Selection and Protection

  • It is recommended that the circuit voltage which is applied on a steady basis to be used at 80% or less of the max. allowable circuit voltage. Exceeding the specification will cause deterioration, shortcircuits and etc.
  • Select proper parts according to the surge energy and the number of the impressions if the varistors are used to absorb the surge for an inductive loading.
  • The rated surge endurance volume is defined in terms of shockwave current wave form (starting up 8μs/wave-tail length 20μs). Please select the suitable component based on the surge life characteristic figure listed in the separate volume catalogue for varistors.
  • Insert fuses or thermo fuses in series with varistors if the size of the surge power cannot be estimated, in order to prevent varistors from bursting due to an excessive surge over the rating.
  • There is a possibility of smoking or flaming in case of excessive surge which is over the ratings so do not fix close to combustible materials.
  • Upon mold sealing, fully confirm the reliability and use the resin which have small contractile stress at stiffening since the protection coat may be peeled off, cracks may occur at the solder connection, and the characteristics of the varistor may change.
  • Perform the withstand voltage test and the insulation resistance test with the varistors removed from an equipment since the test voltage may exceed the varistor voltage.

Failure Mode

  • Varistor voltage will drop and the leakage current will increase when excessive surge which is above the rating is applied to the varistor. The Joule heat will increase along with the increase of leakage current which will cause thermal runaway and it will result in short circuit.


  • For basic precautions, please refer to the technical report ofJEITA EMAJ-R039 Safety application guide for varistors.

Terms and Definitions

Maximum Allowable Circuit Voltage

  • The maximum commercial frequency sinusoidal voltage effective value or maximum DC voltage which can be applied continuously.

Maximum Energy (E)

  • The maximum energy within the varistor voltage change rate of ±10% when a single impulse of 2ms is applied.

Maximum Peak Current (IP)

  • The maximum peak current within the varistor voltage change rate of ±10%, when a single standard impulse of 8/20μs is applied.

Operating Temperature (Topt)

  • The allowable ambient temperature range while the device is operating.

Storage Temperature (Tstg)

  • The temperature range in which the elements do not deteriorate.

Varistor Voltage (VC)

  • The terminal voltages on both ends of the varistor when the specified current is applied.

Clamping Voltage (VP)

  • The peak value of the voltage between two terminals of the varistor when the specified standard wave form impulse current (8/20μs) is applied.

Recommended value of varistor voltage for the power supply voltage

Voltage of Power line Varistor voltage
3.3V d.c. 8.2V
5V d.c. 8.2V, 12V
12V d.c. 24V, 27V
24V d.c. 47V, 56V
48V d.c. 82V, 100V