The great advantage of thermistor is its high sensitivity. They are mainly used for room temperature measurements up to moderate high temperatures. They are popular in research and medical applications, such as electronic medical thermometers.
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Thermistor is a kind of thermistor whose resistance varies with temperature. Resistance is measured by passing a small measuring direct current (DC) through it and measuring the resulting voltage drop.
There are two basic types of thermistors, PTC and NTC:
PTC (positive temperature coefficient) devices show an increase in resistance as the temperature increases. PTC thermistor is a temperature dependent resistor made of barium titanate. It should be selected when the resistance needs to be greatly changed at a specific temperature or current level.
The NTC (negative temperature coefficient) device shows a decrease in resistance as the temperature increases. NTC thermistors are made up of semiconductors. They are made from a mixture of oxides of manganese, cobalt, copper or nickel. Their operating temperature ranges from ' 150 ° C to 600 ° C. For a temperature of about 700 ', devices using zirconia doped with rare earth oxides can be used. NTC should be selected when it is necessary to change the resistance continuously in a wide temperature range. They have mechanical, thermal and electrical stability as well as high sensitivity.
Thermistors are commercially available in various shapes: beads, discs, chips and probes. The most common form is a bead with two wires attached. The bead diameter can be about 0.5mm to 5mm.
Small size thermistors make them very adaptable and are usually included in electronic circuits. They can be encapsulated in epoxy or glass or painted. Thermistors are simple, robust and very reliable
Temperature dependence of resistance
The resistance of CTN thermistor decreases exponentially with the increase of temperature. It is negative and nonlinear, as shown by the following relation:
Where R 0 is the nominal resistance measured at the absolute temperature t0 (usually measured at 25 ° C) and B is the constant of the specific thermistor material. The nonlinear characteristics of resistors may be undesirable, and this can be offset by using two or more thermistors in series or in parallel, packaged in a single device.
The temperature coefficient of resistance α, expressed in% / K (or% / ° C), is defined as follows:
The coefficient α of PTC thermistor is positive, while that of NTC thermistor is negative.
The Steinhart & Hart equation can be used to determine the temperature from the thermistor:
Where R is the thermistor in ohm, t is the absolute temperature, K, a, B and C are constants, which are usually provided by the manufacturer, but can be determined by calibrating at three different temperatures and solving three simultaneous equations.
This equation is very close to the actual device, but it does not always provide the required accuracy over the entire temperature range. This can be corrected by fitting the Steinhart & Hart equations over a range of narrow temperature ranges and then 'stitching' them together to cover the desired range.
Advantages and disadvantages of thermistor
The great advantage of thermistors is that they can be point measured
Compared with other temperature sensors, their high sensitivity allows thermistors to operate in a small temperature range and are very accurate (usually better than 0.05 ° C and 0.1 ° C). The sensitivity of thermistor can be one order of magnitude higher than that of resistance thermometer (RTD)
The most common temperature range is 0 ° C to 100 ° C. At higher temperatures, they drift greatly
The high resistivity of thermistors eliminates the need for a four wire bridge circuit
Thermistors are very nonlinear and not robust, which limits their application
The error source is due to the self heating effect caused by excessive bias current when the thermistor is powered on to provide the output voltage signal. In order to reduce this source of error, a current suitable for measuring resistance must be used.
Due to its performance and moderate cost, thermistors are suitable for temperature measurement and control, temperature compensation, electronics, medical and many other applications.
PTC thermistors are essential components in the realm of electronics and temperature control. This article will explore their definition, working principles, various types, advantages, limitations, popular applications, and the critical distinctions between PTC thermistors and fuses. Join us on this enlightening journey as we explore the world of these crucial electronic devices.
A PTC thermistor, short for Positive Temperature Coefficient thermistor, is a type of resistor that exhibits a significant change in electrical resistance with temperature variations. Unlike conventional resistors, PTC thermistors have a positive temperature coefficient, which means their resistance increases as the temperature rises. This unique characteristic makes them useful in numerous temperature-sensing and control applications.
PTC thermistors work based on the behavior of certain semiconductor materials, typically containing barium titanate or other similar compounds. At lower temperatures, these materials have a crystalline structure that allows a relatively free flow of electrons, resulting in low resistance. However, as the temperature increases, the crystal structure changes, restricting the flow of electrons and causing a significant rise in resistance.
This positive temperature coefficient effect enables PTC thermistors to be excellent temperature sensors. By measuring the changes in resistance, it becomes possible to monitor and control temperature-related processes accurately.
Chip Thermistors:
These PTC thermistors are small, often square or rectangular. Electronic circuits are commonly used for temperature sensing and control.
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They are manufactured using semiconductor materials such as barium titanate and exhibit a positive temperature coefficient, meaning their resistance increases with temperature.
Chip thermistors find applications in devices like temperature sensors, heating elements, and overcurrent protection circuits.
Bead Thermistors:
Bead thermistors are typically small, round components encased in a protective material such as glass or epoxy resin.
They are commonly used for temperature measurement in various industries, including automotive, medical, and HVAC systems.
Bead thermistors provide accurate temperature-sensing capabilities; manufacturers often integrate them into probes or sensors for specific applications.
Disk Thermistors:
Disk thermistors are more extensive compared to chip and bead thermistors. They are suitable for applications that require higher power dissipation.
Electronic devices often use them for motor starting, overcurrent protection, and temperature compensation.
Due to their larger surface area, disk thermistors can handle higher currents and power levels, making them suitable for demanding industrial applications.
Glass-Coated Thermistors:
Manufacturers coat these thermistors with a thin layer of glass to protect them against environmental factors like moisture, chemicals, and mechanical stress.
Glass-coated thermistors are commonly used in harsh environments where temperature-sensing accuracy and reliability are crucial.
They are applicable in automotive, aerospace, and industrial automation industries, where the sensors face exposure to extreme conditions.
Surface Mount Thermistors:
Surface mount thermistors are designed for easy integration onto printed circuit boards (PCBs) using surface mount technology (SMT).
They come in various package sizes and shapes, allowing for flexibility in design and placement on PCBs.
Surface mount thermistors are widely used in consumer electronics, telecommunications, and industrial equipment for temperature sensing and control.
Probe Thermistors:
Probe thermistors consist of a sensing element encapsulated within a metal or plastic housing, often with a pointed tip for insertion into liquids, gases, or solids.
They are used in accurate temperature measurements for food processing, HVAC systems, and scientific research applications.
Probe thermistors offer fast response times and high accuracy, making them suitable for industrial and laboratory environments.
Advantages:
Limitations:
The versatility of PTC thermistors enables their utilization in various applications across different industries. Some notable applications include:
Temperature Sensing and Control: PTC thermistors are widely used in thermostats, HVAC systems, refrigerators, and other devices that require precise temperature control.
Overcurrent Protection: Switching PTC thermistors are employed as self-resetting fuses in electronic circuits to safeguard against overcurrent and short circuits.
Motor Starters: PTC thermistors are utilized in motor starter circuits to provide inrush current protection and prevent damage to the motor.
Heating Elements: Heating PTC thermistors are commonly found in electric heaters, hair dryers, and automotive seat heaters, where they efficiently convert electrical energy into heat.
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