The resistor is a very common electronic component and is widely used in circuits. Voltage-Dependent Resistor, abbreviated as "VDR" or "varistor", is a kind of resistance device with nonlinear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is under overvoltage and absorb excess current to protect sensitive devices.
The material of the varistor is a semiconductor, so it is a kind of semiconductor resistor. A varistor is a kind of ideal protection element, because of its excellent performance, low price, small volume, wide working voltage range, fast response to overvoltage impulse, strong resistance to impulse current, small leakage current (less than a few microamperes to tens of microamperes), small resistance temperature coefficient and so on. Zinc oxide (ZnO) varistors are widely used nowadays. Its main material is composed of bivalent element (Zn) and hexavalent element (o). The following small series introduces the working principle of varistor in the circuit, circuit symbols, and role.
Ordinary resistors obey Ohm's law, while the voltage and current of varistors have a special nonlinear relationship. When the voltage applied at both ends of the varistor is lower than the nominal rated voltage, the resistance value is close to infinity, and there is almost no current flowing inside; When the voltage at both ends of the varistor is slightly higher than the nominal rated voltage, the varistor will break down and turn on rapidly, and the high resistance state will change to the low resistance state, and the working current will increase sharply. Therefore, when the voltage at both ends is lower than the nominal rated voltage, the varistor can return to the high resistance state; When the voltage at both ends of the varistor exceeds its maximum limit voltage, it will be completely broken down and cannot recover itself.
The varistor is represented by the character symbol "RV" or "R" in the circuit, and figure 1 is its circuit graphic symbol. A varistor is different from an ordinary resistor. It is made according to the nonlinear characteristics of semiconductor materials.
The main function of the varistor is to protect the transient voltage in the circuit. Because of the working principle described above, the varistor is equivalent to a switch. Only when the voltage is higher than the threshold, the resistance value is infinitesimal, and the switch is closed, which makes the current flowing through it surge and the influence on other circuits is not changed much, thus reducing the influence of Overvoltage on the subsequent sensitive circuit. This protection function of the varistor can be used repeatedly many times and can be made into a one-time protective device similar to the current fuse.
But because of its volt-ampere characteristic similar to the semiconductor voltage regulator, it also has many circuit element functions. For example, the varistor is a DC high voltage and small current regulator, which can not be achieved by silicon regulators, with the stable voltage of more than thousands of volts; Varistor can be used as voltage fluctuation detection element, DC level shift element, fluorescent starting element, voltage equalizing element and so on.
The protection function of the varistor has been widely used now. For example, the power circuit of the color TV is to uses a varistor to complete the over-voltage protection function. When the voltage exceeds the threshold, the varistor will reflect its clamp characteristics, pull the high voltage down, so that the latter circuit works within the safe voltage range.