Selection of varistor and introduction of main parameters of varistor

Varistor is a voltage limiting protection device. Using the nonlinear characteristics of varistor, when overvoltage occurs between the two poles of varistor, varistor can position the voltage clamp to a relatively fixed voltage value, so as to achieve subsequent power supply maintenance. This article has collected and organized some materials, hoping to have great reference value for readers.

1. Main parameters of varistor

1. Standard varistor voltage (V): refers to the pulse current (generally 1ma). The voltage value at both ends of the varistor that requires a duration is usually less than 400ms.

2. Voltage ratio: refers to the ratio of the voltage value generated when the current of the varistor is 1ma to the voltage value generated when the current of the varistor is 0.1ma.

3. Large voltage limit (V): the voltage peak value on both sides of the varistor under the large pulse peak current IP and wave type that can be accepted by the varistor.

4. Residual voltage ratio: when the current of the varistor is a certain value, the voltage formed on both sides is called the residual voltage of the current value. The residual voltage ratio is the ratio of residual voltage to nominal voltage.

5. Through current capacity (kA): Through current capacity, also called through flow, refers to the large pulse (peak) current value on the varistor, which is allowed under specified conditions (required time interval and frequency, apply standard impulse current).

6. Leakage current (mA): leakage current, also called waiting current, refers to the current flowing through the varistor under the specified temperature and large DC voltage.

7. The voltage temperature coefficient refers to the nominal voltage change rate of the varistor within the specified ambient temperature (20 ℃)~70 ℃), that is, when the current of the varistor remains stable and the temperature changes by 1 ℃, the voltage on both sides of the varistor changes relatively.

8. Current temperature coefficient: when the voltage on both sides of the varistor remains stable, when the temperature changes by 1 ℃, the relative change of the current flowing through the varistor.

9. Voltage nonlinear coefficient: refers to the ratio of static resistance value and dynamic resistance value of varistor under the action of given additional voltage.

10. Insulation resistance: refers to the resistance between the lead (pin) of the piezoresistor and the insulation surface of the resistance.

11. Static capacity (PF): refers to the inherent capacitance of the varistor itself.

12. Maximum power: Work at a specific operating temperature of 85 ℃ for 1000 hours to convert the voltage sensitive voltage to a maximum power of less than 10%.

13. Large impulse current (8/20us): impact the varistor once or twice (the interval between each time is 5 minutes), specific impulse current (8/20us waveform). The voltage sensitive conversion is still within 10% of the larger impulse current.

2、 Selection method of varistor

Before using the varistor, you should know the following relevant performance parameters: the nominal voltage refers to the voltage value at both ends of the varistor. The leakage current refers to the current value flowing through the varistor when applying continuous DC voltage at 25 ℃. When the varistor passes 8/20 level current pulse, the horizontal voltage refers to the voltage peak on both sides. The flow rate represents the application of a specified pulse current (8/20 μ s) The waveform of the peak current. Surge environment parameters include large surge current IPM (or large surge voltage VPM and surge source impedance Zo), Tt surge pulse width, the minimum time interval Tm between the last two surges within the predetermined working life of the varistor, and the total frequency N of the surge pulse.

Generally speaking, varistor is usually used in parallel with the protected device or equipment. Under normal conditions, the DC or AC current at both ends of the varistor shall be less than the nominal voltage. Even if the power fluctuation is the worst, it should not exceed the main continuous working voltage selected in the rated value. The nominal voltage value corresponding to the continuous operation voltage value is the adopted value. For the application of overvoltage protection, the voltage sensitive voltage value should be greater than the actual circuit voltage value. Generally, the following method should be selected: VmA=av/bc, where:

A is the voltage fluctuation index of the circuit; V is the DC working voltage of the circuit (effective value during AC); B is the voltage deviation of the voltage sensitivity; C is the aging index of the equipment,; The specific value of VMA is 1.5 times the DC working voltage, and the peak value should be considered under AC conditions, so the value should be increased by 1.414 times.

In addition, attention should be paid to:

(1) Ensure that the continuous working voltage will not exceed the specified value when the voltage fluctuation is large, otherwise the service life of the varistor will be reduced;

(2) When varistor is used between the power line and the ground, sometimes the voltage between the line and the ground will rise due to poor grounding, so the varistor whose nominal voltage is higher than that of the application place between the lines is generally used.

The surge current absorbed by varistor shall be less than the main flow of commodities.

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