1. Varistor performan≈♦↕ce characteristics:

varistor refers to a resistor whose¶₩ resistance value var$©&ies with voltage within a certain volt£'♣age range, or a resistor whose res£₽istance value is sensi↔♦tive to voltage.

Varistors are semico♠<nductor components. When the₽$ voltage applied at both ends o♣ f the varistor is within 0 →☆®✔to U_1mA (varistor voltage), λ₩€ the current flowing th≥♣∏rough the varistor will be no more '₩than 1mA, and the varistor resistance‍↕β value is so large that we canπ' consider that the v✔₹πaristor is an "insuλ"ε'lator" in this sy≥↕•stem; When the instantaε←±¶neous pulse high voltage intrusion, the∏γ peak pulse voltage exceeds  λδ the pressure-sensit₹π£δive voltage, the varistor resistanc&λe value becomes rapidly smaller, t& ×he reaction time is ≤∑αless than 20*10^-9 seconds, and the varistor "∞∏λ"instantly" becomes “conductor".For eΩ♦$xample, when the peak §✘π₩pulse voltage at both∏↑± ends of the varistor is 2 tim€‌es the voltage of the vari↔↑stor U1mA, the resistφ'ance value of the varistor is uε♣sually less than 1Ω, which can completely considered as a✘× conductor.

What a varistor does: When a varistαα•↑or works, it suppres♦₩&ses a single or intermitteε≈Ωnt surge of multiple transient voltage★</current pulses.

Surge duration: The surge volta<±£ge/current duration is in the scale of ÷∞©∏microseconds and millis≠≥♣econds. Such as common 1.2/50​ 'μs (standard voltage wave≤☆♦), 8/20μs (current short wave), 10/1000μs (current long wave), 2mS (squar§‍'↔e wave) and other such instant pul"♠ses, the impact of components ☆β​ in the line is like ♥δ"waves beating the shor"≥×₩e", suppressing and weaken±♥↑ ing such destructive surge pulses is ☆$₹professional work o a v‌α aristor . For a long pe∞‌riod of over-voltage appli ↕♣βed at both ends, will put®✘σ← varistor  "in danger"®σφ';.

The weakness of varistor: it can not wiπ♣Ωthstand long-time over-voltage. For ↔" ∑the fault over-voltage that appears¶'σ↔ in the AC power grid, if the peak☆♠ of the AC voltage rates above the vari>≠εstor voltage U_1mA, then the current flow£®♣ ing through the varistor w♠‌φ§ill be above 1mA, the varistor is in Ω✔$the on-state, at thi↔→‍s time, the continuous "₹current pouring into the va♠δristor is like a "waves שbeating the shore" (see ✔×Figure 1 below). Because the power oΩ™✔£f voltage-sensitive is very small, the"€®∞ varistor will smoke and burn with¶↕¶in a few seconds to a few hours, and ‍"∏φthe internal porcelain bo•σdy will be perforated, which shouldε®↕& be paid special attention to.

2. Varistor working principle:

There are various descriptio• €>ns of the working principle of the var><☆istor, in order to facilitate every←÷∏₩one's understanding,∑®↕∞ the following is a sim‌ ple description from the perΩ ‌spective of "dividing the curren≠☆©t" and "dividing the voltagλ₹e".

The role of dividing the current: Fr☆φ≈ om the current point of view♣♣​, when the high-voltage pul <&<se is encountered, the varistor resiφ•λstance value instantly becomes m​™≥≠uch less than the overall impe® ≠∞dance value of the protected circu​×it that in parallel, most of the p™₩ulse current flows throug→​εh the varistor to the external enΩ d of the equipment, and the varistor plλ♣< ays a current-dividi£↕♠ng role, which will attract the majoriφ"★¶ty of the high pulse current,γ∏©​ since current always tend to move≈‌‍& towards the way with less ₽↕ε§resistance, protecting th∏β™ e equipment from the intrusion of the≥•≠ instantaneous high curren∞¥←t and high energy. Figu®↑✘re 2 below:

The role of dividing th✘γ∑e voltage:From the v≤  αoltage point of view, due to the so∏εurce impedance R₀ to divide part of the pulse voltagγ≥‌•e, and the pulse voltage on the vari↓ε↔"stor only has a small part of t ↓he voltage - residual voltage (a  ≈n important parameter of vα↓ aristor ), the selectiε™<₽on of the appropriate specification♠¶★s of the varistor so that the ¥©&☆residual voltage is less÷↓ than the insulation ™→¶voltage limit of the protected &₩£circuit components, ">♦♣the equipment will not be dama ‍♥ ged by high pulse vol"≥Ω§tage.

As the pulse voltage d ∏'isappeared, the varistor im≈ΩΩmediately restored the insul∞π✔ation state and continued to stan‌©"₹d by.

Source impedance:

It can be roughly thought of as t££∞→he sum of the impedances of all c≥♦ables, transformers, air↕÷♥ switches, etc， from the location of the✘₹ lightning strike to the varistor.

The source impedance in the‍✘₩ circuit will divide a part of the>₽& pulse voltage to reduce the vol&★ tage of protection devices such as vari♠ stors, so thatexperienc"∞ed circuit engineers will consi♦¥€der the source impedance whil£∏≤¥e designing.