Each grain boundary is the equivalent of a junction with a breakdown voltage of 2 to 3 volts, with the net result equivalent to hundreds of diodes in series and parallel. By varying the grain size, thickness and area, parameters including current and voltage rating can be controlled. Packaging ranges from chips to large mutikiloamp, multikilovolt units. MOV parameters are symmetrically bidirectional. For longer duration transients, thick metallized contacts are required to absorb the heat produced by the transient.
Parts are available as chips, axial lead and surface mount devices.
Long duration pulses can be suppressed by increasing chip size and heat dissipation. Devices are available in unidirectional configuration for dc and bidirectional for voltages with positive and negative swings. Most GDTs have two parallel electrodes in a low pressure inert gas cavity of glass or ceramic. The spacing and size of the electrodes determines the voltage and current ratings respectively. They are small in size for their surge current ratings and are available in axial lead, surface mount, cellular discs or chips.