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Posted on 12 July 2020

Preventing Overvoltages with Varistors

 

 

 

 

 

 

 

Industrial and commercial systems, as well as building, automation, and household appliances, are often destroyed by overvoltages, necessitating their repair or replacement. To avoid this damage, variable resistors, known as varistors, are incorporated into the electrical network or circuit. Varistors are voltage dependent resistors whose resistance decreases with increasing voltage. They are connected in parallel with the electronic circuit to be protected in order to form a low resistance path when the voltage increases, thus preventing damage to equipment in the protected circuit due to overvoltages.

Standard high quality varistors have the following characteristics:

  • High current handling capability 8kA
  • Low protection level  36V
  • High energy absorption capability up to 410J
  • No follow-up current
  • Long-term stability

In contrast, advanced varistors are characterized by:

  • High current handling capability 10kA (+25%)
  • The same lead spacing as the standard series
  • High energy absorption capability up to 440J
  • No follow-up current
  • Long-term stability

Varistors with improved derating have a voltage range from 130 to 680V. Applications include all hard-wired equipment such as drives, convertors, hand-driers, etc.

User benefits include multiple pulse performance with a stable clamping voltage suitable for applications exposed to repetitive low-amplitude surges. Varistors for automotive devices have high energy absorption, particularly for load dumps. They also offer high jump-start strength. PSpice models are available with high operating temperatures up to 125°C and high resistance to cyclic temperature stress.

 

For information, please read:

General Voltage Surge Protection

Overvoltage Protection with Varistors

Varistor Operation - Derating, Temperature, and Overload

Protecting Appliances with Inrush Current Limiters

 

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