Pressure-sensitive resistors

Varistor is a resistive device with a nonlinear volt-ampere characteristic, which is mainly used to clamp the voltage when the circuit is under overvoltage and to absorb excess current to protect the sensitive device. The English name is "Voltage Dependent resistor" abbreviated as "VDR", or called "Varistor". The resistor body material of a varistor is a semiconductor, so it is a variety of semiconductor resistors. Now a large number of "ZnO" (ZnO) Varistor, its main material has two valence element zinc (Zn) and six valence element oxygen (O). So from the material point of view, zinc oxide varistor is a "ⅱ-ⅵ-family oxide semiconductor." In Taiwan, Varistors are called "surge absorbers" and are sometimes referred to as "electric shock (surge) suppressors (absorbers)".

When the voltage on the varistor is below its threshold, the current flowing through it is very small, which is equivalent to a resistor with an infinite resistance. That is, when the voltage above it is below its threshold, it is equivalent to a switch in a disconnected state. When the voltage on the varistor exceeds its threshold, the current flowing through it surges, which is equivalent to a resistor with an infinitely small resistance.  That is, when the voltage above it is above its threshold, it is equivalent to a closed state switch. The varistor is generally used in parallel in the circuit, when the voltage at both ends of the resistor is rapidly changing, the resistance short-circuit fuses the current fuse, play a protective role. Varistors are commonly used for overvoltage protection and regulation of power supplies in circuits.

Calculation of pressure-sensitive voltage:
Generally available in the following formula:
U1ma=kuac type: K for the power quality-related factors, generally take k= (three or three), the power quality of the city preferable smaller, poor quality of the rural areas (especially mountainous areas) preferable. UAC is a valid value for AC supply voltages. For 220v~240v AC power Lightning Protection device, the pressure-sensitive voltage of 470v~620v should be chosen as the appropriate pressure-sensitive resistor. The use of a pressure-sensitive voltage sensitive resistor, can reduce the failure rate, prolong the service life, but the residual pressure slightly increased

Calculation of nominal discharge current:

The nominal discharge current of the varistor should be greater than the required inrush current or the maximum inrush current that may occur each year. The nominal discharge current shall be calculated by pressing the value of more than 10 times in the norm curve of the varistor surge life, which is about 30% of the maximum impact flux (i.e. 0.3 IP)
Steps to read

Varistors in parallel:

When a varistor cannot meet the requirements of a nominal discharge current, multiple varistors should be used in parallel. Sometimes in order to reduce the limiting voltage, even if the nominal discharge current meets the requirements, multiple varistors are used in parallel. It is important to note that in parallel use of varistors, it is necessary to strictly select the same parameters (for example: δu1ma≤3v,δα≤3) for pairing to ensure uniform distribution of current

The temperature insurance tube should have good thermal coupling with the Varistor, when the Varistor failure (high impedance short-circuit), the heat generated by it fuses the temperature fuse, so that the failure of the varistor and circuit separation, to ensure the safety of the equipment. When the higher Power frequency transient overvoltage action on the varistor, it is possible to make the varistor instantaneous breakdown short circuit (low impedance short circuit), and the temperature insurance tube is too late to fuse, may also fire. In order to avoid this phenomenon, it is possible to cascade an impact-resistant frequency fuse on each varistor (single-frequency fuse may not fuse during aging failure)