1. Varistor performance ch‍£‌φaracteristics:

varistor refers to a r≈↑‍>esistor whose resistance val©δue varies with voltage with<₽®in a certain voltage range, or Ω∏♦a resistor whose resistance value is♣‌ sensitive to voltage.

Varistors are semiconducto™∞ r components. When the vol ¶Ωtage applied at both ends <®of the varistor is within 0 to U_1mA (varistor voltage), the cur♦™"♠rent flowing through the varistor wi✘±​↕ll be no more than 1mA,♣  and the varistor resistance value iβ©>≥s so large that we can conside♥σr that the varistor is an​$ "insulator" in this syste‍σ↔m; When the instanta↕₹neous pulse high voltage in₽↔πtrusion, the peak pul<"✘®se voltage exceeds the pre₩→ssure-sensitive voltage, the varistor r₽♠§ esistance value becomes rapidly s×∑←maller, the reaction time is less t¥±♠han 20*10^-9 seconds, and the varistor &q∏↑✔uot;instantly" becomes “conductor".For example, when the π peak pulse voltage at both ♦↑ends of the varistor Ωσ→is 2 times the voltage of t§α♣he varistor U1mA, the resistance value  ​of the varistor is usually l→♣ •ess than 1Ω, which can complete♣↕ly considered as a conductor.

What a varistor does: When a $∏ varistor works, it suppresses a singleδ≈→$ or intermittent surge of mult‌↑→iple transient voltage/current pu≤αβ™lses.

Surge duration: The surge voltage/curr✔&→≥ent duration is in the scale of micro₹≤β seconds and milliseconds. Such as c≈ ≥ommon 1.2/50μs (standard voltage ↔∑×wave), 8/20μs (current short wave), 10/1000μs (current long wave), ‍‌2mS (square wave) and other such insta₹ε©βnt pulses, the impact δ₩¶©of components in the line is liφ<‍ke "waves beating th>♣e shore", suppressing and ​ ₽∏weakening such destruct↔ε® ive surge pulses is professi♥‍onal work o a varistor . For a lonσ∞g period of over-voltage ≈ applied at both ends, will p&∏↑★ut varistor  "in danger&quo ¥t;.

The weakness of varist↕™♠or: it can not withsta→"nd long-time over-voltage. For the fa®Ωult over-voltage that appears in t§¥≠he AC power grid, if the peak of t≤εhe AC voltage rates above the varis∏← tor voltage U_1mA, then the current f"♥♥¶lowing through the vari↕→®stor will be above 1mA,±→α the varistor is in the on-state, at t★£βδhis time, the continuous current po&≠€uring into the varistor is like a &∞€quot;waves beating the shore" (sγ>ee Figure 1 below). Because the power™✘ of voltage-sensitive is very s↕↓✔mall, the varistor will smoke an★€d burn within a few sec↔✘$₽onds to a few hours, an ÷d the internal porcelain body will be pφ₽€erforated, which should φ be paid special attention to.

2. Varistor working principle:

There are various descriptions of th&>e working principle of♣φ∏♦ the varistor, in order to facil✔±itate everyone's understand∏π→↓ing, the following is aγ♠£ simple description from the ±€<perspective of "dividing the curre​★"nt" and "dividing the voltage™§£".

The role of dividing the current: Froπ>‍Ωm the current point of view, when‌∏™☆ the high-voltage pulse is encou∏"↔ntered, the varistor resistance ≥≈value instantly becomes much le &ss than the overall impedance value of ∑≠÷the protected circuit that in par§<allel, most of the pulse curre™$ γnt flows through the varistor to thβ£​e external end of the equi↓₩pment, and the varistor pla≥'♥ys a current-dividing role, whicπ∏h will attract the m÷©ajority of the high pulse cur☆✘λ←rent, since current always tend to mo☆¥₩∑ve towards the way with less resistΩ↔ance, protecting the equipment from th↕⶙e intrusion of the i•¶nstantaneous high current a©∞nd high energy. Figure 2 ​↔☆✔below:

The role of dividing the voltagδᱩe:From the voltage poi ≥÷nt of view, due to the£→δ source impedance R₀ to divide part of the pulse voltage,α→ ↕ and the pulse voltage on the va"≠‌ristor only has a sma♥ ll part of the voltage - residu&♣al voltage (an important para★♦↓♣meter of varistor ), the selection of♥✘☆‍ the appropriate specification←>↓↔s of the varistor so that the residual"®> voltage is less tha ®¥n the insulation voltage limi© →λt of the protected circuit co☆π"mponents, the equipment will not ♥↑≠be damaged by high pulλ£se voltage.

As the pulse voltage disappeare¶≤★¥d, the varistor immediately resto¥×φ≈red the insulation stateπ↕ and continued to stand by.

Source impedance:

It can be roughly thought of as the sumΩ→ of the impedances of all©÷ cables, transformers, air swit×±δεches, etc， from the location of the'© lightning strike to the varistor.

The source impedance i>÷←σn the circuit will divide a part of ∞↔÷∞the pulse voltage to ∏φreduce the voltage of protection d✔₩evices such as varistors, so thatexp®♦★<erienced circuit engi↔ε→←neers will consider the source ®<"₩impedance while desi‍¥☆gning.