Varistor principle
Varistors are passive semiconductor devices used to protect electrical and electronic circuits. Unlike fuses or circuit breakers, which provide overcurrent protection, rheostats provide surge protection through voltage clamping.
When a high-voltage voltage is applied to a circuit, the results are often catastrophic, so variable resistors play an important role in protecting delicate electronic circuits from switching peaks and surge transients.
Transient overvoltages are generated by different circuits and power sources, whether they are powered by AC or DC power, as they are usually generated within the circuit itself or transmitted to the circuit from an external power source.
Transients within a circuit can cause voltage to rise rapidly, to thousands of volts, and these voltage spikes must be prevented from appearing on delicate electronic circuits and components.
One of the most common sources of voltage transients is the L(di/dt) effect (i.e., induced current transient), which is caused by switching the magnetizing current of the induction coil and transformer, the switching application of the DC motor, and the overvoltage or other overvoltage when connecting fluorescent lamps. Transient sources include lightning, static electricity, etc.
In normal operation, rheostats have a very high resistance (megaohms) and have no effect on the circuit as if they were not present. However, if the voltage across the rheostat (regardless of polarity) exceeds the nominal value of the rheostat, its effective resistance will drop dramatically as the voltage increases, as shown in the figure.
After the clamping of the varistor, the spike voltage is absorbed, which protects the subsequent circuit, as shown in the figure below:
The varistor is identified in the circuit as follows:
As can be seen from the identification of the picture, the schematic symbol of the varistor adds a variable meaning to the resistor.
Varistor package types are available in both in-line and chip types.
Figures 1 and 2 are plug-in type, and Figure 3 and Figure 4 are patch types. The shorter the lead, the smaller the parasitic index and capacitance. In the case of strong shocks, the residual voltage of the variable is constant, since the influence of parasitic parameters results in different peak voltages. When used, it is possible to compare the parameter differences of different packages of the same device type. The most common use of variables is to use the package in Figure 1 and place it at a power outlet for better surge protection.
The varistor device structure is shown in the figure below:
As can be seen from the figure above, the varistor is mainly composed of 5 components: 1 insulation layer, 2 element body, 3 conductive silver paste, 4 connecting strips, and 5 pins.
Volt-ampere characteristic curve of varistor
The volt-ampere characteristic curve of a varistor reflects the relationship between the voltage across it and the current passing through it. The volt-ampere curve of the varistor is shown in the figure below, from which it can be seen that the volt-ampere curve of the varistor is symmetrical, which is more suitable for overvoltage protection in AC circuits.
The voltage characteristics of varistors are nonlinear. Depending on the voltage, the voltage curve of the varistor can be divided into three parts: the leakage zone, the operating zone, and the transfer zone.
The following is a description of each of the three districts:
(1) Leakage current area
When the varistor operates in the leakage region, the voltage across the rheostat is low, and the rheostat has a large impedance to the outside, usually at the MΩ level. At this point, the rheostat will only increase the leakage current in the circuit, and the device will not work.
(2) Workspace
The operating area is also a nonlinear region of the rheostat, and the volt-ampere characteristic curve is almost perpendicular at normal operating voltages (below Vc). The current varies over a wide range, but the voltage at both ends of the rheostat does not change significantly, showing good clamping characteristics, which is the range in which the varistor acts. The steeper the nonlinear part of the curve, the better the varistor is protected.
(3) Transfer area
The up-transfer area is also known as the saturated region. As the current carried by the rheostat operating in the nonlinear range continues to increase, the voltage limitation characteristic of the rheostat disappears, the resistance drops dramatically, and the impedance becomes very small. Because a varistor generates heat due to its high power dissipation, it can eventually burn or even explode. Therefore, when a rheostat is used, it cannot reach the saturation zone or the upturn zone upwards and is fundamentally damaged.
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