Voltage spikes are clipped off to safe levels by clamp devices. Crowbar devices conduct when threshold voltages are exceeded and then trigger to an on-state voltage drop of only a few volts, hence the name crowbar. Suppressor Structures Metal oxide varistors are made up of grains of zinc oxide in a matrix of bismuth and other metal oxides. 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.