Categorized | Power Devices, Varistors


Posted on 26 March 2019

Overvoltage Protection with Varistors



Overvoltages are generally categorized according to where they originate.

Internal overvoltages

Internal overvoltages originate in the actual system which is to be protected, e.g. through

  • inductive load switching
  • arcing
  • direct coupling with higher voltage potential
  • mutual inductive or capacitive interference between circuits
  • electrostatic discharge (ESD)

External overvoltages

External overvoltages affect the system that is to be protected from the outside, e.g. as a result of

  • line interference
  • strong electromagnetic fields
  • lightning

In most cases, the waveform, amplitude, and frequency of occurrence of these transients are not known or, if so, only very vaguely. And this, of course, makes it difficult to design the appropriate protective circuitry.
There have been attempts to define the overvoltage vulnerability of typical supply systems (e.g. industrial, municipal, rural) so that the best possible protective device could be chosen for the purpose. But the scale of local differences makes such an approach subject to uncertainty. So, for reliable protection against transients, a certain degree of “overdesign” must be considered. Therefore the following figures for overvoltage in 230 V power lines can only be taken as rough guidelines:

  • Amplitude up to 6 kV
  • Pulse duration 0.1 ms to 1 ms

Where varistors are operated directly on the line (i.e. without series resistor), normally the type series S20 should be chosen. In systems with high exposure to transients (industrial, mountain locations) block varistors are to be preferred. Requirements are stipulated in IEC 61000-4-X. Severity levels are specified in the respective product standards.

Principle of protection and characteristic impedance

The principle of overvoltage protection by varistors is based on the series connection of voltage-independent and voltage-dependent resistance. Use is made of the fact that every real voltage source and thus every transient has a voltage-independent source impedance greater than zero.

This voltage-independent impedance Zsource can be the ohmic resistance of a cable, the inductive reactance of a coil, or the complex characteristic impedance of a transmission line. If a transient occurs, current flows across Zsource and the varistor that, since Vsource = Zsource · i, causes a proportional voltage drop across the voltage-independent impedance. In contrast, the voltage drop across the varistor is almost independent of the current that flows. Since

\begin{equation} V_{VAR}=\Big(\frac{Z_{VAR}}{Z_{SOURCE}+Z_{VAR}}\Big)v\end{equation}

the voltage division ratio is shifted so that the overvoltage drops almost entirely across Zsource. The circuit parallel to the varistor is thus protected.

Equivalent circuit
Figure 1. Equivalent circuit in which Zsource symbolizes the voltage-independent source impedance

Figure 2  shows the principle of overvoltage protection by varistors.

Overvoltage protection principle with varistors
Figure 2. Principle of overvoltage protection by varistors


For selection of the most suitable protective element, you must know the surge current waveform that goes with the transient. This is often, and mistakenly, calculated by way of the (very small) source impedance of the line at line frequency. This leads to current amplitudes of unrealistic proportions. Here you must remember that typical surge current waves contain a large portion of frequencies in the kHz and MHz range, at which the relatively high characteristic impedance of cables, leads, etc. determines the voltage/current ratio. Figure 3 shows approximate figures for the characteristic impedance of a supply line when there are high frequency overvoltages. For calculation purposes, the characteristic impedance is normally taken as being 50 W. Artificial networks and surge generators are designed accordingly.

Supply line impedance

Figure 3. Impedance of a supply line for high-frequency overvoltages


Areas of application for varistors

A wide selection of types is available to cover very different requirements for protective level and load capability. Straightforward conditions of use and an attractive price/performance ratio are vital to ensure success in just about every area of electrical engineering and electronics.

These areas of application include:

Telecommunications, Power supply units, Cellular (mobile) phones, Chargers, Industrial controls, Machine controls, Alarm systems, Lighting controls, LCDs, Bridge rectifiers, Electric welding, Electric vehicles, Switch-mode power supplies, DC/AC converters, Personal computers, EMI/RFI suppression,
Video sets , etc.


For further information please read:

Introduction to Varistors

Terms and Descriptions - Varistors

PSpice Simulation Model

Selection Giuide – Varistors

Design Notes

General Voltage Surge Protection


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