Varistor - what is it? Varistors: the principle of operation, types and applications

Varistor - what is it, where it is applied, and why is it necessary? This element of electronic circuits is rarely used, so its name is not on hearing. Let's fix this and get acquainted with its work and the principle of the device.

general information

Electrical installations have insulation that corresponds to the rated voltage. The actual figure may differ from the theoretical value. But the work will be provided in case the deviation is small and within the allowed range. And yet, electrical equipment often fails due to a voltage pulse. This is called the sharp change in the characteristic at a certain point, when it follows the restoration to the initial level in a short period of time. Pulses can be lightning and switching. To protect against such differences, various devices are used, among which are gate dischargers, filters, chains and many other developments. But the most successful was the varistor. What it is? This is the name of an effective and cheap means of protection against impulses, which is based on nonlinear semiconductor resistors. The principle of their action is simple: the varistor is turned on in parallel to the protected equipment and in normal mode the operating voltage of the protected device influences it. When an emergency situation occurs, it begins to function as an insulator. Their distinctive feature is a symmetrical and well-expressed nonlinear current-voltage characteristic.

Varistor actions

When an impulse occurs, the device, due to the non-linearity of the characteristic, rapidly decreases its resistance (to fractions of Ohm) and shunts the load. Thus, it is protected, and the absorbed energy is dissipated in the form of heat. During such processes in varistors, a current of several thousand amperes can flow. Taking into account the practically inertialess of the device, after the impulse is extinguished, it again becomes a device with a large resistance. Thus, under normal conditions, it does not affect the operation of electrical equipment. But there will be impulses of dangerous tension, then rest assured - they will be cut off. This ensures the preservation of even weak insulation.

Most popular samples

Speaking about the varistor, what is it, you can not bypass the materials from which it is made. The most widespread are those devices that are made with the use of zinc oxide. This is due to several reasons:

1. Ease of manufacture.
2. Zinc has a good ability to absorb high-energy voltage pulses.

They are created according to the "ceramic" technology, which includes pressing, roasting, applying electrodes and electrical insulation, soldering the terminals and installing waterproof coatings. Due to the simplicity of manufacture, they can be created even for individual orders.

Marking

We have already paid enough attention to studying what the varistor is. The marking of this device is complex, and therefore when you purchase a device about it, you can not judge by the data placed on the case. Consider the following example: there is 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 disk varistor. 400 - operating voltage. K - this letter indicates that tolerance for a possible deviation has an error of 10%. If we talk about computer technology, then they varistors are designed for 470V. Agree, a lot. But there is not one varistor! 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 it is necessary to use or help of sellers, or to resort to services of directories.

Picture

If we do not want the technology to burn out, then the varistor is important to us. The designation on the circuit looks like a conventional resistor, only there is an oblique line and the letter U. It says that the performance characteristics directly depend on the voltage value. But maybe the varistor looks different. The designation on the diagram for it is given as RU, after which the digits are indicated. The number is the serial number, but the letters denote the name of the device: resistor-varistor. There may also be information signs. This can be attributed to the popular domestic products, which are manufactured at the plant "Progress" in Ukhta. Their varistor on the diagram can be marked with letters from A to G.

Checking the health of the item

Here we have a varistor in our hands. How to test its performance? It is always necessary to start with an external inspection of the device. It is necessary to look carefully for chipped parts, cracks, blackening or traces of carbon deposits. If there are external defects, then this alone means that the element must be replaced or not used at all. If the inspection did not reveal any problems, then you can start checking with a multimeter. In this case, the tester must be switched to the maximum resistance measurement mode. Here's the easiest way to find out if a varistor is working. How to test its performance, we have already considered, now let's discuss how to select the necessary elements.

Optimal operating mode

Due to the high linearity of the device, finding the best parameters for the circuit is not an easy task. For this, rather complicated and numerous calculations are applied. 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 too low a working current, then the voltage limit will increase, and the device will not perform its basic function. As a "lazy" option, one can adopt such a principle: the working DC voltage should not exceed 0.85 of the varistor threshold. But this simple approach in practice is of little use. After all, the varistor's work is specific, and the desired result, as well as the limits of the constraint, must be selected for each specific case.

Select and install

The fact that the varistors should be placed parallel to the protected electrical equipment, we have already said. The most preferred place for mounting varistors is the place after the switching device (if viewed from the load side, which must be protected). As an example of a ready-made solution, one can cite the products of the previously mentioned Progress plant with the name Impulse-1. This varistor is designed to be fixed on the electrical panel. Thanks to it, you can simply implement a three-phase load protection scheme with a "star" or "triangle" connection. Alternatively, choose the protection of 3 electrical installations that are powered by a three-phase network.

Options

Speaking about the varistor, what is it, you can not ignore its characteristics, which are important in the work:

1. Classification stress. This is the name for the value at which a current of 1 mA flows through the device.
2. The maximum permissible alternating voltage. By this is meant the value at which the varistor is triggered and begins to perform the protective functions assigned to it.
3. The maximum permissible DC voltage. Same as with the previous version. But in this case this parameter concerns the work with direct current.
4. The maximum limitation voltage. This is the value at which the varistor can operate without damage. As a rule, it is indicated separately for different current values. If you exceed this value, then the varistor will crack in half or even break up into pieces.
5. Maximum absorbed energy. It is indicated in joules. It is the value of the maximum energy of the pulse, which can be scattered by the varistor in the form of heat without the threat of destroying the device itself.
6. Response time. This is the interval for which the device passes from one state to another if the maximum allowable voltage has been exceeded. As a rule, it is measured in dozens of nanoseconds.
7. Tolerance. This is a value, the change to which the varistor's qualification voltage is considered the norm. Always specified in percentage. As it was possible to understand from the article earlier, this parameter is indicated by a letter at the end of the marking.

Using

Let's consider, for example, a network of 220 volts. For it, the devices with the operating voltage in the range of 275-420V will be optimal (but there are some technical nuances that we will not touch). Three varistors are used as the network filter . They block the penetration of pulses along the phase and zero circuits. And why are there three? Sometimes it happens that in the news there are reports of problems, due to which thousands of people lost their electronics. This happens when instead of zero and phase, only the last one goes through the wires. For equipment, this is almost always a sure death. But the presence of a varistor at zero makes it possible to successfully protect against such situations. As an example, we can mention mobile phones. That they are not burned out, use miniature multilayer varistors. In addition, they can be found in telecommunications equipment and automotive electronics.