How does a latching relay work

Diagram of a latching relay mechanism - how it works
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The question “what is a latching relay?” is often asked by those venturing into the world of electronics and automated control systems. A latching relay is an electromechanical switch designed to maintain its last commanded state even after the driving power has been removed, thereby operating with exceptional efficiency in power-sensitive environments. In this comprehensive article, we will dissect the inner mechanics and applications of a latching relay, particularly focusing on the dual coil latching relay variant for its unique operation and compare these devices to their non-latching counterparts.

Introduction to Latching Relays

Latching relays are integral to many systems requiring controlled electrical functionality with minimal power wastage. Definition of a Latching Relay: A latching relay, also known as a “bistable latching relay” due to its two stable states, is characterized by its ability to ‘latch’ onto a set or reset position. This type of relay is different from standard relays, which must be continually powered to maintain their position. The latching relay’s distinct feature of state retention without a power supply conserves energy and extends battery life in various applications.

Comparison with Standard Relays

In comparison to standard relays, that behave like temporary push-button switches, a latching relay is similar to a toggle switch that stays in its last position until actuated again. The advantage of a latching relay, particularly a dual coil latching relay, is its energy-saving profile, allowing it to be a preferred choice in battery-powered or remote operations.

 Illustration of a latching relay switch - operation explained

The Mechanics of Latching Relays

The operation of latching relays depends on several mechanical components and their interaction. Components and Their Functions: A dual coil latching relay, as the name implies, incorporates two coils. These coils, when energized with pulses of electricity, can independently set or reset the relay. This dual-coil action provides reliable bistable functionality. Apart from the coils, these relays also contain an armature to switch contacts and a spring mechanism to enable or disable the connection without continuous power.

The Bistable Mechanism

Bistability is a defining feature of a latching relay. This particular aspect enables the relay, especially a bistable latching relay, to maintain either an open or closed state indefinitely after being activated by a pulse until it receives another actuation pulse. It is this bistable characteristic that essentially provides the memory function, making it one of the most reliable types of relays on the market.

The Electrical Operation of a Latching Relay

Exploring further into the electrical side of operation, activating the Relay: initiates with a pulse sent to one of the dual coils in a dual coil latching relay. The pulse induces a magnetic field strong enough to physically shift the internal switch to the new state. For a relay to go from a ‘set’ to ‘reset’ state or vice versa, it depends on the input pulse’s polarity and which coil is energized.

Switching and Maintaining States

Latching relays are celebrated for their ability to switch states with just a pulse and remain there without continuous power. They are the epitome of electrical efficiency in the relay family. After the relay contacts have shifted, the stable state is maintained passively, without the need for power, until an opposite pulse triggers a state change. The process of a typical dual coil latching relay can be seen as follows:

  1. Energize ‘set’ coil with a voltage pulse, the relay switches to a closed state.
  2. When the pulse stops, the ‘set’ state continues without power.
  3. Energize ‘reset’ coil with a voltage pulse, the relay switches to an open state.
  4. Again, when the pulse ends, the ‘reset’ state is held without power supply.

Common Applications of Latching Relays

Latching relays, with their energy-efficient design and reliable bistable operations, are commonly found in a broad spectrum of applications. Industrial Uses: In industrial settings, latching relays are widely utilized to control large loads with precision. They are instrumental in situations where maintaining a specific state is crucial even during power loss. For example, emergency stop circuits in manufacturing rely on these relays to ensure machinery remains off once triggered, regardless of power restoration, until a manual reset occurs.

Domestic and Everyday Uses

In the home, latching relays are often found in smart switches that retain their state after a power cycle. Their energy-saving nature also makes them ideal for battery-operated devices and security systems, where they can hold a circuit open or closed until an authorized signal prompts a change. For instance, they are employed in:

  1. Thermostats, where they maintain the heat or cooling setting after an initial pulse.
  2. Lighting systems that remember the last on/off state despite power outages.

The reliability of latching relays in maintaining circuit states without a constant power supply renders them invaluable in modern electronics and automated systems where energy efficiency is paramount.

 Photo of a latching relay coil - magnetic field activation

Advantages and Challenges

Latching relays come with an array of advantages that make them highly sought after for various applications. Energy Efficiency and Reliability: They are supreme in scenarios where power availability is intermittent or limited, as they do not necessitate a continuous power supply to maintain their state. This factor contributes to significant energy savings, especially in battery-powered or remote devices.

Energy ConsumptionRequires power only during switching operations, conserving energy in steady stateInitial energy requirement for switching can be higher compared to non-latching relays
Mechanical WearLess mechanical wear and tear due to absence of continuous power consumptionStill subject to mechanical failure over time due to moving parts
Switching SpeedCan switch faster than some traditional relaysComplex switching mechanism may introduce latency in some applications
CostMay be more cost-effective in applications with frequent switchingInitial cost can be higher due to the complexity of the relay design
ReliabilityLess prone to accidental disconnection due to power lossDependent on the quality of the relay construction and materials

However, the inherent complexity of latching relays can present challenges. Troubleshooting these devices can be more complicated than their non-latching counterparts due to the need to understand their bistable nature and dual-coil operations. Furthermore, a designer must carefully consider the impact of power disturbances and ensure proper sequence and polarity in control signals to avoid unexpected state changes.

Considerations and Troubleshooting

When implementing latching relays, thoughtful design and setup are essential. Complex control systems need to be programmed to handle the nuances of latching relays. Engineers must account for:

  1. Proper sizing of the relay for the specific load to prevent premature failure.
  2. The inclusion of manual override or reset circuits, particularly for safety-critical applications.

Understanding potential pitfalls and proactively planning for them is crucial for the effective and safe use of latching relays in any system.

 Schematic of a latching relay circuit - electrical connections and function


To recap the functionality of latching relays, we recognize the sophistication and resourcefulness of these components. From industrial machines to household appliances, their bistable nature and ability to latch into a state with no need for power conservation make them an integral part of modern circuit design. As technology evolves, we may see an increased reliance on such relays, with enhancements to their designs for even greater versatility and efficiency.

The Future of Latching Relays in Technology

With the ongoing push for greener technologies and energy conservation, latching relays will likely become even more prevalent. Innovations may lead to smaller sizes, more robust performance, and integration with smart technologies, enhancing their applicability and ease of use in various sectors.


What is the difference between a latching relay and a non-latching relay?

A latching relay maintains its state (set or reset) after being actuated and does not require continuous power to hold that state. In contrast, a non-latching relay needs constant power to maintain its position, reverting to its default state when the power is removed.

How do you reset a latching relay?

To reset a latching relay, specifically, a dual-coil latching relay, you need to apply a pulse of electrical current to the reset coil. This pulse generates a magnetic field that shifts the contacts back to their original position.

Can latching relays work without a continuous power source?

Yes, latching relays are designed to maintain their position without the need for a continuous power source, making them ideal for energy-conserving applications or in systems where power supply is limited.

What kind of applications are not suitable for latching relays?

Applications that require real-time response to power variances or those that cannot accommodate the complexity of the relay’s control circuit may not be suitable for latching relays. Also, scenarios requiring frequent and rapid switching may benefit more from non-latching relays.

Are latching relays more expensive than standard relays?

The cost of latching relays might be slightly higher due to their additional internal mechanism for bistability. However, this price difference is often justified by the energy savings and reliability they provide in suitable applications.