Memoryrelæer

Memoryrelæer, or memory relays, are electromechanical devices that preserve their contact state after the driving signal is removed. They are bistable switches with two stable positions, enabling a circuit to remember a command without continuous power to hold the state.

Construction and operation: The two most common constructions are dual-coil latching relays and single-coil pulse relays employing a permanent magnet. Dual-coil types use a set coil to move the armature to the set contact position and a reset coil to return it to the opposite position. Single-coil versions use a polarity-reversing pulse or a mechanical latch to toggle state; energizing again reverses the state. Contacts are typically Form A (normally open), Form B (normally closed), or Form C (changeover) and come in various configurations such as single- and multi-pole arrangements.

Characteristics: Memory relays consume little power to maintain a state, since the coil need be energized only to change state. They provide robust, mechanical isolation between control and load circuits. Drawbacks include slower switching relative to solid-state devices, wear of moving parts, and potential sensitivity to shock or vibration. Reset and set pulses must be applied carefully to avoid unintended toggling.

Applications: They are used in industrial automation, programmable logic controllers, telecom and safety interlocks where remembering a last state is beneficial or where continuous current to hold a state is undesirable. While many memory tasks are now performed with semiconductor memory and PLC logic, memory relæer remain in use in environments requiring simple, rugged memory with electrical isolation or high-temperature operation.