Varistor we usually see the symbol “VDR”, it is a non-linear volt-ampere characteristics of the resistor device, with the characteristics of the resistance, but also other characteristics, it is mainly used in front of the voltage input to clamp the input voltage, limited to a certain range, while absorbing the excess current, it is the response time of the ns level, very fast, to protect the circuit behind a special component, especially in the surge protection. We often use zinc oxide varistors.
Varistors are often used in parallel with the protection components, if placed at the entrance of the power supply basically behind the fuse, when the varistor fails the fuse can play a protective role. The thermistor in the high-voltage circuit is generally an instantaneous current limit on the power, separate series in the circuit is generally used in the NTC negative temperature coefficient thermistor.
Varistors are generally divided into three types.
we usually often use is the standard type of varistor, sometimes also called surge absorption type, mainly used in is referred to as the input voltage overclamp and lightning transient suppression, this transient overvoltage appears to be random, the peak value of the voltage may be very large. We usually see the vast majority of varistors belong to this category. General body size range in Ф5mm ~ Ф20mm.
Here the pressure sensitive selection time to consider the full range:
① Maximum limiting voltage (V): that is, the maximum voltage value that the two ends of the varistor can withstand. Take 220V mains as an example, assuming that the mains input voltage fluctuation range between 0.8 ~ 1.5, at 1.5, the voltage at both ends of the varistor as high as 220 × 1.5 = 330V, then you can choose the varistor specifications can be 471 or 561 can be, choose the larger because the peak voltage may be more than 330V, or even higher, if the varistor is in the nominal voltage for a long time, then the varistor life expectancy. If the voltage of the varistor is at the nominal voltage for a long time, then the life of the varistor will be affected easily.
② Through-current capacity (kA): It refers to the maximum pulse current value allowed to pass through the varistor when the inrush current is applied in the specified time interval. In practice, the surge current absorbed by the varistor should be less than the maximum throughput of the product.
There are nominal voltage, leakage current (mA) and other aspects may be considered.
As the name suggests, is to absorb the surge of a higher level, greater energy density, this type of pressure sensitive mainly used for cranes, current transformers, generators and other large inductive coils in the magnetic energy, for this pressure sensitive energy absorption capacity is one of its selection criteria, because it is the use of piezoresistors to absorb energy. Now a lot of high-energy type varistor is either not enough high-pressure or not enough high-energy, not enough high-energy, then the absorption of energy density is not large enough, which in the selection of the actual needs of the trade-offs, focus on the protection of which.
This type of pressure sensitive is mainly used to absorb the periodic appearance of a continuous pulse group of a pressure sensitive, such as in the switching power supply varistor, because the switching power supply impact voltage has a periodicity, according to the period of the energy can be calculated, although the peak voltage is not high, but because of the frequency is very high, which leads to the average power is very large, therefore, this time you need to be a high power type of varistor. Selection should be calculated according to the cycle of the pulse appears to choose.
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