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September 24, 2012

MOVs vs GDTs: Which one do I use?

Barry Brents
Field Application Engineer, TE Connectivity

Power Channels: Energy Efficiency, Power Components, Power Quality Protection

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.

Overvoltage devices can be divided into two categories: clamping and crowbar devices. Crowbar devices go to a short circuit when they trip. (The name “crowbar” comes from the concept of dropping a crowbar across a pair of power lines and shorting them out.) Clamping devices don’t go to a short circuit when they trip; however they do go to a low enough resistance to limit the voltage to a rated clamping voltage. In a perfect world, this voltage would be constant no matter what level of current is present. But, in reality, clamping devices (like MOVs) have a higher clamping voltage when the current is higher.

Getting back to GDTs vs MOVs: GDTs are crowbar devices and MOVs are clamping devices. Because GDTs are crowbar devices, they have a very low voltage drop while in the tripped condition. This means that they can conduct a lot of current without dissipating a lot of power (P = V x I, a low voltage multiplied by a high current results in lower power than a high voltage multiplied by a high current). As a result, GDTs are rated for very high current surges. Since MOVs are clamping devices, they dissipate more power in the tripped condition than GDTs, so they typically have lower surge current ratings.

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