What is the working principle of a varistor, how does a varistor work

Varistors. Varistor (VSR) is the abbreviation of voltage sensitive resistor, which is a new type of overvoltage protection component. The varistor is a metal-oxide-semiconductor ceramic element made of zinc oxide as the main material, the core material of the varistor is zinc oxide, and the zinc oxide includes zinc oxide grains and the grain boundary layer around the grains, the resistivity of zinc oxide grains is very low, and the resistivity of the grain boundary layer is very high, and a potential barrier equivalent to a Zener diode is formed between the two grains in contact, which becomes a varistor unit, and the unit is connected in series and in parallel to form a varistor matrix.

When the varistor is working, each varistor unit bears surge energy, and these varistor units are roughly evenly distributed in the entire resistor body, that is, the entire resistor body bears energy, unlike the Zener diode regulator tube that only the junction area bears the electrical power, which is why the ceramic varistor has a much larger flow and energy quota than the Zener diode regulator. Its resistance value varies with the terminal voltage. The main characteristics of varistors are a wide range of working voltage (6-3000 volts, divided into several gears), fast response to overvoltage impulse (a few to tens of nanoseconds), strong ability to resist impulse current (up to 100 amperes-20 thousand amperes), small leakage current (less than a few to tens of microamperes), small resistance temperature coefficient, high performance and low price, small size, is an ideal protection element.

It can be used to form overvoltage protection circuits, noise reduction circuits, spark suppression circuits, and absorption circuits.

The structure of a varistor is like two regulators connected back to back with the same characteristics, and their properties are basically the same. The main characteristics of the varistor are that when the voltage applied at both ends is within the nominal rated value, its resistance value is almost infinite, in a high-impedance condition, its leakage current < 50 microamperes, when the voltage at both ends of it slightly exceeds the rated voltage, its resistance value drops sharply, immediately in the conduction condition, the working current increases by several orders of magnitude, and the reaction time is only in the nanosecond level. Varistors are commonly known as "choppers" and "limiters" abroad, which are named after its actual role.

1, which means that the resistance value changes with the voltage within a certain range of current and voltage, or in other words"The resistance value is sensitive to voltage"of the resistor.

2, What is the use of varistors? The most important feature of a varistor is that when the voltage applied to it is lower than its threshold"un"When the voltage exceeds un, the current flowing through it surges, which is equivalent to the valve opening. With this function, it is possible to suppress abnormal overvoltages that often occur in circuits and protect circuits from damage caused by overvoltages.

For example: now our home color TV power supply circuit is used zinc oxide varistor, the varistor used here varistor voltage is 470V, when the transient surge voltage maximum (non-RMS) exceeds 470V, the varistor is to reflect his clamping characteristics, pull the excessive voltage down, so that the rear circuit works within a safe range.

Principle: When the voltage applied to the varistor is below its threshold, the current flowing through it is extremely small, and it is equivalent to a resistor with infinite resistance. That is, when the voltage applied to it is low and pre-flutters above its threshold, it acts as a switch in the off-state state of the plexus.

When the voltage applied to the varistor exceeds its threshold, the current flowing through it surges, and it acts as a resistor with infinitesimal resistance. That is, when the voltage applied to it is higher than its threshold, it is equivalent to a closed state of repentance disorder switch.

The role of varistors:

The voltage placed on it is below the critical value, and the valve is opened by the current, which allows the suppression circuit to generate a special voltage, thus protecting the circuit from the danger of overvoltage.

How varistors work:

When the voltage applied at both ends of the varistor is lower than the nominal rated voltage value, the resistance value is close to infinity, and there is almost no current passing through it; When the voltage at both ends of the varistor is higher than the nominal rated voltage, the voltage quickly breaks down and turns from a high-resistance state to a low-resistance state, and the working current also increases sharply. When the voltage at both ends is lower than the nominal rated voltage, the varistor returns to the high-impedance state; When the voltage across the varistor exceeds its maximum limiting voltage, the voltage is completely damaged by breakdown and cannot be recovered.

Introduction to varistors

Varistor, referred to as VDR, is a resistive device with nonlinear volt-ampere characteristics, which is mainly used for voltage clamping when the circuit is subjected to overvoltage, and absorbs excess current to protect sensitive devices. The resistive material of the varistor is a semiconductor, so it is a variety of semiconductor resistors, and it is also a voltage-limiting protection device.

The principle of varistors

Taking advantage of the nonlinear characteristics of the varistor, when the overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, so as to realize the protection of the subsequent circuit.

The role of varistors:

Varistors are mainly used for transient overvoltage protection, and due to their volt-ampere characteristics similar to semiconductor regulators, they also enable them to have a variety of circuit element functions, such as being used as:

1. The stable voltage of DC high-voltage and low-current voltage regulator components can be as high as thousands of volts or more, which cannot be achieved by silicon voltage regulators.

2. Voltage fluctuation detection element.

3. DC level shift element.

4. Equalizing pressure element.

5. Fluorescent starting element.

The main uses of varistors: lightning protection, overvoltage protection.

A varistor is a resistive device with nonlinear volt-ampere characteristics, and a resistor is generally called a resistor in daily life. It is a current limiting element, after the resistor is connected to the circuit, the resistance value of the resistor is fixed, generally two pins, which can limit the current through the branch it is connected to.

The response time of the varistor is ns, which is faster than the air discharge tube and slightly slower than the TVS tube, and the response speed of the overvoltage protection used for electronic circuits can meet the requirements in general. The overvoltage refers to the long-term voltage fluctuation phenomenon in which the root mean square value of the AC voltage rises at the power frequency, exceeds 10 of the rated value and lasts more than 1 minute, which is an electromagnetic disturbance phenomenon in the power system.

Introduction to the basic performance of varistors:

1) Protection characteristics, when the impulse strength of the impulse source (or impulse current ISP USP ZS) does not exceed the specified value, the limiting voltage of the varistor is not allowed to exceed the impulse withstand voltage (URP) that the protected object can withstand.

2) Shock resistance, that is, the varistor itself should be able to withstand the specified impulse current, impulse energy, and the average power when multiple impulses occur one after another.

3) There are two life characteristics, one is the continuous working voltage life, that is, the varistor should accompany and work reliably for the specified time (hours) under the specified ambient temperature and system voltage conditions. The second is impact life, that is, the number of times it can reliably withstand the specified impact.

4) After the varistor intervenes in the system, in addition to the protective role of the safety valve, it will also bring in some additional effects, which is the so-called secondary effect, which should not reduce the normal working performance of the system. At this time, there are three main factors to consider, one is the capacitance of the varistor itself (tens to tens of thousands of pf), the second is the leakage current at the system voltage, and the third is the influence of the nonlinear current of the varistor through the coupling of the source impedance on other circuits.