What is Latching Relay : Working & Its Applications

A relay is an electrical switch that includes a set of input and operating terminals used for a single or various control signals. The switch may include a number of contacts in many contact forms like break contacts, make contacts, etc. Relays are applicable where a single circuit or number of circuits needs to control through a separate low power signal. There are different types of relays available based on the requirement in the market like a solid-state relay, reed relay, latching relay, automotive relay, delay relay, differential relay, timer relay, etc. So this article discusses an introduction to latching relay that includes working, types, and its applications.


What is Latching Relay?

A latching relay is one kind of control device that maintains its condition after being activated. That is why these are known as keep relays, stay relays, or impulse relays. This relay is applicable where power consumption & power dissipation needs to be limited. The operating of this relay can be done manually, remotely, by impulses, or using different control inputs. These relays are also referred to as impulse relays or bistable relays. The latching relay symbol is shown below.

Latching Relay Symbol
Latching Relay Symbol

How Does a Latching Relay Work?

The latching relay works by opening or closing its contact every time when a mains voltage signal is provided to the terminals of its coil. The generation of the signal can be done by depressing one of the push-buttons which are parallelly connected. These relays are very helpful in operating the lighting circuit of the zone from different locations.

Using these relays in place of contactors within lighting circuits no coil needs to be power-driven, so 2W power can be saved for each relay. For every relay, the conserving of universal energy for every year is higher than 5 kWh. These relays allow the control of the lighting through various pushbuttons. The understanding of the circuit through parallel keys is extremely simple. So this is particularly utilized in more difficult lighting plants.

How Do You Reset a Latching Relay?

The setting and resetting of a latching relay are possible through applying positive voltage. When a positive voltage is applied through the push button the relay will be set. Similarly, once the reverse voltage is applied through a push-button then the relay will reset.

Latching Relay Types

Latching relays are available in three types magnetic latching, impulse sequencing & mechanical latching.

Magnetic Latching Relays

A magnetic latching relay is an automatic switch, so it can switch ON/OFF the circuit automatically similar to other electromagnetic relays.

In this relay, the NO (normally open) or NC (normally closed) condition completely depends on the permanent magnet action. The switching condition of this relay can be activated through a certain pulse electric signal width.

This relay can be separated into single-phase and three phases. Based on the important information on the market, the highest contact switching current of this relay can reach 150A; the control coil voltage can be separated into DC 9V, DC 12V, etc.

Magnetic Latching Relay
Magnetic Latching Relay

The specifications of magnetic latching relay are; mechanical life is 10,00,000, electrical life is 10000 times & contact voltage drop is below 100mV. The characteristics are small size, stable performance, power saving, bearing capacity is large & its performance is good as compared to normal electromagnetic relay.

These relays are applicable in centralized meter reading systems & IC card prepayment meters. At present, these relays are broadly used in the reactive power compensation device. In this device, this magnetic relay is used in combination with a thyristor to switch large current circuits and is also used in automatic control devices & various household appliances.

Impulse Sequencing Relay

Impulse latching relays are also known as bi-stable relays which change the condition of contact by using every voltage input signal. These relays are very famous due to their shock resistance & high vibration.
Once a power supply is given to this relay, then it determines the relay condition & activates the opposing coil. In this relay, the contacts will transfer & maintain that position whenever the power supply is detached.

Impulse Sequencing Type
Impulse Sequencing Type

When a power supply is again applied, then the contacts of the magnetic relay will transfer to the opposite state & hold that place until the power on or off-cycle gets repeated. These relays are applicable in switching a single device from single or different locations using a momentary switch or push-button for a conveyor or lighting circuit.

Mechanical Latching Relay

These relays use locking mechanisms for holding contacts in the final location until they get information to modify. So, this is typically taking place by strengthening a secondary coil. After that, holding contacts will stay locked in that location until the opposite coil has obtained energy.

Mechanical Latching Relay
Mechanical Latching Relay

Since the mechanical latching relay does not depend on a magnet, then the strength of locking will not degrade eventually or fail throughout thermal cycling. So the contacts in the relay will stay locked within the directed location until the opposite coil has been strengthened. The best example of this is packaging machinery.

Latching Relay Circuit Diagram & Working

The latching relay circuit diagram is shown below. The designing of this circuit can be done with a single push button switch, 12V battery, two relays like RL1 & RL2, 1N4007 diodes like D1 & D2, and load like the bulb. Once you provide the supply to the circuit, the output load will be in OFF condition. When the switch SW1 is pressed for 1 sec then the load used in this circuit-like bulb will be turned on. Again, if we press the same switch is pressed for 1 second then the load will be turned OFF.

Latching Relay Circuit with Single Push Button
Latching Relay Circuit with Single Push Button

First, once the push button switch is not turned ON and the power supply is ON throughout the circuit then-current supplies across the common pin of RL1 relay first, and after that the current supplies to the second relay RL2 coil throughout it. So in this case, the second relay like RL2 is triggered only & the RL1 relay is not triggered.

Once the push button switch SW1 is pressed for a second then the positive supply flows throughout the COM & NO pins of RL2 to RL1 relays cross the diode D1. Now the RL1 relay is triggered & the RL2 relay will be deactivated because the flow of the current of RL1 is detached. So in this way, the current supplies to the load & after that the load will be activated.

Now the RL2 relay will become OFF, consequently, the NO & COM pins of the RL2 relay will be connected. Again if we press the push button, then the short circuit will occur throughout these pins, then the voltage at the RL1 coil will become 0, so the RL1 will be deactivated.

So after deactivating the RL1 relay, the pins COM &NC are connected, then the current flows throughout the RL2 coil then RL2 will be activated. So, in this condition, the output load is again OFF.

Difference between Latching and Non-Latching Relays

The difference between latching and non-latching relay includes the following.

Latching Relay

Non-Latching Relay

A latching relay will stay in the last position when it was powered last. A non-latching relay goes back to its regular position.
This relay is also known as a keep impulse, bi-stable, and lock up the relay. It is also known as a typical mechanical relay.
As compared to a non-latching relay, this relay consumes less power. This relay consumes more power.
These relays have noiseless switching within household applications. These relays have some noise while operating.
Not like a non-latching relay, these relays are not designed to be used like highly sensitive ones. Once the latching relay is anxious, then it does not have very high sensitivity. Non-latching relays have high sensitivity as compared to latching relays.
The latching relays include indicating knobs that are used to control the position of the relay manually. This relay doesn’t have to indicate a knob feature.
The life service of latching relay is no longer. The life service of the non-latching relay is longer.
These relays are more expensive. Non-latching relays are not expensive as compared to latching relays.
These relays are very efficient, so they do not have a broad range of application regions. Non-latching relays are used anywhere in electronics & automation.

Advantages

The advantages of latching relay include the following.

  • It requires excitation of pulse and it can work through the single-coil otherwise double coil
  • Its size is small, so easily connected to PCB.
  • Load capacity is strong
  • Power consumption is less.
  • Reliable, safe, and service life is long.
  • Safe and reliable
  • These relays simply save crossbar switches, so the controlling of the lighting can be done through push buttons in place of a combination of the three-way & crossbar switches.
  • These relays save conductors.
  • They provide higher comfort in controlling all loads while leaving the house.
  • These relays simply control more lamps as compared to contractors with a similar nominal current.
  • The time taken to connect the devices by using this relay is less.
  • It conserves energy.

Disadvantages

The disadvantages of latching relay include the following.

Latching relays need two control signals for turning ON & OFF the load.

  • A high burden range of transformers is necessary for operating the electromagnetic relays as compared to static relays.
  • They use more materials as compared to electromagnetic relays.
  • The directional feature is not present within the relays.
  • It needs periodic maintenance & testing.

Applications

The applications of latching relay include the following.

  • These relays simply allow a consumer for controlling a circuit by providing simply a single pulse to the control circuit of a relay.
  • These are used in a wide range of industrial applications for different purposes like the following.
  • It is used in industrial sorting systems and counting.
  • Used in power supplies.
  • It is used in HVAC, anti-condensation & refrigeration systems.
  • It is used in cleaning equipment in industries like automated car-washing devices.
  • Commercial coffee machines.
  • Automated food preparation devices.

Thus, this is all about an overview of a latching relay and its working with applications. Here is a question for you, what are the features of the relay?

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