Introduction
As mentioned earlier, a relay is an electrical switch consisting of input terminals and operating contacts used to control one or more signals in electrical circuits. These switches can have various control contacts, such as make contacts, break contacts, and more. Relays are commonly used in circuits where a low-power signal controls high-power electrical systems. There are several types of relays available, depending on the specific requirements. In this article, we will focus on the latching relay, discussing its operation, circuit diagrams, various types, and applications in power electronics. Let’s get started!
Latching Relay Definition
A latching relay is an electrically operated switch that can maintain its position without continuous power being applied to its coil. In other words, it is a simple two-position electro-mechanical switch. It is also known by other names, such as keep, bistable, stay, or impulse relay. This type of relay is used in applications where a small control current is used to manage a much larger electric current. The coil of the latching relay only consumes power during the switching process. Once the switch is activated, the relay’s contacts remain in their new position, even after power is removed from the coil.
Circuit Diagram of the Latching Relay
Figure 1: Latching Relay Circuit (Courtesy of Simon Mugo)
The circuit diagram consists of two push buttons. Push-button B1 is used to complete the circuit, while push-button B2 is used to break the current flow in the circuit.
- The latching relay is energized when push-button B1 is pressed. This action closes contacts A to B and C to D. Once the relay is energized and contacts A and B are closed, releasing push-button B1 will allow current to continue flowing from the power supply.
- To de-energize the relay and interrupt the current flow, push-button B2 must be pressed to deactivate the relay coil.
Working of the Latching Relay
Push-button B1 is normally open, while push-button B2 is normally closed. This means that, in its initial state, B1 is open and B2 is closed.
- Pressing push-button B1 energizes the circuit. Current flows through the circuit in the path +VE-B1-A-B- and -VE, which connects contacts A to B and C to D.
- Once push-button B1 is released, the relay coil remains energized, and the circuit will stay in this state until a disconnection is made.
- To disconnect the relay, push-button B2 is pressed. Since B2 is normally closed, pressing it opens the circuit and de-energizes the relay.
Latching Relay Circuit Assembly Steps
Now, let’s go through the step-by-step process of assembling a latching relay circuit.
- Step 1: Choose a relay and a push button, and connect them as shown in the diagram below. Label the push-button as B1. Push-button B1 should normally be open.
Figure 2: Step 1 of Latching Relay Circuit Connection (Courtesy of Simon Mugo)
When push-button B1 is pressed, the relay is activated. Releasing push-button B1 will turn off the relay.
- Step 2: To latch the relay, connect the common point of the relay to the power source via a push-button, as shown in the circuit diagram below.
Figure 3: Latching Circuit with Single Push-Button (Courtesy of Simon Mugo)
In the configuration shown above, pressing the push-button powers the relay. When the button is released, the relay contacts remain in the latched position, unaffected by the button release. The DC power supply continues to provide power to the relay, ensuring that it remains on until further action is taken.
- Step 3: Next, we introduce an additional push-button, B2, which is normally closed, and incorporate it into the circuit with the DC power line, as shown in the diagram below.
Figure 4: Adding Push-Button B2 to the Circuit (Courtesy of Simon Mugo)
The role of push-button B2 is to power off the relay by disconnecting the DC power supply. Pressing it opens the circuit and de-energizes the relay.
In summary, push-button B1 turns the relay on, and push-button B2 turns it off.
Types of Latching Relays
Latching relays are primarily categorized into three types: impulse, magnetic, and mechanical latching relays.
Magnetic Latching Relays
Magnetic latching relays are typically defined as magnetic switches that can automatically switch the circuit on and off. They use a permanent magnet to maintain the open or closed state. A specific pulse or modulated electric signal activates the switching operation. These relays can be further divided into single-phase and three-phase magnetic latching relays.
Magnetic latching relays can switch currents up to 150A, depending on their design, and the control coil voltage is usually 9V or 12V DC.
Specifications of Magnetic Latching Relays
· 1,000,000 mechanical life cycles
· 10,000 electrical life cycles
· Contact voltage drop below 100mV
Characteristics of Magnetic Latching Relays
· Stable and reliable performance
· Compact size
· High load capacity
· Power-efficient
These advantages make magnetic latching relays more efficient compared to standard electromagnetic relays.
Applications of Magnetic Latching Relays
· Used in centralized meter-reading systems and IC card payment systems.
· Ideal for reactive power compensation in transmission lines and other power equipment.
· Suitable for large current applications, such as thyristor switch circuits.
· Commonly used in household appliances and automatic control devices.
Impulse Sequencing Latching Relays
Also known as bi-stable relays, impulse sequencing latching relays change their contact states based on voltage signals. These relays are highly valued for their resistance to shocks and vibrations.
Once the relay is powered, its opposing coil is activated, closing the contact, which remains closed even after the power is disconnected. When power is reapplied, the contact changes direction and remains in that new position. This process can be repeated for the on/off cycle.
Applications of Impulse Sequencing Latching Relays
These relays are used in:
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