NTC stands for 'Negative Temperature Coefficient', which means that the resistance of the NTC probe decreases as the temperature increases. They are mainly used as resistive temperature sensors and current limiting devices.
NTC probes are generally made with ceramics or polymers, and the different materials used in their manufacturing result in different temperature responses, as well as other distinct performance characteristics.
Characteristics of NTC probes
Response to temperature
The majority of NTC probes are usually suitable for use in a temperature range between -55 and 200 °C, where they provide their most accurate readings. There are special families of these that can be used at temperatures close to absolute zero, as well as those specifically designed for use above 150 °C.
The temperature sensitivity of an NTC probe is expressed as "percentage change per degree C" or "percentage change per degree K." Depending on the materials involved and the specifics of the production process, typical values for temperature sensitivities range from -3% to -6%/°C.
Self-heating effect
The self-heating effect is a phenomenon that occurs whenever there is a current flowing through an NTC probe. Since this is basically a resistor, it dissipates energy in the form of heat when there is a current flowing through it.
This heat is produced in the core of the probe and affects the accuracy of the measurements. The degree to which this occurs depends on the amount of current flowing from the environment, the temperature coefficient of the probe, the total area of the thermistor, among other factors.
The fact that the resistance of the NTC sensor and, therefore, the current passing through it, depends on the environment is often used in liquid presence detectors, such as those found in storage tanks.
Thermal capacity
The thermal capacity represents the amount of heat required to raise the temperature of the probe by 1 °C and is usually expressed in mJ/°C. Understanding the precise heat capacity is very important when using an NTC probe as an input current limiting device, as it defines the response time of the NTC temperature sensor.
Types of NTC probes
The materials that are commonly involved in the manufacturing of NTC resistors are platinum, nickel, cobalt, iron, and silicon oxides, used as pure elements or as ceramics and polymers. NTC probes can be classified into three groups, depending on the production process used.
Bead thermistor temperature probe
These NTC probes are made with lead wires of platinum alloy directly sintered into the ceramic body. They usually offer fast response times, better stability, and allow operation at higher temperatures than disk and chip NTC probes; however, they are more fragile.
It is common to cover them in glass to protect them from mechanical damage during assembly and to improve their measurement stability. Typical sizes range from 0.075 to 5 mm in diameter.
Disk and chip thermistor thermal probe
The probes of this type have metal surface contacts. They are larger and, as a result, have slower reaction times than pearl-type NTC resistors. However, due to their size, they have a higher dissipation constant, which is the power required to raise their temperature by 1 °C.
Since the power dissipated by the thermistor is proportional to the square of the current, they can handle higher currents much better than bead-type thermistors. Disk-type probes are produced by pressing a mixture of oxide powders into a round mold and then sintering it at high temperatures.
Chips are typically manufactured using a tape casting process in which a paste of material is spread into a thick film, dried, and cut to shape. Typical sizes range from 0.25 to 25 mm in diameter.
Glass-encapsulated thermistor temperature probe
These are NTC probes covered in a hermetically sealed glass bulb. They are designed for use at temperatures above 150 °C, or for mounting on printed circuit boards, where robustness is a requirement.
Encapsulating a thermistor in glass improves the stability of the sensor and protects it from the environment. They are manufactured by hermetically sealing NTC bead-type resistors in a glass container. The most common sizes range from 0.4 to 10 mm in diameter.
Common uses of NTC probes
NTC probes have a wide variety of applications. They are used to measure, control, and compensate for temperature. They can also be used to detect the absence or presence of a liquid, as current limiting devices in power supply circuits, to control temperature in automotive applications, among others. NTC sensors can be divided into three groups, depending on the electrical characteristic that is utilized in an application.
Resistance-temperature characteristic
The applications based on the resistance-temperature characteristic include measurement, control, and temperature compensation. They also include instances where an NTC probe is used so that the temperature of the NTC temperature sensor is related to some other physical phenomenon.
This group of applications requires the probe to operate under a zero power condition, which means that the current passing through it is kept as low as possible to avoid heating.
Current-time characteristic
The applications based on the current-time characteristic are: time delay, input current limitation, surge suppression, and many more. These characteristics are linked to the thermal capacity and the dissipation constant of the NTC probe used.
The circuit is usually based on the heating of the NTC probe due to the current passing through it. At a given moment, it will cause some type of change in the circuit, depending on the application in which it is used.
Voltage-current characteristic
The applications based on the voltage-current characteristic of a thermistor often involve changes in environmental conditions or variations in the circuit that cause changes in the operating point on a specific curve of the circuit. Depending on the application, this can be used for current limiting, temperature compensation, or temperature measurements.
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