1. Varistor performance characterγ↕≤©istics:

varistor refers to a resisto∑ε<r whose resistance value varies wit♣←'h voltage within a certain v★εoltage range, or a resistor whose $•∏resistance value is sensitive to voltag∞¥£↓e.

Varistors are semiconducε¥×tor components. When the voltage↑© applied at both endsβ≥¶ of the varistor is w&πithin 0 to U_1mA (varistor voltage), the current flowi€£ng through the varistor wi∞↔φll be no more than 1mA, a™↕nd the varistor resistance value is→σβ so large that we can consider that the↓§↑ varistor is an "insu÷>παlator" in this system; ∑<×When the instantaneous&₽ pulse high voltage "∏λσintrusion, the peak pulse voltage exc<εeeds the pressure-sensitive voltag$‍σe, the varistor resisγ₽tance value becomes ✔÷δrapidly smaller, the reaction time is l♠↑©ess than 20*10^-9 seconds, and the varistor₽α< "instantly" become£σs “conductor".For example, whe‍© n the peak pulse voltaγ♣ge at both ends of the vδ&♥aristor is 2 times the‌←£ voltage of the varistor U1mA, the re¶λsistance value of the ✔★∏varistor is usually less than 1Ω, which can completely ∏∞γconsidered as a conductor.

What a varistor does: When a vari$✔✘§stor works, it suppr≥∏esses a single or intermitte∞•÷nt surge of multiple transient voltage/ ≥↓§current pulses.

Surge duration: The surg♣¥<e voltage/current du÷ ration is in the scale of microsec↑φonds and milliseconds¥<. Such as common 1.2/50μs (standard voltage wave), 8/20μs (current short wave), 10/10 ₽00μs (current long wave), 2mS (squa εre wave) and other su±®ch instant pulses, the impaλ&•£ct of components in thΩ​e line is like "waves beat☆♠✘ing the shore", suppressi δ>βng and weakening such destruc♠€↓≥tive surge pulses is p♠ rofessional work o a varistor . ♣←₩For a long period of oveγ≥¶‌r-voltage applied at bΩ"×αoth ends, will put varistor&Ωσ ₹nbsp; "in danger" Ω↕≥.

The weakness of varist<♠ ♥or: it can not withstand l‌•≠ong-time over-voltage. F±€δ'or the fault over-voltageσα that appears in the AC pow₽£er grid, if the peak↕•₹ of the AC voltage raα¶tes above the varistor ♠☆voltage U_1mA, then the current flowing ₹↔¥<through the varistor will be ←÷λ₽above 1mA, the varistor is‍δ in the on-state, at this time, the ∑•₩continuous current pouring into ☆™the varistor is like a &qβ​©uot;waves beating the shore" (s₽βee Figure 1 below). Because t♦₽'€he power of voltage-sensitive i©β∏s very small, the varistor will α✘±₹smoke and burn within a fe↔©γw seconds to a few hours, and the∞>≤ internal porcelain body will be pe≈↔↑♦rforated, which should be paid s≥δ±pecial attention to.

2. Varistor working principle:

There are various descriptions ≠₽of the working principle of the variφα≠πstor, in order to facilitate everyoβ'≥ne's understanding, the followingσλ is a simple description from the pers"♣↓pective of "dividing¥↕ the current" and "dividing t™>€•he voltage".

The role of dividing α☆Ωthe current: From the current pointφ  of view, when the high-voltage pulse i× ∑≥s encountered, the varistor φ♠≤£resistance value instantly becomes m↓≠uch less than the overaα↔β$ll impedance value of the prot₩π₹®ected circuit that in parallel, most σ✔of the pulse current f‍€★lows through the varistor to  β the external end of the equip∑εment, and the varistor p>±αlays a current-divid¶≥↔ing role, which will attrπδ§act the majority of the hδ™↓ igh pulse current, since cur$∏↕£rent always tend to move towa≤σ$±rds the way with less resistance★'±, protecting the equipment ₽♣✔←from the intrusion of↔↑© the instantaneous high currγ®ent and high energy. Figure 2 below:

The role of dividing the voltage:Frγφ<om the voltage point of view, due & to the source impedance R₀ to divide part of th• e pulse voltage, and the₩¥  pulse voltage on the var‍​§istor only has a sma£≥$'ll part of the voltage -™π$ residual voltage (an impo¥®₽rtant parameter of varistor ), the δ©₽≈selection of the appropriate ↔¥δ specifications of the varistor s $o that the residual voltage i♦♠↑↓s less than the insulation voltagε≤>₹e limit of the protected circuit compo♣" σnents, the equipment will not ≥¶γ≥be damaged by high pulse €↑γvoltage.

As the pulse voltage disappeared, the♦Ω✘ varistor immediately restε®ored the insulation sta€✘×'te and continued to stand by.

Source impedance:

It can be roughly thoughtΩβ≤ of as the sum of the impedances of♥÷ all cables, transformers, air sw€"itches, etc， from the location of the¶π☆< lightning strike to ""₽≥the varistor.

The source impedance in the circuit ' ≥<will divide a part of the pulse voltag☆♣¶e to reduce the volta∞★ge of protection devices such as vari↑λ®stors, so thatexperienced circuit engi•‍neers will consider the s‌→ource impedance while designingπ$♥.