There are many overvoltage protection devices available for today’s engineer to use, including zener diodes, TVS diodes, thyristors, and tiny ESD protectors. But the two technologies that I get asked about most often are GDTs (gas discharge tubes) and MOVs (metal oxide varistors). MOV and GDT technologies have been around for many years. Both of these technologies work by going from a normally high impedance to a low impedance when a high fault voltage is detected. The idea is that when they are at a high impedance state, they have no effect on circuit operation. But when there is a high voltage present, high enough to damage circuit components, they activate by becoming low resistance and shunting the energy to ground. The mechanical analogy would be a relief valve on a high-pressure pipe or tank that opens and vents gas or liquid if the pressure gets too high. Keep reading…