This article takes an in depth look at thermistors.
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A thermistor, a shortened version of the term thermal resistor, is a passive component whose resistance changes as the temperature in a system changes. Thus they serve as an inexpensive, accurate, and dynamic method for measuring temperature. The amount of resistance of a thermistor is dependent on the materials used to produce them.
Thermistors are used to monitor the temperature surrounding a device and temperature changes in a device. Changes in temperature, detected by a thermistor, influences equipment and is used for temperature sensing and overload cut outs. Thermistors can be found in an assortment of circuits, equipment, and devices providing a low cost method for temperature measurement.
The precise control of temperature is a crucial aspect of many manufacturing processes. The precision and accuracy of temperature control can determine the success or failure of an application. In most cases, thermistors are at the heart of any temperature measuring device due to their cost and accuracy.
Wherever and whenever temperature needs to be measured, whether it is in industrial applications or home cooking, a thermistor is used to determine, control, and monitor the temperature. A common use for thermistors is as a part of an HVAC system, which are responsible for thermal support and air flow.
Thermostats are used in HVAC systems to monitor the temperature. The central part of a thermostat is a highly sensitive thermistor. The temperature control on an HVAC system consists of simple circuit components that include an operational amplifier, thermistor, and a relay with the thermistor being the main temperature sensor in the circuit.
Thermistors are widely used in automobiles to measure the temperature of oil and coolant. They are the device that lets the driver know if the vehicle is overheating. Thermistors are directly connected to the instruments on the dashboard and gather necessary information about the efficiency of a vehicle's operation.
The location of the thermistor in the image of the engine below can be seen on the left of the image.
Every microwave has a thermistor for determining and maintaining the temperature of the microwave and is designed to prevent microwaves from overheating and catching fire.
The process of recharging a battery produces heat that has to be controlled. Included in recharging units is a low resistance thermistor that monitors the recharging process. If things get too hot, the thermistor stops the charging to prevent any accidents or damage.
As cell phones continue to get smaller, more compact, and technically advanced, they have a greater potential of overheating. Thermistors detect heat within the phone and relay the collected data to the IC. Thermistors in cell phones allow electrical components to operate efficiently and accurately when heat is detected.
The purpose of a thermistor in a washing machine is to determine when the optimum temperature has been reached for proper operation of the machine. When an error code appears on a washing machine’s display regarding a heating error, it indicates a faulty thermistor or a problem with the heating element. Thermistors ensure that the proper temperature is maintained and are an essential component for washer and dryer operation.
Since an electricity overload creates heat, surge protectors are necessary to prevent overloads that could potentially damage equipment. Thermistors are placed in surge protectors to control surges of energy. As an overload occurs, there is a heat buildup. The thermistor identifies the buildup and shuts down the flow of current.
A thermistor in a refrigerator is a method for collecting information regarding the freezer, evaporator, and refrigerator. It monitors the temperature of the refrigerator and sends the collected data to the control board. In the evaporator, a thermistor is attached to the top of the evaporator coil. A refrigerator can have from five to nine different thermistors that monitor every aspect of a refrigerator’s operation.
As with any resistor, a thermistor resists electrical current. However unlike a resistor, thermistors affect electrical current depending on the temperature. The resistance of a thermistor to electrical current changes in accordance with a change in the temperature. The various varieties of thermistors all work using the same principle regarding temperature fluctuations.
The two general categories of thermistors are negative temperature coefficient (NTC) and positive temperature coefficient (PTC) with the NTC version being the most commonly used type.
With NTC thermistors, resistance decreases as the temperature increases. NTC thermistors are made of a semiconductor material with conductivity between electrical and non-electrical conductors. When a component heats up, electrons are loosened from the lattice atoms. They leave and transport electricity easily. As the temperature increases, a thermistor moves electricity quickly and efficiently.
The behavior of a NTC thermistor varies depending on its components. Producers change the mixing ratio of oxides or doping metals to meet the desired requirements. Another factor in the manufacturing process is the oxygen content in the firing and the variations in the cooling rate.
NTC thermistors are made in discs, rods, plates, beads, or chips using a sintered metal oxide. Metallic oxide NTC thermistors are made from a fine power that is compressed and sintered. The materials include manganese, nickel, copper, iron, and titanium, as well as silicon or germanium crystals.
The method of conduction for NTC thermistors varies according to the types of materials used in their manufacture. The choice of materials is determined by the temperature range that will be measured.
Germanium – 1° Kelvin (K) to 100° K or – 272° C to -173° C Silicon – up to 250° K or -23° C Metallic Oxide – 200° K to 700° K or -73° C to 426° CFor higher temperatures, thermistors are made from aluminum oxide (Al2O3), Beryllium oxide (BeO), Zirconium dioxide (ZrO2), Yttrium oxide (Y2O3), and Dysprosium oxide (Dy2O3).
NTC thermistors come in a wide variety of sizes to fit any application. They are an important part of the electronics industry, which uses them in a small bead size. The variations in sizes create variations in a thermistor’s properties and characteristics.
Glass encapsulated NTC thermistors are completely sealed to eliminate the possibility of reading errors. Their encapsulation makes them applicable for severe environmental conditions and few limitations to their use. Glass encapsulated thermistors have an operating range of -55° C to 200° C or -67° F to 392° F. They are exceptionally accurate, have a quick response time, and are very small for easy placement.
As can be concluded, PTC thermistors work in the opposite direction from NTC thermistors. With PTC thermistors, as the temperature increases, the resistance in the thermistor increases. There are two types of PTC thermistors, with one showing linear increase while the other shows sudden changes in resistance. The two types are known as switching and silistor.
Switching PTC thermistors are non-linear. The resistance initially falls a little with an increase in temperature. Once it reaches a certain level, the resistance increases rapidly, which makes it ideal for protective use.
Silistor PTC thermistors have a semiconductor as its base material and are linear. The basic use of PTC thermistors is found in a variety of temperature sensing equipment. They are made from doped silicon, where the level of doping determines their characteristics.
The switching type of PTC thermistor is very widely used. They are made from polycrystalline materials such as barium carbonate or titanium oxide as well as silica, tantalum, and manganese. In the manufacturing process, the materials are ground into a powder and compressed to fit the shape of the thermistor.
The uses for switching PTC thermistors are self heating and sensor.
In the self heating mode, current passes through the thermistor. As the thermistor heats, it achieves the critical temperature for the device, and the thermistor's temperature radically increases. This characteristic makes it an ideal form of a regulator.
The sensor mode switching PTC thermistor has minimum current passing through it but senses the temperature of the surroundings. Its purpose is to ensure that the surrounding temperature does not affect the monitored device. When the environmental temperature reaches a crucial level, the thermistor’s resistance significantly increases.
There are several ways that thermistors are made and include the use of metallic oxides, binders, pressed wafers, and being cut to form chips, discs, or several other shapes. The composite of materials determines a thermistor’s temperature curve, which is precisely controlled for optimum thermistor function.
The term thermistor derives from the combination of thermally sensitive resistors. The materials used to produce thermistors are electrically resistant and dependent on whether the thermistor is NTC or PTC.
The main materials used in the manufacturing of thermistors are manganese, nickel, and cobalt, with resistivities of 100 ohm to 450,000 ohm.
Bead shaped thermistors are fabricated by applying a slurry of metal oxides with a binder onto spaced platinum alloy lead wires. The binder is the essential part of the process and supplies the appropriate level of surface tension to draw the material into the bead shape. Bead thermistors offer high stability, fast response, and operate at high temperatures and exhibit a low dissipation constant. They can be as small as 0.01 mm to 1.2 mm or 0.05 in.
Disc type thermistors are manufactured by pressing oxide powders into a circular mold. The pressed materials are sintered at a high temperature under pressure to form a cylindrical shape with diameters of 2.5 mm to 25 mm or 0.094 in to 0.98 in. The widely varying sizes of disc type thermistors offers a selection of thermistors to fit any application.
Though there are several different thermistor configurations, the three most common are hermetically sealed flexible (HSTH), bolt on or washer thermistors, and surface mounted thermistors.
HSTH thermistor sensors are hermetically sealed at the tip of the sensors as a method of protection against corrosive environments. The sealing material is the plastic polymer PerFluroroAlkoxy (PFA), a transparent and flexible fluoropolymer that is chemically inert for use in chemical and solvent applications. HSTH thermistors are available in three resistance values, which are 2252Ω, 5000Ω and 10000Ω.
A bolt on or washer type thermistor is designed for fast response, harsh environments, and capable of being adapted to any application. They are very easy to install and are inexpensive. Bolt on and washer thermistors are made by pressing the thermistor material under extreme pressure into flat cylindrical shapes with diameters of 3 mm to 25 mm or 0.12 in to 0.98 in.
Surface mounted thermistors have an adhesive material on the bottom of their sensor that can adhere to any type of surface. They are a type of NTC chip thermistor and are ideal for use in temperature compensation networks.
Ceramic switching PTC thermistors are made of polycrystalline ceramic that contains barium titanate and is doped with earth material to give it its positive temperature coefficient resistance. They have a very high non-linear resistance temperature curve.
Polymeric (PPTC) thermistors are made of non-conductive crystalline organic materials that are mixed with black carbon particles. The mixture of these materials causes them to be non-conductive. PPTC thermistors react to changes in the ambient temperature and automatically reset when fault conditions are eliminated.
The basic working principle of a thermistor is that its resistance is dependent on a temperature change. The resistance of a thermistor is measured by an ohm meter, which is a device that measures electrical resistance.
By knowing the relationship between how temperature changes affect the resistance in a thermistor, the acquired data can be used to obtain a temperature reading. The relationship between the two factors is non-linear.
All resistors have a change in temperature when they are in use, an effect that is measured by temperature coefficient resistance, which is a percentage change in resistance when there is a change in temperature. Thus with typical resistors, there is a form of temperature change during performance. For thermistors, a large temperature coefficient resistance change is necessary to be able to measure the temperature.
As a temperature sensor, a thermistor is placed in the body of the device for which it will measure the temperature and is also connected to an electrical circuit. When the temperature in the device changes, the resistance in the thermistor changes. The change is recorded by the directly connected circuit and is calibrated against the set temperature.
Thermistors come with two wires with one wire connected to the excitation source that measures the voltage of the thermistor. The advantage of thermistors is their ability to provide a huge change in resistance value when there is a temperature change, offering a more sensitive and accurate reading.
The concepts of a thermistor are based on the Steinhart-Hart Coefficient that is a mathematical method for deriving precise temperature readings.
The measurement of temperature is very common and one that most people monitor every day. Every home has a large number of temperature measuring devices the majority of which include thermistors. Thermistors can be found in fire alarms, refrigerators, ovens, boilers, and microwaves. Their unique ability to change electrical resistance into temperature readings makes them a very beneficial and accurate tool.
There are several types of sensors available for measuring temperature including thermocouples and resistance temperature detectors (RTD). Though every device provides the same accurate data, many manufacturers choose thermistors over other methods.
Cost is one of the main driving forces behind the popularity of thermistors. They are capable of providing accurate and precise data for a small temperature range at minimal cost.
Thermistors have a compact design and are fabricated in many forms, including beads, discs, and rods. Though they are available in small sizes, thermistors are exceptionally durable and long lasting.
When a device is turned on, it is charged with an abnormally high amount of current, which is referred to as the inrush. Without protection, damage and harmful results may occur to the device. NTC thermistors are used as inrush current limiters (ICLs) to protect sensitive circuits. Inrush currents can damage capacitors, harm power switch contacts, and destroy rectifier diodes.
The flow of current in an electrical circuit produces heat that is dissipated. The created heat increases the temperature of the resistor. With a thermistor, the definite amount of resistance is reached, and the heat is reduced.
Though thermistors are mainly known as components for measuring temperature, they have found use as a means of measuring pressure, liquid levels, and power. They are used as overload protectors and provide warnings of malfunctions.
Thermistors are installed at a measured distance from a circuit. This form of installation avoids errors in the readings due to resistance in the lead. Since thermistors operate over a small temperature range, their readings are more precise. They respond rapidly to small and minute temperature changes.
Since thermistors can respond to slight incremental changes in temperature, they provide data instantly with little delay. This particular property is also due to the small range of temperatures they monitor.
There is an endless variety of thermistors that can be adapted, changed, and configured for any type of temperature application. Their multiple types and sizes allow them to be used in any operation, condition, or situation.
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