Varistor design parameters and classic circuit recording hardware learning notes 5

For ten days , While preparing for an exam , I finished my exam yesterday , I don't feel it , Or study hard , Keep a good record .
The article is only for personal understanding , Learning record , Without any authority .

Varistor, as the name suggests, is a voltage sensitive resistor , Often as surge Use of protective devices .

There are several design parameters of varistor that need attention ：

1： Maximum allowable voltage ： The normal working voltage of the circuit should be less than this voltage , Otherwise, the service life of the varistor will be greatly shortened .
2： threshold ： Generally, the varistor flows 1MA when , Voltage across the varistor , Also called on voltage .
3： Maximum limiting voltage ： In exerting 8/20us When the current surges , The maximum voltage at both ends of the varistor .
4： Maximum capacity ： At the first application 8/20us When the current surges , Conduct the maximum current , The second through capacity will be reduced .
5： Nominal discharge current ： This value should be greater than the surge current during the test .
6： Conduction time ：NS level .

Personal summary ：
The parasitic capacitance of the varistor is relatively large , In the use of AC circuits , Its loss is considerable , Preventing common mode surges is , If used alone , It will form considerable leakage current to the ground , Affect the normal operation of the circuit . There are two failure modes of varistors ：1, Open circuit ： The instantaneous current through the varistor is much larger than its on capacity , Cause the varistor to burst , Form an open circuit .2, A short circuit ： The usual failure mode of varistors is short circuit , The leakage current increases slowly , Until short circuit , Therefore, the leakage current affects the service life of the varistor . Because the parasitic capacitance of the varistor is relatively large , So it is not used in high-speed signal circuit , Will cause signal distortion .

Varistor application circuit record

1： Because the parasitic capacitance of the gas discharge tube is small , However, the series branch capacitance is reduced to PF level , Reduce leakage current , Improve the service life of varistors .

2： When the varistor is connected in series with a gas discharge tube , The conduction time is the sum of the two conduction times . The resistance value of the gas discharge tube is GΩ Level , The varistor is MΩ Level , Therefore, the pilot of the gas discharge pipe is connected , The varistor turns on after .

3： Why? LN No gas discharge tube is added between , And then PE Gas discharge tube shall be added at the end ？
a: stay L-N Adding a gas discharge tube will increase the conduction response time .
b: The unit price of gas discharge tube is too expensive ,L,N and PE When the varistor fails , In case of equipment metal shell , Will make the metal shell live , Form electric leakage , Will threaten the safety of life , stay DFMEA In the design, this failure effect is classified as grade 10, The highest level , So special protection is needed . stay L-N In case of short circuit failure of varistor between , It will form overcurrent , The front level air meeting shall be protected in time , Prevent overheating and fire .

4： The conduction voltage of the series branch depends on the gas discharge tube , The off voltage depends on the varistor .