Product Selection: Surge absorber/VRD

This section introduces the ordinary selection procedure when using a VRD.

How to select a VRD

○ The voltage Vmof the circuit using a VRD should be Vm≦Vs

Vss the rated voltage of the VRD.

○ The withstand voltage Vmaxof the device to be protected should be set Vc so that Vmax≧the clamping voltage (Vc).

Vc is VBmax+α.
VBmax is the maximum breakdown voltage tolerance for each product.
α varies with ambient temperature, surge waveform, surge current, thermal resistance, surge frequency, and other conditions.

– Ambient temperature Ta
The power rating of the VRD should be reduced relative to the ambient temperature.
The change is ⊿VB=VB ×(Ta-25℃)× temperature coefficient.

– Surge waveform, surge current
The figure below shows the clamping voltage characteristics at 10 × 1000 μsec and 8 × 20 μsec.
The clamping voltage VCvaries with the applied surge waveform and current.

Clamping voltage characteristics of SMD-VRD

Figure of VRD

– Repetitive Surge
If surges are expected to occur repeatedly, use the product such that the average power does not exceed the power rating.

○ The VRD is a device to absorb a surge with steep rise time and is not intended for use in constant voltage applications.

Do not use the VRD in a way that keeps it constantly energized.