A thermistor is a device designed to measure temperature, and consists of a semiconductor material, which, with a small change in temperature, greatly changes its resistance. Typically, thermistors have negative temperature coefficients, that is, their resistance decreases with increasing temperature.
General characteristic of thermistor
The word "thermistor" is an abbreviation 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: one with a negative temperature coefficient and one with a positive one. 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. In the manufacture of these devices give the following form:
- disc-shaped;
- pivotal;
- spherical like a pearl.
The principle of a strong change in resistance with a small change in temperature is the basis for the operation of a thermistor. In this case, at a given current 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 the Wheatstone bridge, and measure the current strength and voltage on the device. According to Ohm's simple law, R = U / I determine the resistance. Next, they look at the curve of the dependence of resistance on temperature, according to which it can be accurately said what temperature corresponds to the obtained resistance. When the temperature changes, the resistance value changes sharply, 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) 3 O 4 or (ABC) 3 O 4 , where A, B, C are metal chemical elements. Most commonly used are manganese and nickel.
If it is assumed that the thermistor will operate at temperatures lower than 250 ° C, then the composition of the ceramics includes magnesium, cobalt and nickel. Ceramics of this composition show the stability of physical properties in the specified temperature range.
An important characteristic of thermistors is their specific conductivity (inverse value to resistance). Conductivity is regulated by adding small concentrations of lithium and sodium to the composition of semiconductor ceramics.
Instrument manufacturing process
Spherical thermistors are made by applying them to two platinum wires at a high temperature (1100 ° C). After that, the wire is cut to give the necessary shape to the contacts of the thermistor. For sealing, a glass coating is applied to the spherical device.
In the case of disk thermistors, the contact manufacturing process consists in applying a metal alloy of platinum, palladium and silver to them, and its subsequent soldering to the coating of the thermistor.
Difference from Platinum Detectors
In addition to semiconductor thermistors, there is another type of temperature detector, the working material of which is platinum. These detectors change their resistance when the temperature changes linearly. For thermistors, this dependence of physical quantities is of a completely different nature.
The advantages of thermistors in comparison with platinum analogues are as follows:
- Higher sensitivity of resistance to temperature changes over the entire operating range of values.
- High level of instrument stability and repeatability of readings.
- Small size that allows you to quickly respond to temperature changes.
Thermistor resistance
This physical quantity decreases its value with increasing temperature, while 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, devices with a resistance exceeding 10 kOhm are used.
Unlike platinum detectors and thermocouples, thermistors do not have specific standards for resistance curves depending on temperature, and there is a wide variety of choices for these curves. This is due to the fact that each material of the thermistor, as a temperature sensor, has its own resistance curve.
Stability and accuracy
These devices are chemically stable and do not degrade their performance over time. Thermistors are one of the most accurate temperature measuring instruments. The accuracy of their measurements over the entire operating range is 0.1 - 0.2 ° C. It should be borne in mind that most devices operate in the temperature range from 0 ° C to 100 ° C.
The main parameters of thermistors
The following physical parameters are basic for each type of thermistor (a description of the names in English is given):
- R 25 is the resistance of the device in Ohms at room temperature (25 ° C). Checking this thermistor characteristic is easy with a multimeter.
- Tolerance of R 25 - tolerance 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 R 25 .
- Max Steady State Current - the maximum current in amperes that can flow through the device for a long time. Exceeding this value threatens a rapid drop in resistance and, as a result, the thermistor fails.
- Approx R of Max. Current - this value shows the resistance value in Ohms, which the device acquires when a maximum current passes through it. This value should be 1-2 orders of magnitude less than the resistance of a thermistor at room temperature.
- Dissip. Coef. - a coefficient that shows the temperature sensitivity of the device to the power absorbed by it. This coefficient shows the amount of power in mW that the thermistor needs to absorb in order for its temperature to increase by 1 ° C. This value is important because it shows how much power you need to spend to warm up the device to its operating temperatures.
- Thermal Time Constant. If the thermistor is used as an inrush current limiter, it is important to know how long it can cool down after turning off the power in order to be ready to turn it on again. Since the temperature of the thermistor after it turns off decreases according to the exponential law, the concept of "Thermal Time Constant" is introduced - the time during which the temperature of the device will decrease by 63.2% of the difference between the operating temperature of the device and the ambient temperature.
- Max Load Capacitance in μF - the value of the capacitance in microfarads that can be discharged through this device without damaging it. This value is indicated for a specific voltage, for example, 220 V.
How to check the thermistor for performance?
For a rough check of the thermistor for its serviceability, you can use a multimeter and a conventional soldering iron.
The first step is to turn on the resistance measurement mode on the multimeter and connect the output contacts of the thermistor to the terminals of the multimeter. 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 turn on the soldering iron and bring it to one of the outputs of the thermistor. Care should be taken not to burn the appliance. During this process, you should observe the readings of the multimeter, it should show a smoothly falling resistance, which will quickly be established at some minimum value. The minimum value depends on the type of thermistor and the temperature of the soldering iron, usually it is several times smaller 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, conversely, has fallen sharply, then the device is unsuitable for its use.
Note that this check is crude. 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.
Areas of use
In all areas of electronics, in which it is important to monitor temperature conditions, thermistors are used. These areas include computers, precision equipment for industrial plants and devices for transmitting various data. So, a 3D printer thermistor is used as a sensor that monitors the temperature of a heating table or print head.
One of the common uses for a thermistor is to limit inrush current, for example, when you turn on the 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 amperage in the entire circuit. This current is able to burn the entire chip, so a thermistor is included in the circuit.
This device at the time of inclusion had room temperature and huge resistance. This resistance allows you to effectively reduce the jump in current at the time of start. Further, the device heats up due to the current passing through it and heat generation, and its resistance decreases sharply. Calibration of the thermistor is such that the operating temperature of the computer chip leads to the practical zeroing of the resistance of the thermistor, and there is no voltage drop on it. After turning off the computer, the thermistor cools down quickly and restores its resistance.
Thus, the use of a thermistor to limit the inrush current is cost-effective and quite simple.
Thermistor Examples
Currently, there is a wide range of products on sale, we present the characteristics and areas of use of some of them:
- The nut thermistor B57045-K has a nominal resistance of 1 kΩ with a tolerance of 10%. It is used as a temperature measurement sensor in consumer and automotive electronics.
- The disk device B57153-S, has a maximum permissible current of 1.8 A with a resistance of 15 Ohms at room temperature. Used as an inrush current limiter.