Below are several important design considerations regarding the use of varistors in power electronics applications.
Physical protection - fuses
Due to the unpredictable nature of transients a varistor may be overloaded although it was carefully selected to begin with. An Overload may result in package rupture and expulsion of hot material. For this reason, the varistor should be physically shielded from adjacent components, e.g. using a suitable metal case.
Fuse protection of varistors against excessive surge current is usually not possible because standard fuses are unable to quench surge currents. But fuses can offer protection against damage caused by follow-on currents. Such follow-on currents occur when a damaged varistor is in low resistance mode and still connected to power.
When varistors are operated on standard line impedances, nominal fuse currents and varistor type series should be matched according to product data sheets.
In applications where the conditions deviate from standard power line impedances, better fuse protection of the varistor can be obtained using thermofuses. These thermofuses should be in direct thermal contact with the varistor.
Potting ,sealing, and adhesion
Potting, sealing, or adhesive compounds can produce chemical reactions in the varistor ceramic that will degrade its electrical characteristics.
Prior damage
The values specified only apply to varistors that have not been subjected to prior electrical, mechanical, or thermal damage.
Environmental conditions
Some varistors are designed for indoor applications and must therefore be prevented from exposure to:
- Direct sunlight
- Rain or condensation
- Steam, saline spray
- Corrosive gases
- Atmospheres with reduced oxygen content
Mechanical strength of wire leads of disk-type varistors
Wire leads must comply with the requirements of IEC 62020-2-2. They may only be bent at a minimum distance of 4 mm from the enamel coating end. When bending leads to shape, the lead component junction must be supported. The minimum bend radius should be 0.75 mm.
For more information, please read:
Terms and Descriptions - Varistors
Varistor Operation - Derating, Temperature, Overload
V-I Characteristics - Varistors
Overvoltage Protection with Varistors
