Magnetic latching relays
In the widely used magnetic configuration for latching relays, a single pulse of current to a coil briefly generates an electrical field that will move a reed switch in one or the other direction. When the pulse stops, the latching relay remains electromagnetically held in whichever position it was just moved to, and will not return to the opposite position until another, redirected pulse is sent through the coil(s) to move it back again.
As well as offering the lower power draw common to all latching relays, magnetic latching relays are therefore especially useful in applications where interrupting the current flow to the coils will not produce the undesired effect of moving the switch to a different position between the two contacts.
They can also perform the switching action very quickly, tend to be less bulky than mechanical variants, and generally offer a longer lifespan due to the very limited extent of the physical movement taking place within the switch.
Mechanical latching relays
As opposed to a magnetic latching system, a mechanical latching relay uses a physical locking mechanism to hold the armature in place against the contact in the last position it was moved to. Electromechanical relays bring various pros and cons into play:
- They tend to feature larger, bulkier contacts than electromagnetic versions, and are therefore less flexible units in terms of space requirements
- Mechanical latching relays tend to be better at coping with unexpected surge currents
- Speed of switching is limited due to the extent of mechanical movement required, making them unsuitable for some applications
- The lifespan of mechanical latching relays is typically somewhat shorter than for magnetic versions in terms of overall number of actuations
- However, current size is an equally important consideration in terms of overall longevity for any relay switch
- The projected lifetime of mechanical relays under higher loads will often decrease much more slowly over time than for magnetic reed versions
- Its contacts will be less prone to weakening during thermal cycling than an electromagnetic latching relay
Impulse latching relays
Impulse relays are a form of magnetic latching relay that change the contact state with each subsequent input pulse. Upon application of power, the impulse latching relay determines which position the switch is in automatically, and energises the opposite coil to actuate or move it each time.
The impulse latching relay typically does this via the use of a solid state steering circuit, which allows for the input pulse to be unidirectional, with no need to redirect the control pulse or reverse the polarity. Impulse switches are therefore well suited to applications that involve the need to turn a single device on or off, from one or more locations, with a single momentary switch or push button.