Latching Relay Definition, Basic Functions and Selection

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Author: Kynix

Published: 2021-12-18 | Last Updated: 2021-12-18

Contents

Introduction

Latching relay is a new type of relay and also an automatic switch. Like other electromagnetic relays, it turns on and off the circuit automatically. The difference is that the normally closed or normally open state of it is completely dependent on the action of permanent magnets, and the switching state of it is triggered by a pulse electric signal of a certain width. It has the characteristics of power saving, stable performance, small size, large carrying capacity, and superior performance than general relays.

Latching Relay Basics in 2 Minutes

Catalog

Ⅰ Working Principle

1.1 Action Principle

The on and off state of the latching relay is usually held by the magnetic force generated by the permanent magnet. When the contacts of the relay need to be opened or closed, only the positive (reverse) DC pulse voltage is needed to excite the coil, and the relay completes the state transition between opening and closing in an instant. Usually when the contact is in the holding state, the coil does not need to continue to be energized, and the state of the relay can be maintained unchanged only by the permanent magnet force.

1.2 Action Process

When the contacts of the relay need to be set, it is only necessary to excite the coil J2 with a positive DC pulse voltage. The magnetic poles generated by the coil J2 after excitation interact with the magnetic poles of the permanent magnet. As we all known, the same polarities repel each other, but the opposite polarities attract each other. The state transition from reset to set is completed in an instant. The following schematic diagrams demonstrates the specific state transition process. The process of the latching relay changing from the set state to the reset state, which are the same.

Reset Latching Relay

Figure 1. Reset Latching Relay

Constant-Current Pulse Monment

Figure 2. Constant-Current Pulse Monment

Constant-Current Pulse Monment

Figure 3. Constant-Current Pulse Monment

Latching Relay Reset

Figure 4. Latching Relay Reset

Ⅱ Main Functions and Advantages

2.1 Average View

Latching relay is an automatic switching element with isolation function. It is widely used in remote control, telemetry, communication, automatic control, mechatronics and power electronic equipment. It is one of the most important control elements in electricity.
Magnetic latching relays generally have an induction mechanism (input part) that can reflect certain input variables (such as current, voltage, power, impedance, frequency, temperature, pressure, speed, light, etc.). It has the ability to turn on/off the controlled circuit. In addition, between the input part and output part of the relay, there is an intermediate mechanism (drive part) for coupling and isolating the input, functional processing and driving the output part.

2.2 Function Lists

The latching relay has the following functions:
1) Expansion
For example, when the control signal of a multi-contact relay reaches a certain value, multiple circuits can be switched, disconnected, and connected at the same time according to different forms of contact groups.
2) Amplification
For example, magnetic latching relays can control a large-power circuit with a very small control quantity.
3) Integration
For example, when a plurality of control signals are input to a multi-winding relay in a prescribed form, they will be compared and integrated to achieve a predetermined control effect.
4) Automation, remote control and monitoring
For example, on the automatic device and other electrical appliances, magnetic latching relays can form a program control circuit to realize automatic operation.

2.3 Application Area Lists

1) Smart meters: IC card meters, prepaid meters, single-phase meters, three-phase meters.
2) Reactive power compensation: synchronous switch, composite switch, smart capacitor.
3) Intelligent control: smart home, solar street light control, automation equipment, etc.

2.4 Latching Relay Advantages

1) They only need pulse excitation, and can work with single and double coils.
2) Small size, easy installation.
3) Low power consumption and strong load capacity.
4) Safe and reliable, long service life.

General Relay Structure

Figure 5. General Relay Structure

Ⅲ Tech Parameters

1) Rated Voltage
It refers to the voltage required by the coil when the relay is working normally. Depending on the model of the relay, it can be AC voltage or DC voltage.
2) DC Resistance
It refers to the DC resistance of the relay coil, which can be measured by a multimeter.
3) Pull-in Current
It refers to the minimum current that the relay can produce the pull-in action. In normal use, the given current must be slightly larger than the pull-in current, so that the relay can work stably. As for the working voltage applied to the coil, generally do not exceed 1.5 times the rated working voltage, otherwise it will generate a larger current and burn the coil.
4) Release Current
It refers to the maximum current that the relay generates to release the action. When the current in the pull-in state of the relay is reduced to a certain level, it will return to the unpowered release state. The current at this time is much smaller than the pull-in current.
5) Switch Voltage and Current
This refers to the voltage and current that the relay allows to load. It determines the magnitude of the voltage and current that the relay can control, and you cannot be exceeded this value during use, otherwise it is easy to damage the contacts of the relay.
6) Coil Resistance R
The resistance produced by winding a T circle with an enameled wire with a diameter of Φ: R=￡*(T/Φ).
7) Temperature
The higher the temperature, the greater the resistance, and the lower the resistance, the smaller the resistance. Generally speaking, the coil resistance refers to the coil resistance at 20°C. When the temperature is higher or lower than 20℃, there is a calculation formula: Rt=R20[1+(T℃-20℃)×0.004].
8) Contact Resistance CR
It is the resistance between the contacts of the relay. And it is the sum of the shrinkage resistance Re and the surface film resistance Rf: Rk=Re+Rf.
Note:
🔺The factors that form shrinkage resistance Re:
♦️The size of the contact pressure determines the size of the shrink resistance. The relationship between the two is inversely proportional, the greater the contact pressure, the smaller the shrinkage resistance.
🔺The factors that form the surface film resistance Rf:
♦️Dust in the air.
♦️Harmful gases in the air: H2S, SO2, etc.
♦️Organic vapor in the air: plastic, glue and rosin.
♦️Oil stains on the contact surface during the production process.
9) Pull-in Voltage
The minimum voltage required to close the relay contacts.
10) Release voltage
The minimum voltage required to open the relay contacts.
11) Insulation Resistance
The resistance value that appears when measured with a specified DC voltage between conductive parts that are insulated from each other.
12) Medium Withstand Voltage
Also known as dielectric degree, it refers to the maximum voltage that can withstand between two conductive parts without breakdown.
13) Reversing Retention
The minimum force applied on the armature assembly handle to open or close the contacts.
14) Contact Pressure
The interaction force generated between the moving and static contacts is equal to the contact pressure equal to the reaction force generated by the over-travel of the reed head.

latching relay product

Figure 6. Latching Relay Product

Ⅳ Latching Relay Test

4.1 Measuring Contact Resistance

Use the resistance profile of the universal meter to measure the resistance of the normally closed contact and the moving point, which should be 0. The contact resistance can be measured in a more accurate way within 100 milliohms, while the resistances of the normally-open contact and the moving point should infinite. From this, it can be distinguished which is a normally-closed contact or a normally-open contact.

4.2 Measuring Coil Resistance

The resistance value of the relay coil can be measured with the universal meter R×10Ω, so as to judge whether there is an open circuit phenomenon in the coil.

4.3 Set Voltage and Current

Get an adjustable regulated power supply and an ammeter, input the set voltage to the relay, and insert an ammeter in the power supply loop for monitoring. Slowly increase the power supply voltage, and when you hear the sound of the relay picking up, write down the set voltage and set current. In order to be accurate, you can try several times. Pay attention to, the setting current should be the average value, and the setting voltage should be the maximum value.

4.4 Reset Voltage and Current

Repeat the above operation. But there is a little difference that the reset current is the average value, and the reset voltage is the maximum value.

Ⅴ Relay Selection

5.1 The Necessary Conditions

① The power supply voltage of the control circuit, the maximum current that can be provided.
② The voltage and current in the controlled circuit.
③ How many sets and types of contacts are needed for the controlled circuit. When selecting a relay, the power supply voltage of the general control circuit can be used as the basis for selection. The control circuit should be able to provide enough working current to the relay, otherwise the relay will be unstable when it is closed.

5.2 Relevant Information Search

After consulting the relevant information to determine the conditions of use, you can search for the relevant information to find out the model and specification number of the required relay. If you already have a relay on hand, you can check whether it can be used based on the data. Finally, consider whether the size is appropriate.

5.3 Installation Layout Consideration

If it is used for general electrical appliances, in addition to considering the volume of the case, it is necessary to consider the installation layout of the circuit board. For small electrical appliances, such as toys and remote control devices, ultra-small relay products should be used.

Ⅵ FAQ

1. What is magnetic latching?
The design of a Magnetic Latching Relay is such that when a short pulse of electrical energy is applied to the solenoid coil, sufficient magnetic force is generated to over-come the force of the relay' return mechanism.

2. How does a latching relay work?
One latching type has two opposing coils with an over-center spring or permanent magnet hold the contacts in position after the coil is de-energized. A pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off.

3. Where is latching relay used?
This type of relay is most suitable in applications like ON/OFF devices from multiple places with push-button or momentary switch. For example, it is used in a lighting circuit or conveyer to control from different locations.

4. What is a magnetic latching relay used for?
A magnetic latching relay is a device in which the solenoid principle is applied to open and close light-current electrical circuits. The same device applied in heavy-current circuits is called a contactor, or circuit breaker.

5. How does a latching relay reset?
Resetting is very simple. The coil circuit needs to be opened and shortened to the coils ground potential. The capacitor will be discharged through the coil and drives a current pulse with opposite polarity through the coil.

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