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.
Figure 1. AC/DC single-phase protection
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.
Figure 3. Single-phase protection 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.
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.
Burst
According to IEC 61000-4-4, 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 61000-4-5 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 61000-4-5. 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 61000-4-5 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 61000-4-5 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
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