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Mechanical relays and contactors depend upon magnetism generated by an electric current running through a wire coil. When the current stops, the magnetic field collapses. But the relay does not know the difference between a wire coil moving in a magnetic field (as in a generator) or a magnetic field moving in a wire coil (as in a collapsing magnetic field). Thus a large voltage—1000V to 1500V typically—is induced in the coil. This current goes the same direction the original current did—so it slows the contact opening—allowing arcing, chatter, bouncing, contact welding and even re-closure! Yikes! This coil-craziness has to be suppressed for things to function at all. Perihelion Design sells 18V 600W Bi-Directional Zener Transient Voltage Suppressors (for 14.5V systems) that provide the most modern technical solution to relay coil suppression. We sell these as a set of twelve with the typical hardware needed to retrofit a typical small airplane.
US$29.89/Set