Varistor - what is it, where is it used, and why is it necessary? This element of electronic circuits is rarely used, so its name is not at the hearing. Let's fix it and get acquainted with its work and the principle of the device.
general information
Electrical installations have insulation that corresponds to the rated voltage. Actual performance may differ from theoretical value. But the work will be provided if the deviation is small and is within the allowed range. Nevertheless, electrical equipment often fails due to a voltage pulse. This is the name for a sharp change in the characteristic at a certain point, when restoration to the initial level follows in a short period of time. Impulses can be lightning and switching. To protect themselves from such differences, they use various devices, including valve arresters, filters, chains and many other developments. But the varistor turned out to be the most successful. What it is? So they call an effective and cheap means of protection against pulses, which is based on non-linear semiconductor resistors. The principle of their operation is simple: the varistor is connected in parallel to the protected equipment and in normal mode it is affected by the operating voltage of the protected device. When an emergency occurs, it begins to function as an insulator. Their distinguishing feature is a symmetrical and well-defined nonlinear current-voltage characteristic.
Varistor Actions
When an impulse occurs, the device, due to the non-linearity of the characteristic, quickly reduces its resistance (to Ohms fractions) and shunts the load. Thus, it is protected, and the absorbed energy is dissipated in the form of heat. During such processes, a current of several thousand amperes can flow in the varistors. Given the practically inertialessness of the device, after the pulse is extinguished, it again becomes a device with high resistance. Thus, under normal conditions, it does not affect the operation of electrical equipment. But there will be impulses of
dangerous voltage, then be sure - they will be cut off. This ensures the safety of even poor insulation.
Most popular samples
Speaking about the varistor, what it is, you can not ignore the materials from which it is made. The most widely used are those devices that are made using zinc oxide. There are several reasons for this:
- Easy to manufacture.
- Zinc has a good ability to absorb high energy voltage pulses.
They are created using the "ceramic" technology, which includes pressing, firing, applying electrodes and electrical insulation, soldering the terminals and installing moisture-proof coatings. Due to the simplicity of manufacture, they can be created even for individual orders.
Marking
We have already paid enough attention to the study of what a varistor is. The marking of this device is complicated, and therefore, when purchasing a device, it cannot be judged by the data placed on the case. Consider this example: there is a CNR-06D400K. CNR is the name of the type, in this case we have a metal oxide varistor. 06 - it has a diameter of 6 millimeters. D - before us is a disk varistor. 400 - tripping voltage. K - this letter indicates that the tolerance of a possible deviation has an error of 10%. If we talk about computer technology, then their varistors are designed for 470V. Agree, a lot. But there is more than one varistor! The marking of these parts is carried out by each major manufacturer in its own way, so there are no universal and standardized recognition rules. Therefore, you need to use either the help of sellers, or resort to the services of directories.
Picture
If we do not want the equipment to burn out, then the varistor is important to us. The designation on the diagram looks like a conventional resistor, but there is also a slanting line and the letter U. It indicates that the performance characteristics directly depend on the voltage value. But the varistor may also look different. The designation on the diagram for it is set as RU, after which the numbers are indicated. The number is a serial number, but the letters indicate the name of the device: resistor-varistor. There may also be informational signs. This can be attributed to the popular domestic products, which are manufactured at the Progress factory in Ukhta. Their varistor in the diagram can be marked with letters from A to G.
Check item health
Here we have a varistor in our hands. How to check its performance? It is always necessary to start with an external inspection of the device. It is necessary to carefully look for chips, cracks, blackening or traces of soot on the body. If there are external defects, then this alone indicates that the element must be replaced or not used at all. If during the inspection there were no problems, then you can proceed to check with a multimeter. In this case, the tester must be switched to the maximum resistance measurement mode. Here is the easiest way to find out if a varistor is working. How to check its performance, we have already considered, now let's discuss how to select the necessary elements.
Optimum operating mode
Due to the high linearity of the device, finding the best parameters for the circuit is not an easy task. For this, quite complex and numerous calculations are used. Of great importance in this case is the operating current, the value of which should be minimal and not lead to overheating of the device. But here you have to balance. After all, if you use a too small operating current, then the voltage limit will increase, and the device will not perform its main function. As a "lazy" option, you can take on board the following principle: working constant voltage should not exceed 0.85 from the varistor threshold. But this simple approach is inapplicable in practice. After all, the varistor's work is specific, and the desired result, as well as the limits of the restriction, must be selected for each specific case.
Selection and installation
About the fact that varistors should be placed parallel to the protected electrical equipment, we have already said. The most preferred mounting location for varistors is considered to be the place after the switching device (when viewed from the load side, which must be protected). As an example of a ready-made solution, we can cite the products of the previously mentioned Progress plant with the name Impulse-1. Such a varistor is designed to be mounted on an electrical panel. Thanks to him, you can simply implement a three-phase load protection circuit with a star or delta connection. Or, as an alternative, choose the protection of 3 electrical installations that are powered by a three-phase network.
Parameters
Speaking about the varistor, what it is, you can not ignore its characteristics, which are important in the work:
- Classification voltage. So called the value at which a current of 1 mA flows through the device.
- The maximum permissible alternating voltage. This is understood as the value at which the varistor is triggered and begins to fulfill the protective functions assigned to it.
- The maximum permissible constant voltage. The same as with the previous option. But in this case, this parameter applies to DC operation.
- Maximum voltage limit. This is the value at which the varistor can operate without damage. As a rule, it is indicated separately for different current values. If this value is exceeded, then the varistor will crack in two or even fly to pieces.
- The maximum absorbed energy. Indicated in joules. It is the value of the maximum pulse energy that can be dissipated by the varistor in the form of heat without threatening to destroy the device itself.
- Response time. This is the period during which the device switches from one state to another if the maximum allowable voltage has been exceeded. Typically measured in tens of nanoseconds.
- Tolerance. This is the value by which a change in the qualification voltage of the varistor is considered normal. It is always indicated as a percentage. As could be understood from the article earlier, this parameter is indicated by a letter at the end of the marking.
Using
Let's consider, for example, a 220 Volt network. For it, devices whose triggering voltage is in the range 275-420V (but there are some technical nuances that we will not touch) will be optimal for it. Three varistors
are used as a
line filter . They block the penetration of pulses along the phase and zero circuits. Why are there three of them? Sometimes it happens that in the news there are messages about problems resulting in thousands of people who have lost electronics. This happens when instead of zero and phase, only the last goes through the wires. For equipment, it is almost always certain death. But the presence of a varistor at zero allows you to successfully protect against such situations. An illustrative example is mobile phones. So that they do not burn out, they use miniature multilayer varistors. In addition, they can be found in telecommunications equipment and automotive electronics.