Latching relays, or ‘bistable’ relays, are an increasingly popular switch type for applications where energy efficiency is a key priority. They are a relatively low-power consumption and high-temperature option that allows for a reliable switch action to be made without the use of a continuous voltage input to the coil, making them well suited for use in situations where a significant amount of power is not available or can’t be afforded.
How to wire a bistable relay
A latching relay is a type of switch that operates with a’switching’ pulse of current in order to move the armature (a strip of metal suspended between two contacts) between one state and the other. Its key feature is that, unlike most other types of relay switches, it can hold and retain a given position indefinitely between each’switching’ pulse, enabling a lower level of input current to be used for its operation over longer periods of time.
There are many different latching relay types, each with its own particular set of advantages and disadvantages to consider when selecting the most appropriate device for your application. As a general rule, the higher-power latching relays tend to have a larger coil, which means that they can be used with higher voltage inputs to their circuits (and therefore require a higher current input).
Latching relays have the ability to remember the last position it was moved to and continue holding that position if a further’switching’ pulse is applied to the coil(s) again. This is known as a’memory function’ and enables them to be’set’ and’reset’ by the momentary-acting switches that control them.
To be able to achieve this’memory’, a latching relay switch needs to have a pair of momentary-acting switches that each’sets’ and’resets’ it. Depending on the specific switch type and model, this can be achieved with either a single’set’ or’reset’ switch, or in the case of an electromagnetic relay, by using a series of ‘poles’ or ‘pole-disconnecting’ switches between the two coils of the relay.
This is important because if a relay switch is only’set’ once, it may not be able to re-set itself properly when required. This can have negative implications for the operation of a circuit or device it’s connected to, and can cause it to behave very unpredictably when attempting to’remember’ its last position or’state’, which could be a potential problem in some environments.
Alternatively, latching relay switches can be operated with a single’set’ switch or sensor. This can be especially beneficial in applications where a relay is often triggered by manual instructions, such as an automatic switch that requires the relay to remember the last position it was switched to.
A latching relay can also be configured to’remember’ which switch was pressed first, or even which switch it was’set’ by (for example, in an MRD1 train detector where the ‘Set’ and ‘Reset’ switches have priority). This is commonly achieved with a pair of’set’ and’reset’ switches, and can be achieved with a range of other ‘pole-disconnecting’ or ‘pole-switching’ types of switch, including both electromagnetic and mechanical ‘pole-disconnecting’ types of relay.
