What Is A Latching Relay And Variants

Thursday, April 26th, 2018 - Active Components
Latching relay is a generic term that is used to describe a relay that maintains its contact position after the control power has been removed. Latching relays allow a customer to control a circuit by simply providing a single pulse to the relay control circuit. Latching relays are also desirable when the customer needs to have a relay that maintains its position during an interruption of power.

In a single-coil latching relay, the polarity of volt­age applied to the coil determines which pair of contacts will close. In a dual-coil latching relay, a second coil moves the armature between each of its two states.

dual-coil latching relaySchematic symbols for a two-coil latching relay

Schematic symbols for a dual-coil latching relay are shown in the figure above. Some symbol styles do not make it clear which switch position each coil induces. It may be necessary to read the manu­ facturer’s datasheet or test the relay by applying its rated voltage to randomly selected terminal pairs while testing for continuity between other terminal pairs.

Variant Of Latching Relay

There are three main types of Latching relays. Magnetic latching, Mechanical Latching and Impulse Sequencing.

Magnetic Latching Relays

Magnetic Latching relays require one pulse of coil power to move their contacts in one direction, and another, redirected pulse to move them back. Repeated pulses from the same input have no effect. Magnetic Latching relays are useful in applications where interrupted power should not be able to transition the contacts.

Magnetic Latching Relays

Magnetic Latching relays can have either single or dual coils. On a single coil device, the relay will operate in one direction when power is applied with one polarity, and will reset when the polarity is reversed. On a dual coil device, when polarized voltage is applied to the reset coil the contacts will transition. AC controlled magnetic latch relays have single coils that employ steering diodes to differentiate between operate and reset commands.

Mechanical Latching Relays

Mechanical Latching Relays

Mechanical latching relays use a locking mechanism to hold their contacts in their last set position until commanded to change state, usually by means of energizing a second coil.  Since the relay does not rely on a magnet, the locking strength will not degrade over time or weaken during thermal cycling. The contacts will remain locked in the directed position until the opposing coil has been energized. Packaging machinery that places several units into a single container would be a good example.

Impulse Sequencing Latching Relays

Impulse relays are a form of latching relay that transfers the contacts with each pulse.  Many impulse relays are made up of a magnetic latch relay and a solid state steering circuit that, upon application of power, determines which position the relay is in and energizes the opposite coil.  The contacts transfer and hold that position when power is removed.  When reenergized, the contacts transfer again and hold that position, and so on.  In order to transfer the contacts, one simply provides a singleunidirectional pulse.  There is no need to redirect the control pulse or reverse the polarity.

Impulse Sequencing Latching Relays

Impulse relays can be used as wear equalizers.  They are well suited for applications such as turning a single device on or off from one or more locations with a single momentary switch or push button at each station.  Using a impuls latching relay a conveyor could be started and or stopped from multiple locations by means of a single button at each position.

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