PCB News - The main points and principles of the selection of varistor for pcb manufacturers

The main points and principles of the selection of varistor for PCB manufacturers

1. Application principle of zinc oxide varistor
Varistor is a voltage-limiting protection device. Using the non-linear characteristics of the varistor, when an overvoltage appears between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby protecting the subsequent circuit and allowing the PCBA board to By a certain degree of self-protection. The main parameters of varistor are: varistor voltage, current capacity, junction capacitance, response time, etc.

The response time of the varistor is ns, which is faster than the air discharge tube and slightly slower than the TVS tube. Under normal circumstances, the response speed of the overvoltage protection for electronic circuits can meet the requirements. The junction capacitance of varistor is generally in the range of several hundred to several thousand pF. In many cases, it is not suitable to be directly applied to the protection of high-frequency signal lines. When applied to the protection of AC circuits, because its large junction capacitance will increase leakage Current needs to be fully considered when designing the protection circuit. The current capacity of the varistor is larger, but smaller than that of the gas discharge tube.

The varistor is used in parallel with the electrical equipment or components to be protected. When lightning overvoltage or transient operation overvoltage Vs occurs in the circuit, the varistor and the protected equipment and components are exposed to Vs at the same time. Because the varistor has a fast response speed, it appears quickly in nanoseconds. Excellent non-linear conductivity characteristics (see the breakdown zone in Figure 3). At this time, the voltage across the varistor drops rapidly, much less than Vs, so that the actual voltage on the protected equipment and components is much lower than the excessive Voltage Vs, so that the equipment and components are protected from the impact of overvoltage.

2. Selection of varistor voltage for zinc oxide varistor
Select the voltage V1mA under the specified current of the varistor according to the voltage of the protected power supply. The general selection principles are:

For DC circuit: V1mA≥2.0VDC

For AC circuit: V1mA≥2.2V effective value

In particular, it is pointed out that the selection criteria for the varistor's varistor voltage is higher than the supply voltage. While being able to protect the device that needs to be protected, choose a varistor with a high varistor voltage as much as possible, so that it can not only protect the device, but also Can improve the service life of the varistor. For example, the withstand voltage of the device to be protected is Vdc=550Vdc, and the working voltage of the device is V=300Vdc, then we should choose a varistor with a varistor voltage of 470V. The varistor voltage range is (423-517). The negative error of the varistor voltage of 470-47=423Vdc is greater than the device's power supply voltage of 300Vac, and the positive error of 470+47=517Vdc is less than the device's withstand voltage of 550Vdc.

You must also pay attention to:

(1) It must be ensured that the continuous working voltage will not exceed the allowable value when the voltage fluctuates, otherwise the service life of the varistor will be shortened;

(2) When a varistor is used between the power line and the ground, sometimes the voltage between the line and the ground rises due to poor grounding. Therefore, a varistor with a higher nominal voltage than the line-to-line use is usually used.

3. Selection of the flow rate
Usually the flow rate given by the product is the current value that the product can withstand when the pulse test is carried out according to the waveform, the number of impacts and the gap time given by the product standard. The number of shocks that the product can withstand is a function of the waveform, amplitude and gap time. When the amplitude of the current waveform is reduced by 50%, the number of shocks can be doubled, so in practical applications, the surge current absorbed by the varistor Should be less than the flow rate of the product.

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