Expert: cleggsan - 3/13/2007

Question
Dear Mr Adam

The most commonly use and already proven technology to reduce the transient voltage ie from lightning is the usage of MOV. Under normal condition it will have high impedance but when there is surge, it switch into low impedance mode and divert the high surge ,20 kV to the geround.

My question is, how a single component switch from high to low to high impedance mode. How it do that?


Answer
It is a form of semiconductor.  The device has a threshold voltage that, when a transient spike voltage reaches that point, causes the device to conduct.  You can think of it as a semiconductor high speed switch.  It is not unlike a transistor that is biased OFF and when the bias voltage gets over the threshold voltage or current, it turns the transistor all the way ON to a low resistance.  When the threshold voltage is gone, the device quickly goes back to its original state of high resistance.

www.nemasurge.com/news/metoxvar.doc

is a good, but simple explanation of how they work.

Have a great day,
C