1. Varistor performance characteristics:

varistor refers to a resistor whose resistance value varies with voltage within a certain voltage range, or a resistor whose resistance value is sensitive to voltage.

Varistors are semiconductor components. When the voltage applied at both ends of the varistor is within 0 to U_1mA (varistor voltage), the current flowing through the varistor will be no more than 1mA, and the varistor resistance value is so large that we can consider that the varistor is an "insulator" in this system; When the instantaneous pulse high voltage intrusion, the peak pulse voltage exceeds the pressure-sensitive voltage, the varistor resistance value becomes rapidly smaller, the reaction time is less than 20*10^-9 seconds, and the varistor "instantly" becomes “conductor".For example, when the peak pulse voltage at both ends of the varistor is 2 times the voltage of the varistor U1mA, the resistance value of the varistor is usually less than 1Ω, which can completely considered as a conductor.

What a varistor does: When a varistor works, it suppresses a single or intermittent surge of multiple transient voltage/current pulses.

Surge duration: The surge voltage/current duration is in the scale of microseconds and milliseconds. Such as common 1.2/50μs (standard voltage wave), 8/20μs (current short wave), 10/1000μs (current long wave), 2mS (square wave) and other such instant pulses, the impact of components in the line is like "waves beating the shore", suppressing and weakening such destructive surge pulses is professional work o a varistor . For a long period of over-voltage applied at both ends, will put varistor  "in danger".

The weakness of varistor: it can not withstand long-time over-voltage. For the fault over-voltage that appears in the AC power grid, if the peak of the AC voltage rates above the varistor voltage U_1mA, then the current flowing through the varistor will be above 1mA, the varistor is in the on-state, at this time, the continuous current pouring into the varistor is like a "waves beating the shore" (see Figure 1 below). Because the power of voltage-sensitive is very small, the varistor will smoke and burn within a few seconds to a few hours, and the internal porcelain body will be perforated, which should be paid special attention to.

2. Varistor working principle:

There are various descriptions of the working principle of the varistor, in order to facilitate everyone's understanding, the following is a simple description from the perspective of "dividing the current" and "dividing the voltage".

The role of dividing the current: From the current point of view, when the high-voltage pulse is encountered, the varistor resistance value instantly becomes much less than the overall impedance value of the protected circuit that in parallel, most of the pulse current flows through the varistor to the external end of the equipment, and the varistor plays a current-dividing role, which will attract the majority of the high pulse current, since current always tend to move towards the way with less resistance, protecting the equipment from the intrusion of the instantaneous high current and high energy. Figure 2 below:

The role of dividing the voltage:From the voltage point of view, due to the source impedance R₀ to divide part of the pulse voltage, and the pulse voltage on the varistor only has a small part of the voltage - residual voltage (an important parameter of varistor ), the selection of the appropriate specifications 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 damaged by high pulse voltage.

As the pulse voltage disappeared, the varistor immediately restored the insulation state and continued to stand by.

Source impedance:

It can be roughly thought of as the sum of the impedances of all cables, 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 voltage of protection devices such as varistors, so thatexperienced circuit engineers will consider the source impedance while designing.