:
These thermistors are used in overcurrent protection circuits and temperature compensation circuits. They are flat, disk-shaped, and have a high resistance.
Chip thermistors:
These thermistors are used in electronic circuits, such as voltage regulators and temperature-controlled oscillators. They are small and compact, and can be integrated into electronic devices.
Glass-encapsulated thermistors:
These thermistors are used in high-temperature applications and have a glass coating that protects them from damage.
Epoxy-coated thermistors:
These thermistors are used in harsh environments and have an epoxy coating that protects them from moisture, dust, and other contaminants.
Interchangeable thermistors:
These thermistors are designed to have a specific resistance at a specific temperature and can be used interchangeably with other thermistors of the same type.
Applications of Thermistors:
Thermistors are used in a variety of applications that require precise temperature measurement and control. Some of the most common applications of thermistors are discussed below.
Temperature Sensors:
Thermistors are commonly used as temperature sensors in thermostats, refrigerators, and other temperature control systems. They are also used in medical devices, such as thermometers and blood pressure monitors, to measure body temperature.
Overcurrent Protection:
Thermistors are used in overcurrent protection circuits in power supplies and motor controllers. When the current through a circuit exceeds a certain threshold, the thermistor heats up and its resistance increases, which reduces the current flow and protects the circuit from damage.
Temperature Compensation:
Thermistors are used in temperature compensation circuits to stabilize the output of electronic components, such as transistors and diodes. The resistance of a thermistor changes with temperature, which can be used to compensate for variations in the characteristics of these components due to temperature changes.
Automotive Applications:
Thermistors are used in automotive applications, such as engine temperature sensors and airbag sensors. They are also used in battery management systems to monitor the temperature of the battery and prevent overheating.
