Contents
- What is a thermistor, general provisions
- Where it is used (scope)
- Device and types
- Types according to the principle of action
- Thermal response classification
- By type of heating
- The main parameters of thermistors
- Basic characteristics of thermistors
- General operating principle
- NTC
- PTC
- How to check with a multimeter
- How to connect
- Where is it on the diagram
- SMD and built-in thermistors
People who are far from radio electronics vaguely imagine the purpose and principle of operation of the thermistor. What is the function of this element? Is it provided for him? How is it marked? What subtleties of verification and connection do you need to know about? What are the types, and what are their features? These and other questions will be discussed below.
What is a thermistor, general provisions
Thermistor is a semiconductor element with varying characteristics (in terms of resistance) depending on temperature. The product was invented in 1930, and the famous scientist Samuel Ruben is considered its creator.
Since its inception, the thermistor has become widespread in radio electronics and has been successfully used in many related fields.
The part is manufactured using materials that have a high temperature coefficient (TC). It is based on special semiconductors, which are superior in characteristics to the most pure metals and their alloys.
When obtaining the main resistive element, oxides of some metals, halides and chalcogenides are used. Copper, nickel, manganese, cobalt, germanium, silicon and other substances are used for manufacturing.
In the manufacturing process, the semiconductor will have a different shape. On sale you can find thermistors in the form of thin tubes, large washers, thin plates or small round elements. Some parts have dimensions of several microns.
The main types of thermistors are thermistors and posistors (with negative and positive TCR (temperature coefficient of resistance), respectively. In thermistors, with increasing temperature, the resistance drops, and the posistors, on the contrary, increase.
Where it is used (scope)
Thermistors are actively used in various fields closely related to electronics. They are especially important in the implementation of processes that depend on the correct setting of the temperature regime.
This approach is relevant for computer technology, information transmission devices, high-precision industrial equipment, etc.
A common way to use thermistors is to limit the currents that occur during the start-up of devices.
When voltage is applied to the PSU, the capacitor quickly gains capacitance, which leads to the flow of increased current. If this parameter is not limited, the risk of damage (breakdown) of the diode bridge is high.
To protect an expensive node, a thermistor is used – an element that limits the current in case of sudden heating. After normalization of the mode, the temperature decreases to a safe level, and the resistance of the thermistor returns to its original level.
Device and types
The thermistor is a semiconductor element that, depending on the type, changes resistance with an increase / decrease in temperature. Today there are two types of products:
- Thermistors are negative temperature coefficient (NTC) parts. Their peculiarity is the drop in resistance with increasing temperature.
- Thermistors are elements that have a “plus” temperature coefficient (PTC). Unlike the previous species, with an increase in T, the resistance, on the contrary, increases.
Depending on the type of semiconductor, different elements are used in its production. As noted, when creating resistive elements, oxides, chalcogenides, and halides of various metals are used, and the design may vary depending on the intended use.
Types according to the principle of action
Thermistors differ in the principle of operation. There are two types:
- CONTACT. This category includes thermocouples, temperature sensors, filled thermometers and bimetal type thermometers.
- CONTACTLESS. This group includes thermistors built on the infrared principle of operation. They are actively used in the defense sector, due to the ability to detect thermal radiation of infrared and optical rays (released by gases and liquids).
Thermal response classification
Thermistors differ in the temperature to which they react when triggered. From this position, the following types of parts are distinguished:
- LOW TEMPERATURE. Such elements work at temperatures below 170 Kelvin (minus 1020C). 1 Kelvin = minus 272,150C.
- MEDIUM TEMPERATURE. Here the operating range is higher and lies between 170 and 510 Kelvin.
- HIGH TEMPERATURE. Thermistors of this class operate at temperatures from 570 Kelvin.
- SEPARATE CLASS. An individual group of high-temperature thermal resistors operating in the range from 900 to 1300 K will also stand out.
Regardless of the type (posistors, thermistors), thermistors can operate in different temperature conditions and environmental conditions. When operating in conditions of frequent temperature changes, the initial parameters of the part may change.
We are talking about two parameters – the resistance of the part at room temperature and the coefficient of resistance.
By type of heating
According to the method of heating, thermistors are divided into two types:
- DIRECT HEATING. It implies a change in the temperature of the part under the influence of ambient air or current flowing through the part. Devices with direct heating are most often used to solve two problems – changing the temperature or restoring normal operation. Such thermistors are used in thermometers, chargers, thermostats and other devices.
- INDIRECT HEATING. Unlike the previous type, here the heating occurs due to the elements in the immediate vicinity of the resistor. The nodes are not connected in any way. With this approach, the resistance of a semiconductor is determined by the change in current that passes through the nearby elements. Thermistors operating on an indirect principle have found application in multimeters (combined instruments).
The main parameters of thermistors
When choosing a part, it is important to focus on its performance and characteristics, which vary depending on the type, manufacturer, source material and other indicators.
When choosing a product, you need to find out the main parameters and determine whether they are suitable for solving the task or not.
Thermistor parameters:
- DIMENSIONS. When buying, you need to be sure that the part is the right size and will fit on the board (in the circuit).
- RESISTANCES RT and RT. The parameters are measured in Ohms and are indicated in relation to the current temperature in degrees Celsius or Kelvin. If the part is designed to operate at temperatures from -100 to +200 degrees Celsius, the temperature regime for the environment is taken at the level of 20-25 degrees Celsius.
- TIME CONSTANT Τ (SEC). The parameter reflects the thermal inertia. The calculation takes into account the time required to change the temperature of the thermal resistor by 63% of the difference t between the part and the ambient air. In most cases, this parameter is taken equal to 100 degrees Celsius.
- TKS (in % per degree Celsius). As a rule, this indicator is prescribed for the same temperature t as cold resistance. In such a situation, other numbers are used in the designation – at.
- Dissipation power Pmax (maximum permissible parameter), W. According to this indicator, one can judge the limit, until which no irreversible changes occur in the semiconductor (the parameters remain the same). In this case, the temperature exceeding tmax when Pmax is reached is excluded.
- Temperature tmax is the maximum allowable parameter at which the characteristics of the thermistor remain unchanged for a long time (at the level set by the manufacturer).
- Energy sensitivity coefficient (measured in W/percent*R). Designation – G. The indicator reflects the power that needs to be dissipated on the parts to reduce the R parameter by one percent.
- Dissipation factor (measured in watts per degree Celsius). The symbol is H. The parameter reflects the power that is dissipated by the thermal resistor with a difference in the temperature conditions of the part and the ambient air by one degree.
The coefficients (G and H) discussed above depend on the characteristics of the semiconductor used and the characteristics of the heat exchange between the product and its environment. The parameters are related to each other through a special formula – G=H/100a.
- Heat capacity (measured in Joules per degree Celsius). The symbol is C. The indicator reflects the amount of heat (energy) required to heat the thermistor by one degree.
Some of the considered parameters are related to each other. In particular, the time constant τ is equal to the ratio between the heat capacity and the dissipation factor.
When buying a positron, in addition to the above parameters, it is necessary to take into account the range of positive temperature resistance and the multiplicity of changes in R in the positive TCR sector.
See also:
Basic characteristics of thermistors
When evaluating thermistors, it is necessary to take into account and analyze their characteristics:
- The current-voltage characteristic is a curve on a graph showing the dependence of the voltage on the sample on the current passing through the thermistor. The graph is drawn taking into account thermal equilibrium with the surrounding nature. For posistors and thermistors, the graphs are different.
- Temperature characteristic. When plotting a graph, the dependence of resistance on temperature in a certain mode is removed. On the R axis, the parameter is set according to the principle of a tenfold increase (10X), and on the time axis, a section in the range from zero to 223 Kelvin is skipped.
- heating characteristic. Using the graph, you can see the parameters of thermal resistors operating on an indirect principle. In other words, the curve reflects the dependence of the resistance of the part on the power supplied to it. When specifying a graph, the scale for resistance is taken taking into account 10X.
General operating principle
Thermistors are made as sensitive as possible to changes in temperature, because they work on this principle. In the absence of heat, the atoms that make up the part are in the correct order and form long rows.
In the case of heating, the number of active charge carriers increases. The more such units, the higher the conductivity of the material.
When studying the curve of resistance versus temperature, one can see a characteristic of a non-linear type. At the same time, the thermistor shows the best characteristics in the range from -90 to +130 degrees.
It is important to consider that the principle of operation of such parts is based on the correlation between the temperature regime and the metals in the composition of the part.
The thermistor itself is made using semiconductor compounds (oxides, manganese, copper, nickel, silicates, iron, and others). Such components are able to respond to the slightest change in temperature.
The generated electric field pushes the electron, which moves until it hits the atom. For this reason, the movement of the electron is slowed down.
As the temperature rises, the atoms move more actively. Under such circumstances, the original act will more quickly collide with another element. The result is additional resistance.
After the operating temperature decreases, the electrons “fall” into the lower valence levels and pass into the unexcited state. In other words, they move less and don’t create as much resistance.
In the case of an increase in temperature, the R indicator also increases. But here it is necessary to take into account the type of thermistor, on which the principle of increasing and increasing resistance depends on changing the temperature regime.
NTC
NTC thermistors are negative temperature coefficient products. Their peculiarity is increased sensitivity, high temperature coefficient (one or two orders of magnitude higher than that of metal), small dimensions and a wide temperature range.
NTC semiconductors are easy to use, stable in operation and capable of withstanding high overload.
The peculiarity of NTC is that their resistance increases with decreasing temperature. Conversely, if t decreases, R increases. In the manufacture of such parts, semiconductors are used.
The principle of operation is simple. As the temperature rises, the number of charge carriers increases sharply, and the electrons are directed to the conduction band. In the manufacture of parts, in addition to semiconductors, transition metals can also be used.
When analyzing NTC, you need to take into account the beta coefficient. It is important if the product is used for temperature measurement, graph averaging and microcontroller calculations.
As a rule, NTC thermistors are used in the temperature range from 25 to 200 degrees. Therefore, they can be used for measurements within the specified limit.
Separately, you need to consider the scope of their use. Such parts have a low price and are useful for limiting inrush currents when starting electric motors, for protecting Li batteries, and reducing the charging currents of the power supply.
The NTC thermistor is also used in the car, a sensor used to determine the shutdown point and turn on the climate control in the car.
Another application is engine temperature control. If the safe limit is exceeded, a command is given to the relay, and then the engine is turned off.
See also:
An equally important element is a fire sensor that detects a rise in temperature and triggers an alarm.
NTC thermistors are identified by letters or are color-coded with stripes, rings, or other markings. Marking options depend on the manufacturer, product type and other parameters.
An example of the designation is 5D-20, where the first digit indicates the resistance of the thermistor at 25 degrees Celsius, and the number next to it (20) is the diameter.
The higher this parameter, the greater the dissipation power of the product. In order not to be mistaken in marking, it is recommended to use the official documentation.
PTC
Unlike the thermistors discussed above, PTCs are thermistors that have a positive resistance coefficient. This means that when the part is heated, its resistance also increases. Such products were actively used in old televisions equipped with color telescopes.
Today, two types of PTC thermistors are distinguished (from the number of leads) – with two and three taps. The difference between three-terminal products is that they include two positrons in the form of “tablets” installed in one housing.
Outwardly, it may seem that these elements are identical, but in practice they are not. One of the “tablets” is smaller. The resistance is also different – from 1,3 to 3,6 kOhm in the first case, and from 18 to 24 Ohm for the second such tablet.
Two-terminal thermistors are manufactured using a semiconductor material (most often Si – silicon). Externally, the product looks like a small plate with two leads at different ends.
PTC thermistors are used in a variety of applications. Most often they are used to protect power equipment from overload or overheating, as well as to maintain the temperature in a safe mode.
Main areas of application:
- Protection of electric motors. The task of the product is to protect the winding from burnout in the event of a wedge of the rotor or in the event of a breakdown in the cooling system. The posistor plays the role of a sensor connected to a control device with an executing relay, contactors and starters. When a force majeure situation occurs, the resistance increases, and the signal is sent to the control element, which gives the command to turn off the motor.
- Protection of transformer windings against overheating or overload. In such a circuit, a posistor is installed in the primary winding circuit.
- Heating unit in glue guns.
- In machines for heating the intake tract.
- Demagnetization of CRT kinescopes, etc.
How to check with a multimeter
An important issue in the operation of thermistors is knowledge of the principles of their verification. When assessing health, you need to understand that thermistors are of two types – with positive and negative temperature coefficient (this was mentioned above). Therefore, the resistance of the part decreases or decreases as the temperature rises.
Given this fact, only two elements are required to check the thermistor – a soldering iron for heating and a multimeter.
Algorithm of actions:
- Switching the device to the resistance measurement mode.
- Connect probes to the thermistor terminals (location does not matter).
- Fixing resistance on paper and bringing a heated soldering iron to the part.
- Resistance control (it rises or falls depending on the type of thermistor).
- If the resistance decreases or increases, the semiconductor is working properly.
An NTC thermistor type MF 72 can be used as an example. In normal mode, it shows a resistance of 6,9 ohms at normal temperature.
After bringing the soldering iron to the product, the situation changed – the resistance went down and stopped at the level of two ohms. From this test, we can conclude that the thermistor is working.
If the resistance changes abruptly or does not move at all, we can talk about the failure of the part.
It should be noted that such a check is very rough. For accurate control, you need to check the temperature and resistance of the thermistor, and then compare the data with the official parameters.
How to connect
The principle of connecting thermistors is simple (for example, Arduino). This will require a circuit board, a part, and a 10 kΩ resistor. Since the product has a high resistance, this parameter for conductors does not affect the final result.
One resistance pin is connected to the 5V pin and the other to the thermistor pin.
The second tap of the thermistor must be connected to ground. The center of the two resistors is connected to the Analog 0 pin.
Where is it on the diagram
The display of the thermistor on the diagram may vary. The product is easy to find by designations t and t0. Externally, it is reflected as a resistance through which a strip passes diagonally with a “stand” under t0 from below. The main designations are R1, TH1 or RK1.
If there is doubt about the application, the thermistor can be heated and its behavior observed. If the resistance will change, this is the desired element.
Thermistors are used almost everywhere – in the charger board, in car amplifiers, PC power supplies, in Li-Ion batteries and other devices. Finding them on the map is not difficult.
SMD and built-in thermistors
There are also two more types of thermistors that you should pay attention to:
- SMD – parts with a special type of mounting (for external mounting). Externally, they do not differ much from SMD capacitors made of ceramics. Dimensions correspond to the standard range – 1206, 0805, 0603, etc. It is almost impossible to distinguish such products from SMD thermistors by appearance.
- Embedded. They are used in soldering stations (to control the temperature of the tip), including hot air type.
See also:
In addition, it should be said that in electronics, along with thermistors, thermal relays and thermal fuses are used, which work on a similar principle and are also installed in electronic devices.
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