What are thermistors and what are they for?
Device and types
A thermistor is a semiconductor device whose resistance depends on its temperature. Depending on the type of element, the resistance may rise or fall when heated. There are two types of thermistors:
- NTC (Negative Temperature Coefficient) - with a negative temperature coefficient of resistance (TCS). Often they are called "Thermistors."
- PTC (Positive Temperature Coefficient) - with positive TKS. They are also called "Posistors."
Important! The temperature coefficient of electrical resistance is the dependence of resistance on temperature. Describes how much Ohm or percent of the nominal value changes the resistance of the element with an increase in its temperature by 1 degree Celsius. For example, ordinary resistors positive TKS (when heated, the resistance of the conductors increases).
Thermistors are low-temperature (up to 170K), medium-temperature (170-510K) and high-temperature (900-1300K). The body of the element may be made of plastic, glass, metal or ceramic.
The conditional graphic designation of the thermistors in the diagram resembles ordinary resistors, and the only difference is that they are crossed out by a strip and the letter t is indicated next to it.
By the way, any resistors are indicated in this way, the resistance of which changes under the influence of the environment, and the type of acting quantities is indicated by the letter, t is the temperature.
- Nominal resistance at 25 degrees Celsius.
- Maximum current or power dissipation.
- The range of operating temperatures.
Interesting fact: The thermistor was invented in 1930 by the scientist Samuel Ruben.
Let's take a closer look at how it works and what each of them is for.
The resistance of NTC thermistors decreases when heated, their TCS is negative. The temperature dependence of the resistance is shown in the graph below.
Here you can see if the resistance of the NTC thermistor decreases when heated.
Such thermistors are made of semiconductors. The principle of operation is that with increasing temperature, the concentration of charge carriers increases, electrons pass into the conduction band. In addition to semiconductors, transition metal oxides are used.
Pay attention to such a parameter as a beta coefficient.It is taken into account when using a thermistor to measure temperature, to average the resistance graph versus temperature and perform calculations using microcontrollers. The beta equation for approximating the curve of the resistance change of the thermistor you see below.
Interesting: in most cases, thermistors are used in the temperature range of 25-200 degrees Celsius. Accordingly, they can be used for measurements in these ranges, while thermocouples operate at 600 degrees Celsius.
Where is used
Negative TCS thermistors are often used to limit the starting currents of electric motors, starting relays, to protect lithium batteries from overheating and in power supplies to reduce the charging currents of the input filter (capacitive).
The diagram above shows an example of using a thermistor in a power supply. This application is called direct heating (when the element itself warms up when current flows through it). On the power supply board, the NTC resistor is as follows.
In the figure below, you see what the NTC thermistor looks like. It can differ in size, shape, and less often in color, the most common are green, blue and black.
The limitation of the starting current of electric motors with the help of an NTC thermistor is widespread in household appliances due to the ease of implementation. It is known that when starting the engine, it can consume current many times and tens of times higher than its nominal consumption, especially if the engine is started not under idle, but under load.
The principle of operation of such a scheme:
When the thermistor is cold, its resistance is high, we turn on the motor and the current in the circuit is limited by the active resistance of the thermistor. Gradually, this element heats up and its resistance drops, and the engine enters the operating mode. The thermistor is selected so that in the hot state the resistance is close to zero. In the photo below, you see a burned-out thermistor on the board of the Zelmer meat grinder, where such a solution is used.
The disadvantage of this design is that when restarting, when the thermistor has not cooled down yet, current limitation does not occur.
There is a not quite familiar amateur use of a thermistor to protect incandescent lamps. The diagram below shows the option of limiting the current surge when such lamps are turned on.
If a thermistor is used to measure temperature, this mode of operation is called indirect heating, i.e. It is heated by an external heat source.
Interesting: thermistors do not have a polarity, so they can be used both in DC and AC circuits without fear of polarity reversal.
Thermistors can be marked both in the alphabetic way and contain color marking in the form of circles, rings or stripes. At the same time, there are many ways of letter marking - it depends on the manufacturer and the type of specific element. One of the options:
In practice, if it is used to limit the inrush current, the most common are disk thermistors, which are marked as follows:
Where the first digit indicates resistance at 25 degrees Celsius - 5 Ohms, and “20” - the diameter, the larger it is - the more power it can dissipate. You see an example of this in the figure below:
To decipher the color marking, you can use the table below.
Due to the abundance of marking options, you can make a mistake in decoding, therefore, for decryption accuracy, it is better to look for technical documentation for a specific component on the manufacturer's website.
The posistors, as was said, have a positive TCS, that is, their resistance increases with heating. They are made on the basis of barium titanate (BaTiO3) The posistor has such a graph of temperature and resistance:
In addition, you need to pay attention to its current-voltage characteristic:
The operating mode depends on the choice of the operating point of the posistor on the I-V characteristic, for example:
- A linear section is used to measure temperature;
- The downstream section is used in starting relays, time relay, measuring the power of electromagnetic radiation on microwave, fire alarm and other things.
The video below describes what posistors are:
The scope of posistors is wide enough. They are mainly used in protection schemes for equipment and devices from overheating or overload, less often for temperature measurement, and also as a self-stabilizing heating element. Briefly list examples of use:
- Motor protection. Installed in the frontal part of each motor winding (for single-speed three-phase 3, for two-speed 6, etc.), the PTC thermistor prevents the winding from burning out in case of jamming of the rotor or in case of failure of the forced cooling system. How does this circuit work? The posistor is used as a sensor connected to a control device with actuating relays, starters and contactors. In case of emergency, its resistance rises and this signal is transmitted to the governing body, the engine is turned off.
- Protect the transformer windings from overheating and (or) overload, then the resistor is installed in series with the primary winding.
- System for the demagnetization of kinescopes for CRT televisions and monitors. By the way, this part often fails and you have to deal with this case during the repair, while the failure of the fuse is characteristic.
- Heating element in glue guns. In cars for heating the intake tract, for example, the photo below shows the heater channel XX of the Pierburg carburetor.
Thermistors are a group of devices that can convert temperature into an electrical signal, which is read by measuring the voltage drop or current in the circuit where it is installed. Or they themselves can be a regulatory body, if its parameters allow it. The simplicity and accessibility of these devices allows them to be widely used both for the professional design of devices and for amateur radio practice.
Finally, we recommend watching a video on which it is described in detail what a thermistor is, how it works and where it is used:
Surely you do not know: