Relays Part 8: Latching Relays

Introduction

As discussed earlier, a relay is defined as an electrical switch made up of inputs and terminals of operation used to carry out single or multiple signal controls in electrical circuits. The switch can involve many control contacts that come in many forms such as make contacts, break contacts, etc. They are involved in electrical circuits where a low-power single circuit controls various electrical circuits. Several types of relays are in existence based on customer requirements. In this article, we will introduce the latching relay from its working, circuit diagrams, various types, and where they find use in power electronics. Let us get started!

Latching Relay Definition

A latching relay is an electrically actuated electric switch that can maintain its position without power being applied to its coil. In other words, it can be defined as a simple two-positional electro-mechanical switch. It is also known as keep, bistable, stay, or impulse relay. It is used in applications where we need to control a very large electric current using a very small current. The latching relay coil consumes power only during switching ON and even after releasing the switch, its movable contacts remain in the switching mode position.

Circuit Diagram of the Latching Relay

Figure 1: Latching Relay Circuit Courtesy of Simon Mugo

The circuit diagram is made up of two push buttons. The push-button B1 is suitable for completing the circuit, while push-button B2 is for breaking the circuit's current flow.

• The latching relay will energize when pressing the first push-button labeled B1. This action closes contacts A to B and contacts C to D. Once the latching relay circuit is energized and contacts A and B are closed, on releasing the push-button B1, the flow of current from the supply continues.
• You must de-energize the latching relay coil to interrupt the current flowing in the circuit. This is where push-button B2 comes into play and is pressed to initiate relay coil de-energization.

The Working of the Latching Relay

Push-button B1 is normally open from the circuit, while push-button B2 is normally closed. This means that in the initial position, push-buttons B1 and B2 are in open and closed positions, respectively.

• Push-button B1 is pressed to energize the circuit. The current flow follows the circuit's +VE-B1-A-B- and -VE path. This connects contact A to B and contact C to D.
• On releasing the push-button B1, the coil of the latching relay remains in the energized form, and we need to create a disconnection to stop this energization.
• For disconnecting the relay, push-button B2 is used in the process. The push-button B2 is normally closed. Pressing it converts it to open normally, breaking the circuit and de-energizing it.

Latching Relay Circuit Making Steps

At this stage, we focus on the step-by-step process of designing latching relay circuitry.

• Step 1: Pick a relay and a push button and connect the circuit shown below. Label the push-button B1. Push-button B1 should normally be open

Figure 2: Step 1 of Latching Relay Circuit Connection Courtesy of Simon Mugo

Pressing the push button B1 above turns on the relay. When you release the push button B2, the relay gets off.

• Step 2: For latching the connected relay in step 1 above, the relay common point must connect to the power source through push-button as displayed in the circuit connection below.

Figure 3: Latching Circuitry with Single Push-button Courtesy of Simon Mugo

 Under the created condition in the circuit above, press the push-button power on the relay. On releasing the push button, the relay contacts do not move and remain in the exact position. Pressing the push-button B1 A1 supply from a push-button remains disconnected, but the power supply is continuously present as supplied by the DC power line. Therefore, this condition is aimed at powering the relay ON, and it should never go off even if the switch is released.

• Step 3: Here, we introduce an extra pushbutton B2, normally closed with the relay and the DC power line, as indicated in the circuit diagram below.

Figure 4: Introducing Push-Button B2 on the Circuit Courtesy of Simon Mugo

The purpose of push-button two is to power OFF the main DC supply. Pressing it creates a disconnection between the relay and the DC supply, hence de-energizing the relay.

In short, push-button B1 power ON the circuit and push-button B2 power OFF the same circuit.

Types of Latching Relays

Latching relays are classified into three main types, namely impulse, magnetic, and mechanical latching relays.

Magnetic Latching Relays

These are generally defined as magnetic switches that automatically switch the circuit ON and OFF. A permanent magnet principle makes the normally closed and open conditions possible. Using a specific pulse with a modulation electric signal activates its switching condition. It can be classified into single and three-phase magnetic latching relays.

Magnetic latching relay can have to switch current of up to 150A depending on operations and the voltage of the control coil can be 9V or 12 V DC.

Specifications of the Magnetic Latching Relays

·      1000,000 times mechanical life

·      10000 times electrical life

·      Below 100mV contact voltage drop

Characteristics of the Magnetic Latching Relays

·      Stable and good performance

·      Smaller in size

·      Large bearing capacity

·      Power-saving ability

All these characteristics are compared to the functioning of the normal electromagnetic relay

Application of the Magnetic Relay

·      Used in the construction of centralized meter-reading systems. Also suitable in the IC card system used for payments.

·      They are suitable for reactive power compensation in transmission lines and other power equipment.

·      Magnetic latching relays with the thyristor switch circuits involving large currents.

·      They also find use in household appliances and automatic control gadgets.

Impulse Sequencing Latching Relay

Also referred to as bi-stable latching relays, they change the contact condition using voltage signals. They are very popular due to their advantage of resisting shocks and working under high vibrations.

Once the relay is powered, the power activates the relay’s opposing coil. The contact closes and remains closed even after disconnecting the power supply. On applying the power to the circuit again, the magnetic relay contacts move in the opposite direction and remain in that position. There is the repetition of the power on the OFF-cycle.

Application of the Impulse Sequencing Latching Relay

The relays are applicable in:

·      Switching of the single device.

·      Conveyor circuits

·      Lightings circuits

Mechanical Latching Relays

They are described as locking mechanisms that keep the contacts in the movable contacts in the final moved position until they are given information to move. The action is made possible through the reinforcement of the secondary coil. After the action, the relay holding contacts stay locked until the other opposite coil gets ignition power. A good example of the mechanical latching relay is the relays used in the area of packaging machines.

Latching Relays Advantages

Listed below are the advantages of latching relays.

·      Easy to connect to printed circuit boards because of their smaller size.

·      It has a strong loading capacity

·      Low power consumption

·      The relays are very safe and can be relied on.

·      They are cheaper to use as they save the conductors

·      Easy to connect, hence saving time

Disadvantages of Latching Relays

Below is a list of the latching relays disadvantages.

• Requires that it is keenly serviced and tested periodically.
• more burdened transformers are needed to operate them than static relays require.
• They do not have directional features.
• More material is needed when related to electromagnetic relays.

Application of Latching Relays

The latching relay application includes.

• Used in industrial counting and sorting systems.
• Find applications in the area of power supply.
• used in refrigeration and anti-condensation systems.
• Find use in the food industry by automating food preparation devices.
• Used in car washing machines and coffee preparation machines.

Key Take Aways

• A latching relay is an electrically actuated electric switch that can maintain its position without power being applied on its coil.
• The circuit comprises two push-button switches, one normally closed and the other normally open with a relay.

·      It is operated by pressing the normally open push button to start the relay and the normally closed push button to stop the relay operation.

• Latching relays can be classified into three types, namely magnetic, impulse sequencing, and mechanical impulse relays.
• Latching relay’s advantages include easy connection, strong loading capacity, low power consumption, safety and reliability, and cheaper conductor saving.
• Latching relay’s disadvantages include close servicing and testing attention, more high-burdened transformers, lack of directional features, and more material needed in its construction.
• The applications of latching relays are industrial counting and sorting systems, power supply, refrigeration and anti-condensation, the food industry, and car washing machines.