The Complete Guide To Latching Relays

This guide is part of our Industrial Automation hub where you can discover more about AI, automation and control.

To determine which specific kind of latching relay switch would be ideal for use in a given application or environment, some more detail about the precise nature of the task will need to be considered. In this guide, we’ll look more closely at what each of the different latching relay circuit switch types available, as well as how they work, and in what sorts of scenarios they might be put to best use.

When subjected to a brief pulse of relatively low input current, the coil(s) in a latching relay switch generates a magnetic field, pushing or pulling the armature - often known as a ‘reed switch’ in electromagnetic relays - suspended between them. This causes the strip to move from one to the other terminal accordingly. The switching action can either be set up to complete or break a single circuit or as a method of switching power between two separate circuits.

As discussed in previous sections, the unique advantage of a latching relay, as opposed to general-purpose or non-latching relays, is that the armature on a latching relay will remain in the last position it was moved to until forced to change states (i.e. to move back in the opposite direction again via the application of a further pulse of current).

Owing to this key characteristic, latching relay switches are known as being ‘bistable’. Because it only requires an input current for those brief voltage pulses necessary to switch it between one state and the other, a latching relay will offer a lower power draw over extended use periods than most other types of non-latching relay.

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.

Again, this can be especially advantageous in scenarios where a higher degree of energy efficiency is of value. This is often the case where the relay is likely to be in fairly constant use, and in particular when it’s frequently being asked to ‘remember’ its position or state (i.e. to switch and hold between two distinct positions without returning automatically to a default open or closed position in between each manual instruction).

Latching relay applications

There are a great many day-to-day uses for latching relay switches found across a diverse array of environments and sectors. Some of the more widespread industrial applications that benefit from the unique latching relay function include:

• Industrial counting and sorting systems
• Power supplies
• HVAC, refrigeration and anti-condensation systems
• Industrial cleaning equipment (including automated car-washing devices)
• Commercial coffee machines and other automated food preparation equipment