Thermistor is a device designed to measure temperature, and consisting of a semiconductor material, which greatly changes its resistance with a small change in temperature. Generally, thermistors have negative temperature coefficients, meaning their resistance decreases with increasing temperature.
General characteristic of thermistor
The word "thermistor" is short for its full term: thermally sensitive resistor. This device is an accurate and easy-to-use sensor for any temperature changes. In general, there are two types of thermistors: negative temperature coefficient and positive temperature coefficient. Most often, the first type is used to measure temperature.
The designation of the thermistor in the electrical circuit is shown in the photo.
The material of thermistors are metal oxides with semiconductor properties. During production, these devices are given the following form:
- disc;
- rod;
- spherical like a pearl.
The thermistor is based on the principle of strongchange in resistance with a small change in temperature. At the same time, at a given current strength in the circuit and a constant temperature, a constant voltage is maintained.
To use the device, it is connected to an electrical circuit, for example, to a Wheatstone bridge, and the current and voltage on the device are measured. According to Ohm's simple law R=U/I determine the resistance. Next, they look at the curve of dependence of resistance on temperature, according to which it is possible to say exactly what temperature the resulting resistance corresponds to. When the temperature changes, the resistance value changes dramatically, which makes it possible to determine the temperature with high accuracy.
Thermistor material
The material of the vast majority of thermistors is semiconductor ceramics. The process of its manufacture consists in sintering powders of nitrides and metal oxides at high temperatures. The result is a material whose oxide composition has the general formula (AB)3O4 or (ABC)3 O4, where A, B, C are metallic chemical elements. The most commonly used are manganese and nickel.
If the thermistor is expected to operate at temperatures less than 250 ° C, then magnesium, cob alt and nickel are included in the composition of ceramics. Ceramics of this composition show the stability of physical properties in the specified temperature range.
An important characteristic of thermistors is their specific conductivity (the reciprocal of resistance). Conductivity is controlled by adding smallconcentrations of lithium and sodium.
Instrument manufacturing process
Spherical thermistors are made by applying them to two platinum wires at high temperature (1100°C). The wire is then cut to shape the thermistor contacts. A glass coating is applied to the spherical instrument for sealing.
In the case of disc thermistors, the process of making contacts is to deposit a metal alloy of platinum, palladium and silver on them, and then solder it to the thermistor coating.
Difference from platinum detectors
Besides semiconductor thermistors, there is another type of temperature detectors, the working material of which is platinum. These detectors change their resistance as the temperature changes in a linear fashion. For thermistors, this dependence of physical quantities has a completely different character.
The advantages of thermistors compared to platinum counterparts are as follows:
- Higher resistance sensitivity to temperature changes over the entire operating range.
- High level of instrument stability and repeatability of readings.
- Small in size to respond quickly to temperature changes.
Thermistor resistance
This physical quantity decreases with increasing temperature, it is important to consider the operating temperature range. For temperature limits from -55 °C to +70 °C, thermistors with a resistance of 2200 - 10000 ohms are used. For higher temperatures, use devices with a resistance greater than 10 kOhm.
Unlike platinum detectors and thermocouples, thermistors do not have specific standards for resistance versus temperature curves, and there is a wide variety of resistance curves to choose from. This is because each thermistor material, like a temperature sensor, has its own resistance curve.
Stability and accuracy
These instruments are chemically stable and do not degrade over time. Thermistor sensors are among the most accurate temperature measuring instruments. The accuracy of their measurements over the entire operating range is 0.1 - 0.2 °C. Please note that most appliances operate within a temperature range of 0 °C to 100 °C.
Basic parameters of thermistors
The following physical parameters are basic for each type of thermistor (decoding of names in English is given):
- R25 - resistance of the device in Ohms at room temperature (25 °С). Checking this thermistor characteristic is simple using a multimeter.
- Tolerance of R25 - the value of the resistance deviation tolerance on the device from its set value at a temperature of 25 °C. As a rule, this value does not exceed 20% of R25.
- Max. Steady State Current - maximumthe value of the current in amperes that can flow through the device for a long time. Exceeding this value threatens with a rapid drop in resistance and, as a result, failure of the thermistor.
- Approx. R of Max. Current - this value shows the value of resistance in Ohms, which the device acquires when the maximum current passes through it. This value should be 1-2 orders of magnitude less than the resistance of the thermistor at room temperature.
- Dissip. Coef. - a coefficient that shows the temperature sensitivity of the device to the power absorbed by it. This factor indicates the amount of power in mW that the thermistor needs to absorb in order to increase its temperature by 1 °C. This value is important because it shows how much power you need to spend to heat the device to its operating temperature.
- Thermal Time Constant. If the thermistor is used as an inrush current limiter, it is important to know how long it will take to cool down after the power is turned off in order to be ready to turn it on again. Since the temperature of the thermistor after it is turned off decreases according to an exponential law, the concept of "Thermal Time Constant" is introduced - the time during which the temperature of the device decreases by 63.2% of the difference between the operating temperature of the device and the ambient temperature.
- Max. Load Capacitance in ΜF - the amount of capacitance in microfarads that can be discharged through this device without damaging it. This value is indicated for a specific voltage,e.g. 220 V.
How to test the thermistor for operation?
For a rough check of the thermistor for its serviceability, you can use a multimeter and a regular soldering iron.
First of all, turn on the resistance measurement mode on the multimeter and connect the output contacts of the thermistor to the multimeter terminals. In this case, the polarity does not matter. The multimeter will show a certain resistance in ohms, it should be recorded.
Then you need to plug in the soldering iron and bring it to one of the thermistor outputs. Be careful not to burn the device. During this process, you should observe the readings of the multimeter, it should show a smoothly decreasing resistance, which will quickly settle to some minimum value. The minimum value depends on the type of thermistor and the temperature of the soldering iron, usually it is several times less than the value measured at the beginning. In this case, you can be sure that the thermistor is working.
If the resistance on the multimeter has not changed or, on the contrary, has fallen sharply, then the device is unsuitable for its use.
Note that this check is rough. For accurate testing of the device, it is necessary to measure two indicators: its temperature and the corresponding resistance, and then compare these values with those stated by the manufacturer.
Applications
Thermistors are used in all areas of electronics in which it is important to monitor temperature conditions. These areas includecomputers, high-precision equipment for industrial installations and devices for transmitting various data. So, the 3D printer thermistor is used as a sensor that controls the temperature of the heating bed or print head.
One of the most common uses for a thermistor is to limit inrush current, such as when turning on a computer. The fact is that at the moment the power is turned on, the starting capacitor, which has a large capacity, is discharged, creating a huge current in the entire circuit. This current is capable of burning the entire chip, so a thermistor is included in the circuit.
This device at the time of switching on had room temperature and a huge resistance. Such resistance can effectively reduce the current surge at the time of starting. Further, the device heats up due to the current passing through it and the release of heat, and its resistance decreases sharply. The thermistor's calibration is such that the operating temperature of the computer chip causes the thermistor's resistance to practically zero, and there is no voltage drop across it. After turning off the computer, the thermistor quickly cools down and restores its resistance.
So using a thermistor to limit inrush current is both cost-effective and fairly simple.
Examples of thermistors
Currently, a wide range of products is on sale, here are the characteristics and areas of use of some of them:
- Thermistor B57045-K with nut fastening, has a nominal resistance of 1kOhm with a tolerance of 10%. Used as a temperature measurement sensor in consumer and automotive electronics.
- B57153-S disc instrument, has a maximum current rating of 1.8 A at 15 ohms at room temperature. Used as an inrush current limiter.
