Each electronic device, which is plugged into network, needs protection from exceeding of threshold values of current or voltage. Various fuses and circuit breakers are used for current protection, but varistors are most commonly used for overvoltage protection. In this article we will consider the principle of varistor operation, its characteristics, advantages and disadvantages of this electronic component.
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What is a varistor and where to use it
Varistor - A varistor is a variable resistor made from semiconductor material, which is able to change its electrical resistance depending on the voltage applied to it.
The principle of operation of this electronic component is different from that of a normal resistor or potentiometer. The standard resistor has a constant resistance at any time regardless of the voltage in the circuit, the potentiometer allows to change the resistance manually by turning the control knob. A varistor, on the other hand, has a non-linear symmetrical volt-current characteristic and its resistance is fully dependent on the circuit voltage.
Because of this property, varistors are widely and effectively used to protect electrical networks, machinery and equipment, as well as radio electronic components, boards and chips, regardless of the type of voltage. They have a low manufacturing price, are reliable in use and capable of withstanding high loads.
Varistors are used in both high voltage applications up to 20 kV and low voltage applications from 3 to 200 V as a voltage limiter. At the same time, they can operate both in AC and DC networks. They are used to regulate and stabilize current and voltage, as well as in overvoltage protection devices. They are used in the construction of mains filters, power supplies, cell phones, SURGE PROTECTORS and other devices.
Types and principle of operation
Under normal conditions, varistor has a huge resistance, which can decrease when the voltage exceeds the threshold value. That is, if the voltage in the circuit rises significantly, the varistor transitions from an insulating state to an electrically conductive state and, due to the avalanche effect in the semiconductor, stabilizes the voltage by passing a large amount of current through itself.
Varistors can operate with high and low voltage and, accordingly, are divided into two groups of devices, which have the same principle of operation:
- High-voltage: capable of operating in circuits with current values up to 20 kV (used in protection systems of networks and equipment, in surge voltage protection devices).
- Low-voltage: rated voltages for this type of components range from 3 to 200 V (used to protect electronic devices and equipment components with a current of 0.1 to 1A and installed at the input or output of the power supply).
Time of operation of varistor in case of voltage jump is about 25 ns, which is an excellent value, but in some cases insufficient. Therefore electronic component manufacturers have developed a technology to manufacture a smd resistor which has a response time of 0.5 ns or better.
All types of varistors are made of silicon carbide or zinc oxide by sintering this material with a binder (resin, clay, glass) at high temperature. After the semiconductor element is obtained, it is metallized on both sides by soldering the metal leads for connection.
Marking, basic characteristics and parameters
Each manufacturer of varistors marks its product in a certain way, so there are quite a lot of variants of designations and their deciphering. The most common Russian varistors is K275, and popular foreign components are 7n471k, kl472m and others.
Decipher the designation of the varistor CNR-10d751k as follows: CNR - metal-oxide varistor; d - means that the component is in the shape of a disk; 10 - is the diameter of the disk; 751 -is the response voltage for this device (the calculation is done by multiplying the first two digits by 10 to the power of the third digit, i.e. 75 times 10 to the power of 750 V); k - the permissible deviation of the rated voltage, which is 10% in either direction (l - 15%, M - 20%, P - 25%).
The main characteristics of varistors are as follows:
Classification voltage - Voltage at a certain value of current flowing through the varistor (this value is usually 1 mA). This parameter is relative and does not affect the choice of device;
The maximum permissible voltage - voltage range (RMS or rms value) at which the varistor begins to decrease its resistance;
Maximum absorption energy - a characteristic that indicates the amount of energy a varistor dissipates and does not fail when subjected to a single pulse (measured in joules);
Maximum Impulse Current - normalizes the rise time and duration of the current pulse (measured in Amperes);
Capacitance - is a very important parameter that is measured in closed state and at a given frequency (falls to zero if a high current is applied to the varistor);
The permissible deviation - the deviation from the nominal potential difference in both directions (is given as a percentage).
Tripping time - Time interval within which the varistor changes from the closed to the open state (Usually a few tens of nanoseconds).
Advantages and disadvantages of varistors
Important advantages of a non-linear resistor (varistor) are its stable and reliable performance at high frequencies and high loads. It is used in many devices operating with voltages from 3 V to 20 kV, it is relatively simple and cheap in production and effective in operation. Additional important advantages are:
- High response speed (nanoseconds);
- long service life;
- ability to track voltage drops (inertia-free method).
Despite the fact that this electronic component has quite a few advantages, it also has disadvantages that affect its application in various systems. They can include:
- low-frequency noise during operation;
- aging of the component (loss of parameters over time);
- high capacitance: depends on the voltage and type of element, is in the range from 70 to 3200 pF and affects the performance of the device;
- no power dissipation at maximum voltages - significantly overheats and fails at prolonged maximum voltages.
Varistor selection
In order to select the correct varistor for a particular device, it is necessary to know the characteristics of its power supply: resistance and transient pulse power. The maximum allowable current value is determined, among other things, by the duration of its exposure and the number of repetitions, so if a varistor is installed with a lower peak current value, it will fail rather quickly. In brief, in order to effectively protect the device, it is necessary to choose a varistor with a voltage that has a small margin to the nominal.
Also very important to the trouble-free operation of such an electronic component is the rate of dissipation of absorbed thermal energy and the ability to quickly return to a state of normal operation.
Variistor schematic symbols and connection variants
At circuits varistors are usually is usually designated .As an ordinary resistor, but with the letter U added next to the slash. This slash indicates in diagrams that this element has a dependence of resistance on the circuit voltage. Also on the wiring diagram this component is marked with two letters R and U with an ordinal number (RU1, RU2 ... etc.).
There are many variants of varistors connection, but common to all methods is that this component is connected in parallel to the power supply circuit. Therefore, in the absence of dangerous voltage pulses, the current that flows through the varistor is small (due to the large resistance values) and does not affect the performance of the system. When an overvoltage occurs, the varistor changes its resistance to small values, the load is bypassed, and the absorbed energy is dissipated into the surrounding space.
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