Semiconductor devices are called varistors, the resistance of which decreases sharply (by several orders of magnitude) when the voltage applied to them exceeds a certain threshold value. This feature of these devices determines their use in systems for protecting electrical circuits against overvoltage (by connecting a varistor in parallel to the protected circuit). The current-voltage characteristic of the varistors is symmetrical, so they limit the voltage regardless of its polarity, including the ability to work in alternating voltage circuits.
As a rule, they are metal oxide or zinc oxide. If you look at the current-voltage characteristics of the varistor, it can be noted that it has a non-linear symmetrical shape, that is, it can work not only at constant, but also at alternating voltage. Such an element is connected in parallel with the load. How does a varistor work?
With increasing voltage in the network, the current does not pass through the equipment, namely through the varistor. Such a device is capable of distributing energy in the form of heat. Its main features are repeated use and quick recovery time, that is, its resistance has an initial indicator when relieving stress.
What is the varistor principle of operation? The part is no different from a conventional resistor, that is, with the normal functioning of the electronics, it has an ohmic resistance. So, we will consider what the varistor principle of work has.
The indicator of such resistance is quite high, and can reach 100,000 ohms. When the voltage is turned on, it can decrease as soon as the need arises to protect the level. The resistance drops from 100,000 ohms to 100. If the value drops to a low limit or equals zero, a short circuit may occur . In this case, the fuse that is in the electrical circuit in front of the varistor fails. After that, the electric circuit closes, and the voltage is completely turned off.
As mentioned earlier, in the absence of voltage, the varistor can fully recover and work in the previous mode. For its operation, it is necessary to replace the blown fuse. Further, the electronic device will function correctly. The varistor is connected in parallel with the power source. Consider the varistor principle of operation, for example, a conventional personal computer. Since it has two outputs, the connection is parallel to phase and zero.
What does the element look like?
Such a device as a varistor, the photo of which is in our article, resembles a conventional resistor, that is, it has the shape of a rectangle. But still has a slight difference.
In the middle of it is a diagonal, the end of which is curved.
How is a varistor labeled?
To date, you can find different designations of these devices. Each manufacturer has the right to install it independently. Markings differ because the technical characteristics of varistors are different from each other. Examples include indicators such as allowable voltage or the required current level.
Currently, each manufacturer sets its marking on these types of devices. This is because the manufactured devices have different specifications. For example, the maximum allowable voltage or the current level necessary for functioning. The most popular marking is CNR, to which a designation such as 07D390K is attached. What does this mean? So, the CNR designation itself indicates the type of instrument. In this case, the varistor is a metal oxide.
Further, 07 is the size of the device in diameter, i.e. equal to 7 mm. D is the disk device, and 390 is the maximum allowable voltage indicator.
Main parameters of varistors
These parameters include:
- rate of stress;
- the most admissible indicator of alternating and direct current;
- peak energy absorption;
- possible errors;
- item run time.
Diagnostics
To check this electronic device, use special equipment called a tester. So, for the test, you will need a varistor, the principle of which is to change the resistance parameters, and a testing device. Before it starts, you must turn on the device and switch to resistance mode. Only then will the device meet all the necessary technical requirements, and the magnitude of the resistance will be huge.
Before starting the test it is necessary to check the technical condition of the device. First of all, you should look at its appearance. There should be no cracks on the appliance, nor signs of burnout. Do not treat the inspection of the device negligently, since any small breakdown can lead to unpleasant circumstances.
Varistors: application
Such devices play an important role in human life.
From all of the above, it can be said that a varistor, the principle of which is to protect the electronics from high voltage in the network, helps to prevent the breakdown of many electrical devices and keep the wiring intact. The main place is electric circuits in various equipment. For example, they are found in trigger elements of lighting, which are also called ballasts. Special varistors are also installed in the electrical circuits, the use of which is necessary to stabilize the voltage and current.
Such devices are still used in power lines. But there they are called surge arresters, the operating voltage of which is more than twenty thousand volts.
Varistors can operate in a wide voltage range, which starts with a very small value of 3 V and ends with 200 V. As for the current strength of the element, here the range is from 0.1 to 1 A. Such current indicators are valid only for low-voltage technical equipment .
Positive aspects of varistors
This type of apparatus has many positive qualities when compared with other devices, for example, with a spark gap. These important benefits include:
- high speed of the element;
- the ability to track current drops without inertia;
- the ability to use at a voltage level in the range from 12 to 1800 V;
- long term of operation;
- relatively low cost due to the simplicity of the design.
Negative sides
Along with so many advantages over other devices, there are also significant disadvantages, among which we can distinguish such.
- Varistors have a huge size of their own capacity, which affects the operation of the electrical network. Such an indicator can range from 80 to 3000 pF. It depends on many points: the design and type of the varistor, as well as the maximum value of the voltage level. It is worth noting that in some cases such a significant drawback can turn into the main advantage. But this is possible quite rarely, for example, if you use a varistor in the filters. In such a situation, a large capacitance will serve as a voltage limiter in the network.
- Compared to arresters, varistors are not able to dissipate power at maximum voltage.
To increase the dispersion index, it is necessary to increase the size of the elements, which many manufacturers do.
Installation Recommendations
If there is a need to include a varistor in the electrical network, it is necessary to remember these important points:
- You should always keep in mind that this device is not eternal, and such conditions will come that will lead to its explosion. To avoid this, it is necessary to use special protective screens in which the entire varistor can be placed.
- It should be noted that flint technical devices are significantly inferior to oxide analogues in their characteristics. Therefore, it is best to use this particular type of varistor.
Conclusion
Varistor plays an important role in the functioning of many electrical circuits. As mentioned earlier, this type of semiconductor resistor serves to reduce the resistance with an increase in voltage or current.
Due to this possibility, they are installed in many electrical appliances. With power surges, the varistor, the purpose of which is aimed at changing the resistance, does not allow the devices to break. It also prevents wiring burnout. Thus, these elements provide reliable protection during power surges in the network.