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Reed relays are a type of electromechanical relay composed of coils wrapped around reed switches. The reed switch has two overlapping ferromagnetic blades hermetically sealed within a glass capsule that is filled with an inert gas. When the coil is energized, the two reeds physically contact one another to complete a path through the relay. When the coil is deenergized, the spring force in the reeds pulls the reeds apart. The following figure represents an open reed relay.
The following figure represents a closed reed relay.
The reeds are generally smaller and therefore can actuate much faster than the armatures in armature relays. However, reeds are also more susceptible to damage from arcing than are armatures in armature relays. When a spark jumps across the contacts, it can melt a small section of the reed. If the contacts are still closed when the molten section resolidifies, the contacts may weld together. The spring force in the reeds is often insufficient to mechanically break the weld.
When using switch modules with reed relays, consider the effects of switching capacitive loads to protect the reed relays.