Posted on 16 July 2019

Varistor Applications

Varistors as protection devices








Varistors in protective circuits

Varistors are commonly used in protective circuits. Varistors used in applications must always be connected in parallel to the electronic circuits to be protected. Typical examples of this principle are given by the cicuit diagrams below.

AC-DC single-phase protection with varistors

Figure 1. AC/DC single-phase protection

AC three-phase protection with varistors

Figure 2. AC Three-phase protection

When varistors are used in line-to-ground circuits (figures 3 and 4), the risk must be considered that a current type fuse may possibly not blow if the grounding resistance is too high, and in this way the current is limited. In such cases, various international and local standards do not allow the line-to-ground application of varistors without taking adequate safety countermeasures. One possible solution is to use thermal fuses in series, which are thermally coupled with the varistor, as indicated in figures 3 and 4.

Single-phase protection with varistors including line-to-ground protection

Figure 3. Single-phase protection including line-to-ground protection

Three-phase protection with varistors including line-to-ground protection

Figure 4. Three-phase protection including line-to-ground protection

Applications of varistors used as a freewheeling circuit

Below are several examples of how varistors are used as freewheeling circuits.

Single-phase protection with varistors with working condition indicator

Figure 5,6, 7, 8. Single-phase protection with working condition indicator

When employing varistors in applications, numerous requirements must be taken into consideration. A few of the important requirements that must be taken into account when using varistors in applications are the following.


According to IEC 62019-4-4, burst pulses are low-energy transients with steep edges and high repetition rate. Thus, for equipment to pass burst testing successfully, design (line filter, grounding concept, case) is as critical as the choice of varistor. If IEC 62019-4-5 has been taken into account when selecting varistors, they will normally also handle the burst pulse energy without any problems. Due to the steepness of the pulse edges, the varistors must be connected in a way that keeps parasitic circuit inductance low.

Surge voltages

Immunity to interference from surge voltages (high energy) is tested in accordance with IEC 62019-4-5. The transient is generated using a combination wave (hybrid) generator. The severity level to be applied in the immunity test must be defined as a function of installation conditions. In most cases the respective product standards demand five positive and five negative voltage pulses. Standard IEC 62019-4-5 specifies severity level 3 (line-to-line, 2 kV applied via 2 W) as being the highest energy load.

For the immunity testing line-to-earth of power supplies, IEC 62019-4-5 specifies 12 W as the internal resistance of the test generator. The energy content, which is considerably lowered due to this, permits use of the “small” type varistors. For all other types of line, the internal resistance of the generator should be set to 42 W.

Note: Connection of varistors to ground may be subject to restrictions. This must be clarified with the respective authorization offices.

Interference emission

Switching off inductive loads can lead to overvoltages that may become sources of line interference as well as of inductively and/or capacitively coupled interference. This kind of interference can be suppressed using varistors connected as a flywheel circuit (figures 5-8).


For more information, please read:

Introduction to Metal Oxide Varistors

Protection of Automotive Electrical Systems Using Varistors

Design Notes - Varistors

PSpice Simulation Model


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