Thermistor: device, principle of operation, purpose, types

When repairing household appliances, you have to deal with a wide variety of parts and components. Often, beginners do not know what a thermistor is and what they are. These are semiconductor components whose resistance changes with temperature. Due to these properties, they have found a wide range of applications. Starting from thermometers, ending with inrush current limiters. In this article, we will answer all your questions in simple words.

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 (TCR). They are often called "Thermistors".
• PTC (Positive Temperature Coefficient) - with a positive TCS. They are also called "Posistors".

Important!

The temperature coefficient of electrical resistance is the relationship between resistance and temperature. Describes by how many ohms or percent of the nominal value the resistance of the element changes when its temperature rises by 1 degree Celsius. For example, ordinary resistors positive TCS (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 cell body can be made of plastic, glass, metal or ceramic.

The conventional graphic designation of thermistors in the diagram resembles ordinary resistors, and the only difference is that they are crossed out with a stripe and the letter t is indicated next to it.

By the way, this is how any resistors are denoted, the resistance of which changes under the influence of the environment, and the type of influencing quantities is indicated by the letter, t is the temperature.

Main characteristics:

• Rated resistance at 25 degrees Celsius.
• Maximum current or power dissipation.
• Operating temperature range.
• TCS.

Interesting fact: The thermistor was invented in 1930 by the scientist Samuel Ruben.

Let's take a closer look at how each of them is arranged and what it is for.

NTC

Basic information

The resistance of NTC thermistors decreases when heated, their TCR is negative. Resistance versus temperature is shown in the graph below.

Here you can make sure that the resistance of the NTC thermistor decreases when it heats up.

These thermistors are made from semiconductors. The principle of operation is that as the temperature rises, the concentration of charge carriers increases, the electrons pass into the conduction band. In addition to semiconductors, transition metal oxides are used.

Pay attention to such a parameter as beta coefficient. It is taken into account when using a thermistor to measure temperature, for averaging the resistance versus temperature graph and for calculating using microcontrollers. The beta equation to approximate the thermistor resistance curve is shown below.

Interesting: in most cases thermistors are used in a temperature range of 25-200 degrees Celsius. Accordingly, they can be used for measurements in these ranges, while thermocouples work at 600 degrees Celsius.

Where is used

NTC thermistors are often used to limit the starting currents of electric motors, starting relays, for protection against overheating of lithium batteries 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 heats up by itself when current flows through it). On the power supply board, the NTC resistor looks like this.

In the picture below, you can see what an NTC thermistor looks like. It can differ in size, shape, and less often in color, the most common are green, blue and black.

Limiting the starting current of electric motors using an NTC thermistor has become widespread in household appliances due to its ease of implementation. It is known that when starting the engine, it can consume current several times and tens of times higher than its nominal consumption, especially if the engine is started not idling, 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 in such a way 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 is still hot, no current limiting occurs.

There is not quite the usual amateur use of a thermistor to protect incandescent lamps. The diagram below shows a variant of limiting the current surge when such bulbs 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 have no polarity, so they can be used in both DC and AC circuits without fear of polarity reversal.

Marking

Thermistors can be labeled both in letters and color-coded in the form of circles, rings or stripes. At the same time, many methods of letter marking are distinguished - it depends on the manufacturer and the type of a particular element. One of the options:

In practice, if it is used to limit inrush current, disk thermistors are most often found, which are labeled as follows:

5D-20

Where the first number denotes resistance at 25 degrees Celsius - 5 ohms, and "20" is 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 coding, you can use the table below.

Due to the abundance of labeling options, you can make a mistake in decoding, therefore, for the accuracy of decoding, it is better to look for technical documentation for a specific component on the manufacturer's website.

PTC

Basic information

Posistors, as mentioned, have a positive TCR, that is, their resistance increases when heated. 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:

• The linear section is used to measure temperature;
• The descending section is used in starting relays, time relay, measuring the power of EMP on microwave, fire alarm and other things.

The video below describes what posistors are:

Where is applied

The scope of application of posistors is wide enough. They are mainly used in circuits to protect equipment and devices from overheating or overload, less often for temperature measurement, and also as an auto-stabilizing heating element. Let's briefly list examples of use:

1. Protection of electric motors. Installed in the frontal part of each winding of the electric motor (for single-speed three-phase 3, for two-speed 6, etc.), The PTC thermistor prevents burn-out of the winding in the event of a jammed rotor or in the event of a failure of the forced cooling. How does this scheme work? The posistor is used as a sensor connected to a control device with executive relays, starters and contactors. In the event of an abnormal situation, its resistance increases and this signal is transmitted to the control body, the engine is turned off.
2. To protect the transformer windings from overheating and (or) overload, then the PTC thermistor is installed in series with the primary winding.
3. Demagnetization system for CRT TVs and monitors. By the way, this part often fails and you have to deal with this case during repairs, while the fuse fails.
4. Heating element in glue guns. In cars for heating the intake tract, for example, the photo below shows the heater for channel XX of the Pierburg carburetor.

Thermistors are a group of devices capable of converting temperature into an electrical signal, which is read by measuring the voltage drop or current in the circuit where it is installed. Alternatively, they themselves can be a regulatory body, if its parameters allow it. The simplicity and availability of these devices allows them to be widely used both for professional design of devices and for amateur radio practice.

Finally, we recommend watching a video that explains in detail what a thermistor is, how it works and where it is used:

You probably don't know:

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