In many applications, saving energy is critical. Latching relays are able to keep its last state (on or off) without having to have its coil powered. Electromechanical relays can be classified into two groups in terms of operating principle. One is mono stable or single-side stable type relay: the monostable relays keep the contacts in “ON” position only while the coil is powered. To stay “ON” this kind of relay needs to be powered at all times. The other is the latching relay. Latching relays can keep the contacts ON even after de-energizing the coil and only another inverted signal can switch them OFF. Therefore these types have the power saving advantage by not requiring power at all times.

The chart on this slide shows the operating principals of both single side stable type and latching type relays. As it is shown here, the latching type relay has two types in terms of operating principals, single winding latching and double winding latching. Here is how they work: when a voltage is applied to a coil terminal in the input side, the coil will become magnetized. For single winding latching relays, voltage needs to be applied from the correct direction; one direction to turn on, reverse direction to turn off. For double winding latching relays, as these relays have two coils, voltage needs to be applied on the set coil to turn on and reset coil to turn off. Then the magnetized coil generates electromagnetic force and armature is pulled towards the core. Next, the armature pushes the movable contact towards stationary contact by armature movement, and then contact switches on. Even after voltage to the input side is removed, the moving contact remains contacted to stationary contact. In case of mono stable, once the voltage to the input side is removed, the movable contact moves away from the stationary contact.  This is the difference between monostable and latching relays, in a nutshell.