The working principle and characteristics of the Metal Oxide Varistor

The Metal Oxide Varistor is a zinc oxide as the main component of the metal oxide semiconductor nonlinear pressure limiting resistor.

The volt-ampere characteristics of the Metal Oxide Varistor is a continuous and increasing, so it does not exist continued flow blocking problem.

Its working principle is the zinc oxide and additives of Metal Oxide Varistors "sintering" under certain conditions, the resistor will be affected by the strong influence of the voltage, the sharp rise in the current as the voltage rises, the rising curve of a nonlinear index. When the normal working voltage, a Metal Oxide Varistor is a high resistance state. When the arrival of the surge, it is in the path state, a strong current flow through itself leaked into the earth. Surge After this he immediately restored to the high resistance state.

Metal Oxide Varistor several important parameters:

A: varistor voltage: varistor voltage is generally considered to be at a temperature of 20 degrees on the Metal Oxide Varistor 1mA current flowing corresponding to the voltage across the resistor.

Varistor voltage in the AC power grid, in general, to be higher than the peak voltage of the grid is 0.7 times the peak voltage and the peak voltage is generally considered to √ 2 times the AC grid voltage (DC peak voltage is 1.2 times the rated voltage). Expressed by the formula:

VN = VNH × √ 2 ÷ 0.7

Where VN varistor voltage; VNH rated voltage for the grid.

B: leakage current: leakage current refers to the current under normal circumstances by the order of magnitude of the Metal Oxide Varistor microamps. Leakage current is smaller the better.

Special emphasis should be placed on the leakage current must be stable, not allowed to work automatically increased if it is found that the leakage current is automatically elevated, it should be immediately eliminated, because of the instability of the leakage current is accelerated aging SPD and SPD explosion the direct cause. Select leakage current parameters, you can not blindly pursue the smaller the better, as long as it is in the grid within the allowed value range, select the leakage current is relatively slightly larger than some of the lightning, but more stable.

C: response time: response time is equal to the time required by the varistor voltage, the voltage at both ends of the SPD and SPD this time completely turned. The Metal Oxide Varistor about the response time of 25ns.

D: parasitic capacitance: a Metal Oxide Varistor generally have a large parasitic capacitance is a parasitic capacitance generally between a few hundred picofarads to several thousands of picofarads, and thus it is not conducive to the protection of high-frequency electronic system. Because this parasitic capacitance of the high-frequency signal transmission will be distorted, thus affecting the normal operation of the system. Thus the protection of high-frequency system, should choose low parasitic capacitance of Metal Oxide Varistor type lightning.

Its advantages:

1, residual depression.

2, fast response time, 25ns or so.

3, free wheeling.

4, Notices telesignalling function can achieve deterioration instructions and failure, therefore, its protective effect safe and reliable. It is commonly used in the current power supply system products, especially electricity, telecommunications, power supply field, is thriving.

Its drawbacks: leakage current; parasitic capacitance larger, is not conducive to the protection of high-frequency electronic circuits.