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Posted on 29 May 2020

Varistor Operation - Derating, Temperature, Overload

 

 

 

 

 

 

 

Below is a description of several factors that limit the operational life of a varistor.

Derating

Derating is the intentional reduction of maximum ratings in the application of a device. With metal oxide varistors, derating is of particular interest under the following conditions:

  • Derating for repetitive surge current and energy absorption
  • Derating at increased operating temperatures

Derating for repetitive surge current

A typical feature of metal oxide varistors is the dependence of the maximum permissible ratings for surge current, and thus for energy absorption, on the pulse shape, pulse duration, and the number of times the load is repeated during the overall lifetime of the varistor. The derating for a particular maximum permissible surge current can be derived from the derating curves for a particular type series in repetition figures graded 10x. The surge derating curve is mainly dependent on the varistor size but also voltage rating. Such derating curves can be found for all individual varistors.

The maximum permissible energy absorption can also be calculated from the derating curves by

Derating at increased operating temperatures

For operating temperatures exceeding 85 °C or 125 °C, the following operating conditions of varistors have to be derated according to the product tables.

  • Voltage
  • Surge current
  • Energy absorption
  • Average power dissipation

Operating and storage temperature

The upper limits of the operating and storage temperature ranges for an individual type series can be deduced from the 100% and 0% values found in product data sheets.

Climatic categories

The limit temperatures according to IEC 60068 are stated in the product tables as LCT (lower category temperature) and UCT (upper category temperature).

Overload response

Moderate overload

Surge currents or continuous overload of up to approximately one and a half times the specified figures can lead to a change in varistor voltage by more than ±10%. In most cases the varistor will not be destroyed, but there may be an irreversible change in its electrical properties.

Heavy overload

Surge currents far beyond specified ratings may puncture the varistor element. In extreme cases, the varistor will burst. Excessive steady-state overload fuses the ZnO grains and conducting paths are formed with the bulk resistance of ZnO, which is considerably lower than the resistance of the undamaged varistor.

The overload can overheat the varistor ceramic with the result that it becomes unsoldered from the electrodes.

 

For more information, please read:

Introduction to Varistors

Terms and Descriptions - Varistors

Design Notes - Varistors

V-I Characteristics - Varistors

PSpice Simulation Model

 

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