What Is a Latching Relay? How It Holds State

A latching relay latches to the commanded contact position even after the control input has ceased. This type of relay “remembers” the last contact state rather than the load power being supplied to it. A momentary command can result in the same physical operation; however, the storage location for the contact state varies between these devices.

What Is a Latching Relay?

A true latching relay is a bistable electromechanical switch with contact-state memory. The bistable design of a latching relay means it retains two separate contact states. A brief energization of control energy will move the relay contacts into a commanded state; the internal retention will keep the relay contacts in position without needing to continuously energize the coil. A later valid command will move the relay contacts to the other commanded position. The Omron Corporation provides a definition for “latching relay” through its Relay Basics: Technology. The Omron definition distinguishes a latching relay from relay models that return to their original position after the input power to the coil is removed.

The control input to the latching relay, the switched contacts, and the load supply source are three distinct parts that work in conjunction with each other. The control input to the winding energizes the winding, while the contacts are used to establish or interrupt another electrical path to the load supply source. The load supply source provides the electrical energy required for powering the load, as illustrated in Omron’s Relay Basics. For this reason, the contact state that is maintained by a latching relay should not be assumed to also retain the load power that is being supplied from the load supply source. In the event of losing power to the load supply source, the load would be turned off even if the relay still remains in a latched position.

What command will be given next will depend on the characteristics of the latching relay being used; commands could be a reset pulse, reversed polarity on one winding, energizing a second winding, or toggling the state back to the previous On/Off state. Although the term “latching relay” indicates that it retains two separate states (bistable), the name does not indicate how this occurs.

How Does a Latching Relay Hold Its State?

A set input energizes the control system, allowing the armature and contacts to move. Once in the new position, the latching relay holds the contacts in position without energizing the coil. To return to the other commanded position, it requires a reset action to cause the armature to return or to change the state of the relay.

Some types of latching relays may use a mechanical or magnetic method of retaining the armature in its last commanded position after the energizing coil state returns to zero. An example found in Omron’s General Relay Technical Guide is a latching relay that retains an armature position using residual magnetism created from the last energization of the coil. Some other latching relay constructions use mechanical catches or stepping mechanisms to hold the armature in the commanded position after the energizing input has ceased.

Resetting also requires energy, although many latching designs require it only during the transition. The following topics are discussed in Omron’s Latching Relay FAQ: single-winding and double-winding latching relays; set operation and reset operation; and reset-pulse duration, polarity, and timing, which are attributes of the specific latching relay you selected.

State Memory Can Live in the Relay, Seal-In Circuit, or Controller

This “stays on” behavior for a given application may result from memory retention in the latching relay, the seal-in circuit, or the controller.

When energizing an ordinary coil (or relay coil), you can activate the relay using a momentary Start command provided the relay is equipped with a seal-in circuit. This means that the momentary Start command will energize the coil. Once you have energized the relay coil, the seal-in circuit provides an alternative feedback path for continued current flow through the relay coil. You can view these seal-in circuits in use by looking at Schneider Electric’s explanation of the holding circuit and Rockwell Automation’s Micro800 Starter Pack Quick Start instructions. With the seal-in circuit, it is possible to use an installed relay that is monostable.

A relay module may carry a label indicating it is a “latching relay module”, but it could contain an ordinary relay plus a flip-flop, microcontroller, or driver circuit as part of its design. It is possible for the output of the module to appear to be latched while it is powered on; however, its blackout-and-restart characteristics will depend on the relay module design.

How Do One-Coil and Two-Coil Latching Relays Differ?

Configuring a device according to some type of universal wiring instructions is not indicated by these diagrams and other schematic depictions of relays. Panasonic’s Relay Terminology separates single-side stable, one-coil latching, and two-coil latching arrangements. It is possible to have multiple coil arrangements for latching and non-latching categories. A two-coil configuration may be beneficial if reversing polarity is difficult.

Holding Mechanism and Contact Form Are Separate Specifications

While the method of holding does not define the contact form, there are different methods of holding the relay position, magnetically or mechanically. SPST, SPDT, DPDT, normally open, normally closed, etc. describe the switched-contact arrangement. There are other specifications, such as winding configuration, coil voltage, timing characteristics, enclosure, and pinout.

For example, the phrase “12 V Latching Relay” indicates a nominal coil or control voltage of 12 V, but does not describe whether the relay has one winding or two. You will still need to know the terminal order, the command polarity, the pulse width, the contact rating, and the reference state of the diagram.

TE Connectivity’s Schrack Relay Family includes monostable and bistable variants, while OMRON’s G6B datasheet distinguishes multiple coil configurations and terminal arrangements. The shared package outline does not imply that these devices are interchangeable.

What Can the Symbol and Part Number Tell You?

Drawing a schematic may show you the coil and contact arrangement for the relay; however, it does not necessarily mean that the relay is a latching relay. Since manufacturers, symbol libraries, and module vendors do not offer a consistent way of identifying latching relays, a reliable way to tell if a relay is a latching type is through the manufacturer’s exact part number and terminal diagram.

There are five checks to make when determining the type of relay:

1. Gather all characters from the part number.
2. Find the manufacturer’s datasheet for that specific ordering code, including any suffixes.
3. Search the manufacturer’s terminology for labeling, e.g., latching, bistable, set, reset, or equivalent states.
4. Count the windings and identify any dedicated coil terminals in the schematic diagram.
5. Prior to interpreting the relay contact position, ensure a match of the documented terminal layout to the package drawing and the actual component.

Documents frequently show that contacts are in a reset, initial, or de-energized state. Each document defines its own reference point. With shipping shock or handling of a latching relay, the relay may be left in an unanticipated state. Therefore, Panasonic’s Relays Cautions for Use direct the user to refer to the product-specific instructions to determine how to handle and initialize the product, rather than assuming a starting position.

When Does Retained State Help, and When Is Default Return Better?

In some instances, state retention could be beneficial for a user because it allows the relay to keep the last commanded contact state without continuous coil energization. If the hold period is lengthy and switching occurs infrequently, state retention will reduce the amount of energy and heat generated by the relay’s coil.

If the loss of control power should always return the output to a known default, then it is usually easier to define and verify with a monostable relay or other fail-return architecture.

Latching vs Non-Latching Relay

Typical Latching Relay Applications

The useful question to ask, then, is not just why that state must persist, but rather what should happen when control or load power is reapplied after being interrupted.

Since relay commanded switching events may be separated by months, the utility’s disconnect and billing logic assumes that once a relay is switched by a command request, the contact’s state will remain unchanged until the next command request is sent, while the coil draws no standby power to hold this position. The issue is compounded if the relay control power goes out after the disconnect command is sent, prior to the relay finishing the transition. When the shutoff command pulse is incomplete, it can leave a magnetic latching relay in an indeterminate state. After control power returns, the controller will have to re-read the contact state via the supervisory input rather than assume the relay’s last commanded state took effect.

Which Datasheet Specifications Matter for a Latching Relay?

The first step before making any wiring decisions about a latching relay is to identify its exact part number from the datasheet. The datasheet should contain the following specifications for the relay: voltage of the coil, winding configuration of the relay, terminal diagram of the relay, polarity of the commands for the relay, minimum pulse width required for set and reset, time to set and reset for the relay, contact bounce (if specified), load ratings for the relay’s contacts, load type or in-rush ratings for the relay, mechanical and electrical life for the relay (where applicable), environmental limits of the relay, and its documented initial state.

Be careful to note that timing terms cannot be used interchangeably; set time and reset time describe the contact’s response after receiving a valid command, whereas minimum pulse width determines the minimum time the coil must be powered in order to perform the action reliably. Panasonic’s General Application Guidelines provide a manufacturer-specific rule of thumb: the relay coil is driven with a rectangular pulse at rated voltage for a minimum width of at least 5 times the relay’s set time or reset time, and then it is verified that the relay will still operate properly. Other relays must follow their own datasheet requirements.

Matching the part’s package family is not sufficient. Nearby suffixes within the same family can produce different winding configurations, coil voltages, contact configurations, and terminal assignments. If a relay is mounted within a module, refer to the schematic or manual for that module to confirm the specifics of the control electronics that exist between the external connector and the relay component.

A relay’s exact variant must match the schematic symbol, terminal assignment, and footprint of the PCB layout. Until the circuit has been completely assembled and tested in operation, it cannot be confirmed that the relay assembly changes and retains its state correctly under the intended sequence and known starting conditions.

Conclusion: Read the State Before You Read the Pins

The memory location is the most direct way to distinguish the relay types. Winding count, contact configuration, voltage rating, timing ratings, load ratings, and pinouts can then be checked as required.