Wiring and Operating Latching Relays in Electronic Circuits

You wire a latching relay by connecting the coil terminals to a control switch and the relay contacts to your load in the circuit. A latching relay holds its state after you remove power from the coil, unlike a standard relay that resets. You need to know how each relay type works because choosing the right relay affects your circuit’s performance. Understanding the wiring steps helps you build reliable electronic projects.

Key Takeaways

• Latching relays keep their state without continuous power, saving energy and improving reliability.

• You can control latching relays using set and reset pushbuttons or microcontrollers for automation.

• Choose between single-coil and dual-coil latching relays based on your wiring and control needs.

• Use flyback diodes with DC relays to protect your circuit from voltage spikes.

• Follow safety standards and use proper wiring, labeling, and insulation to prevent accidents.

• Latching relays work well in lighting, alarms, industrial automation, and motor control applications.

• Test your circuit carefully before powering up to avoid wiring mistakes and ensure proper operation.

• Use self-holding circuits or 8-pin relays to build reliable latching relay setups with clear control.

Latching Relay Basics

What Is a Latching Relay

A latching relay is a special type of relay that keeps its position even after you remove power from its coil. You can think of it as a switch that remembers its last state. When you press a button to activate the relay, it stays on until you press another button to turn it off. This feature makes the latching relay very useful in circuits where you want to save energy or keep a device on during a power loss.

Tip: A latching relay only uses power when you change its state. This means it does not waste energy by staying energized all the time.

Here are some important facts about latching relays:

1. A latching relay is a bistable electromechanical switch.

2. It holds its contact position without continuous power.

3. You can use a push-button to set the relay and another to reset it.

4. The relay stays in its last position until you send a new signal.

Latching vs Standard Relay

You might wonder how a latching relay is different from a standard relay. The main difference is how they use power and how they behave when you stop sending power to the coil.

A standard relay needs power all the time to stay on. If you let go of the button, the relay turns off. In contrast, a latching relay stays on or off until you tell it to change. This makes the latching relay a better choice for circuits where you want to save energy or keep a device running during a power cut.

How Latching Relays Work

You control a latching relay with short pulses of current. When you press the set button, the relay switches on. It stays on even after you release the button. To turn it off, you press the reset button, which sends another pulse to change the relay’s state.

Some latching relays use one coil, while others use two. In a single-coil relay, you reverse the current to change the state. In a dual-coil relay, you have separate coils for setting and resetting. Both types keep the circuit simple and reliable.

• Latching relays come in many types, such as magnetic, mechanical, and electronic.

• You can choose a relay based on size, mounting method, voltage, and the number of contacts.

• These relays work well in circuits where you need to keep the load on or off, even if the power goes out.

Note: Latching relays help you build circuits that use less energy and produce less heat. They also make your projects safer and more reliable.

Latching Relay Types

Electrically Latched

You will find electrically latched relays in many modern control systems. These relays use an electric coil to change and hold their contact position. When you press a set button, the relay coil receives a short pulse of current. This pulse moves the contacts and keeps them in place, even after you release the button. To reset the relay, you press another button that sends a new pulse to the coil.

Electrically latched relays save energy because they only use power during switching. They do not need continuous power to hold their state. This makes them ideal for applications where you want to reduce power consumption and heat. You can see these relays in industrial machines, power supplies, and automated systems.

Note: Electrically latched relays often meet strict industry standards. For example, OMRON MMK relays offer coil voltages from 6 V to 200 V AC/DC. They also provide high mechanical life, sometimes up to 100 million cycles.

Here is a table showing some technical figures for electrically latched relays:

You can rely on these relays for strong loading capacity and long-term reliability.

Mechanically Latched

Mechanically latched relays use a physical locking mechanism to hold their contacts in place. When you activate the relay, a small mechanical part locks the contacts. The relay stays in this position until you apply a reset signal. This design means the relay does not need any power to keep its state.

You will often use mechanically latched relays in safety circuits and emergency systems. These relays work well in places where you want to keep a device on or off, even if the power fails. The mechanical latch ensures the relay stays in its last position, which adds an extra layer of safety.

Some models include features like transparent covers for inspection and snap locks for easy installation. You can find these relays in industrial automation, food processing, and even car washing machines.

• Mechanically latched relays provide high reliability.

• They offer long electrical life, sometimes over 10 million operations at low currents.

• Many designs include safety contacts and magnetic arc blowout for longer contact life.

Single-Coil vs Dual-Coil

You can choose between single-coil and dual-coil latching relays based on your circuit needs.

• Single-Coil Latching Relay:
  This relay uses one coil. You change the relay state by reversing the current direction through the coil. One pulse sets the relay, and a pulse in the opposite direction resets it. This type is simple and uses fewer wires.

• Dual-Coil Latching Relay:
  This relay has two separate coils—one for setting and one for resetting. You send a pulse to the set coil to move the contacts to the ON position. You send a pulse to the reset coil to move them back. Dual-coil relays make wiring easier because you do not need to reverse current.

Tip: Dual-coil relays are common in circuits where you want clear and separate control for ON and OFF actions.

You will see both types in industrial and home automation. The choice depends on your wiring preferences and the level of control you need.

How to Make a Latching Relay Circuit

Required Components

Before you start building a latching relay circuit, you need to gather the right parts. Choosing the correct components helps your circuit work safely and reliably. Here is a list of what you should have:

1. Relay: Pick a latching relay that matches your voltage and current needs. Check the coil voltage, contact rating, and number of poles.

2. Pushbuttons: You need two momentary pushbuttons—one for setting (ON) and one for resetting (OFF) the relay.

3. Power Supply: Use a DC or AC power source that matches your relay coil.

4. Flyback Diode: Place a diode across the relay coil if you use a DC relay. This protects your circuit from voltage spikes.

5. Load: This can be a lamp, motor, or any device you want to control.

6. Resistor (optional): Sometimes, you add a resistor to limit current in the holding circuit.

7. Wires and Connectors: Use insulated wires for safe connections.

8. Perforated Board or PCB: Mount your components securely.

9. Screw Terminals: These help you connect and disconnect wires easily.

10. Multimeter: Test your connections and check for continuity.

When you select your relay, look at the performance requirements. Make sure it can handle the voltage and current of your load. Check the insulation, temperature range, and durability. Some relays, like the TP41-24 or TP41-27SH, meet strict standards for safety and reliability. Always follow IEC standards for best results.

Tip: Always double-check the relay’s datasheet for coil voltage, contact ratings, and wiring diagrams before you start.

Pushbutton Set and Reset

You can control a latching relay circuit with two pushbuttons. One button sets the relay, and the other resets it. This setup lets you turn on the circuit or turn off the circuit with a simple press.

Here is a step-by-step procedure to wire the pushbutton set and reset:

1. Place the relay and screw terminals on your board.

2. Position the relay so the terminals fit well. Enlarge the holes if needed.

3. Place the flyback diode next to the relay coil if you use a DC relay.

4. Arrange the pushbuttons—one for SET (ON) and one for RESET (OFF).

5. Connect the SET pushbutton to the relay coil and power supply. This button should be normally open (NO).

6. Connect the RESET pushbutton in series with the coil. This button should be normally closed (NC).

7. Solder the relay terminals to the screw terminals using thick solder traces.

8. Solder the rest of the components. Use insulated wires to avoid crossovers.

9. Test the circuit with a multimeter before powering up.

10. Power the module and connect your load.

11. Press the SET button to energize the relay and turn on the circuit. The relay stays latched even after you release the button.

12. Press the RESET button to de-energize the relay and turn off the circuit.

You can also control the relay with a microcontroller by connecting its outputs to the SET and RESET pins. Some relay modules, like the Azatrax MRAPR, come with flyback protection and work with both AC and DC circuits.

Note: Always insulate exposed relay terminals to prevent shocks or short circuits.

Self-Holding Circuit

The self-holding, or seal-in, method is a classic way to keep a relay latched after you press the start button. This method uses the relay’s own contacts to maintain coil power.

• Wire a normally open (NO) contact of the relay in parallel with the start (SET) pushbutton.

• When you press the start button, you energize the relay coil. The NO contact closes and keeps the coil powered even after you release the button.

• Use a normally closed (NC) pushbutton as the stop (RESET) button. Pressing this button breaks the coil circuit and de-energizes the relay.

• You can add a resistor to reduce the holding current if needed.

• This setup works well for switching AC loads, like lamps, using a DC coil relay.

The self-holding circuit ensures that the relay remembers its state. You only need a short press to turn on the circuit, and the relay stays latched until you press the stop button to turn off the circuit.

Tip: The Square D Wiring Diagram pamphlet offers detailed circuit diagram examples for latching relay circuits and motor controls.

8-Pin Relay Example

You can use an 8-pin relay to build a reliable latching relay circuit. This type of relay is common in control systems because it offers flexibility and strong isolation between control and load sides. The 8-pin design makes wiring easier and helps you keep your circuit organized.

When you look at an 8-pin relay, you will see two coil pins, two pairs of common (COM) pins, and four contact pins for normally open (NO) and normally closed (NC) connections. Each pin has a specific role in the circuit diagram. You must connect the coil pins to your control voltage. The NO and NC contacts let you control the flow of current to your load.

Here is a step-by-step guide on how to make a latching relay circuit using an 8-pin relay:

1. Place the 8-pin relay on your board. Make sure you know the pinout from the relay’s datasheet.

2. Connect the coil pins to your power supply through the set and reset pushbuttons. Use a flyback diode across the coil if you use DC voltage.

3. Wire the NO contact in parallel with the set pushbutton. This creates a self-holding path for the relay coil.

4. Connect the NC contact in series with the reset pushbutton. This lets you break the circuit and de-energize the relay.

5. Attach your load to the common and NO contacts. When the relay latches, the load turns on and stays on until you press the reset button.

6. Double-check all connections for proper insulation and secure mounting.

Tip: Always use the correct voltage for the relay coil. Too much voltage can damage the relay, while too little may prevent it from latching.

The 8-pin relay works well in latching relay circuits because it can maintain the state of the circuit after you remove the control signal. This means your load stays on or off until you decide to change it. The relay uses very little energy since it only needs power during switching. You also get reliable state maintenance, which is important for safety and energy savings.

Glass relays, often found in 8-pin packages, provide excellent reliability. They can handle frequent switching cycles without wearing out. This makes them a good choice for projects that need long-term performance. The relay also protects your control unit from overloads and simplifies the wiring process.

You should always consider safety when working with relays. Use proper insulation, check voltage levels, and add overload protection if needed. These steps help prevent accidents and keep your latching relay circuit running smoothly.

A simple circuit diagram for an 8-pin relay latching circuit might look like this:

[Power Supply]---[Set Button]---+---[Coil Pin 1]---[Coil Pin 2]---[Reset Button]---[Ground]
                               | 
                        [NO Contact]---+
                               |       |
                            [Load]   [COM Pin]

This diagram shows how the set and reset buttons control the relay coil, and how the NO contact keeps the relay latched. You can adapt this basic design for many applications, such as lighting control, alarms, or automation systems.

If you want to learn how to make a latching relay circuit that is both safe and reliable, start with an 8-pin relay. Follow the wiring steps, use a clear circuit diagram, and always check your work before powering up. This approach gives you a strong foundation for building more advanced latching relay circuits in the future.

Latching Relay Circuit Diagram

Reading Pinouts

Before you start wiring a latching relay circuit, you need to read the relay pinout. The pinout tells you which pin does what. Each relay has coil pins, common (COM) pins, normally open (NO) pins, and normally closed (NC) pins. You can find this information in the relay’s datasheet or on the relay case.

Here is a table to help you understand common relay pinouts and related control components:

Tip: Always double-check the wiring diagram before you connect power. This helps you avoid mistakes and keeps your circuit safe.

Basic Circuit Diagram

A basic latching relay circuit diagram shows how to connect the relay, pushbuttons, and load. You use a set button to energize the relay and a reset button to de-energize it. The relay holds its state until you press the other button.

Here is a simple example using a single-coil relay:

[Power Supply]---[Set Button]---+---[Coil Pin 1]---[Coil Pin 2]---[Reset Button]---[Ground]
                               | 
                        [NO Contact]---+
                               |       |
                            [Load]   [COM Pin]

In this circuit diagram, pressing the set button sends power to the coil. The relay latches and the load turns on. The NO contact keeps the coil powered even after you release the button. Pressing the reset button breaks the circuit and turns the load off.

You can also build a dual-coil latching relay circuit. In this case, you have two separate buttons—one for each coil. Each button sends a pulse to either set or reset the relay. This setup makes wiring easier because you do not need to reverse the current.

DPDT Relay Circuit

A DPDT (Double Pole Double Throw) relay lets you control two circuits at once. You can use it for more complex latching relay circuit diagrams, such as reversing a motor or switching two loads. The DPDT relay has two sets of COM, NO, and NC pins.

DPDT relays work well in control systems. For example, a relay coil might draw only 15 mA at 10 V, but its contacts can switch 15 A at 100 V. This means you can control large loads with a small signal. The relay also provides strong electrical isolation, with insulation resistance up to 1 GΩ and dielectric strength up to 5 kV. These features make DPDT relays reliable for demanding latching relay circuit applications.

Here is a basic DPDT latching relay circuit diagram:

[Power Supply]---[Set Button]---+---[Coil]---[Reset Button]---[Ground]
                               | 
                        [NO1]---[Load 1]---[COM1]
                        [NO2]---[Load 2]---[COM2]

You can use the DPDT relay to switch both loads at the same time. When you energize the relay, both NO contacts close and both loads turn on. When you de-energize the relay, both loads turn off. This setup is useful for controlling motors, alarms, or other devices that need synchronized switching.

Note: Always use the correct circuit diagram and check the relay’s ratings before you connect your loads. This helps you prevent damage and ensures safe operation.

Tips and Configurations

Best Practices

You should always follow industry standards when wiring a relay in your circuit. The industrial control panel design guide gives you clear rules for safe and reliable wiring. You need to ground your relay circuits properly. NEC 250 explains how to ground electrical systems to prevent shocks and damage. Use the right wire size for your relay based on the load current. This keeps your wires from overheating.

Group your relay components neatly inside the panel. Label each wire and use color-coding to avoid confusion. NFPA 79 tells you how to wire control circuits for machines. UL 60947-4-1 sets the rules for control panel safety. You should also add arc flash labels and emergency stop buttons for extra safety.

Tip: Always double-check your relay wiring before you power up the circuit. Careful planning and neat wiring help you find problems quickly.

Common Mistakes

Many people make simple mistakes when working with a relay. You can avoid these problems if you know what to look for:

• You might connect the relay coil to the wrong voltage. This can burn out the relay or make it fail to switch.

• Some people forget to use a flyback diode with a DC relay coil. This can damage other parts of your circuit.

• Loose or messy wiring can cause short circuits or make the relay work poorly.

• You may use wires that are too thin for the load. This can lead to overheating and fire risks.

• Skipping labels or color codes makes troubleshooting hard later.

Microcontroller Integration

You can control a relay with a microcontroller like Arduino or Raspberry Pi. This lets you automate your circuit and add smart features. Use a relay module with built-in protection for easy setup. Connect the control pin from your microcontroller to the relay input. Make sure the relay coil voltage matches your microcontroller output.

Write a simple program to send a HIGH or LOW signal to the relay. This will turn your circuit on or off. Always use a separate power supply for the relay if your load needs more current than the microcontroller can provide.

Note: Use opto-isolated relay modules to protect your microcontroller from voltage spikes.

You can use relays to control lights, fans, or alarms in your projects. This makes your circuit safer and more flexible.

Troubleshooting Latching Relay Circuits

Wiring Issues

Wiring mistakes often cause problems in relay circuits. You might connect the wrong pin or use a loose wire. These errors can stop your relay from working. Always check the relay pinout before you start. Use a multimeter to test for continuity. Make sure each wire goes to the correct terminal. If you see the relay not switching, look for broken wires or cold solder joints. Sometimes, a wire may touch another part and create a short. You can avoid this by keeping wires neat and using insulation.

Tip: Label each wire as you build your circuit. This helps you find problems faster if something goes wrong.

Not Latching or Resetting

Sometimes, your relay does not latch or reset as expected. This can happen if the control buttons do not send a strong enough signal. Check if your power supply matches the relay coil voltage. If the voltage is too low, the relay will not energize. Also, look at the pushbuttons. A stuck or faulty button can block the signal. Try pressing each button and listen for a click from the relay. If you do not hear a click, test the button with a multimeter.

You should also check for a missing flyback diode in DC circuits. Without this diode, voltage spikes can damage the relay or other parts. If your relay uses two coils, make sure you connect each button to the correct coil. Swapping the wires can stop the relay from working.

Noise and Isolation

Electrical noise can cause your relay to switch on or off by itself. This noise may come from nearby motors or other circuits. You can reduce noise by using shielded wires and keeping signal wires away from power lines. Adding a capacitor across the relay coil can help filter out spikes. If you use a microcontroller, choose a relay module with opto-isolation. This keeps the control side safe from high voltages.

⚡ Alert: Never touch relay terminals when the circuit is powered. High voltage can cause serious injury.

Good isolation protects your devices and makes your circuit more reliable. Always follow safety rules when you work with relays.

Applications and Safety

Real-World Uses

You can find latching relays in many places where you need reliable control. These relays help you save energy and keep devices running even if the power goes out. Here are some common ways you might use latching relays:

• Lighting Control: You can use a latching relay to turn lights on or off with a single button press. This works well in homes, offices, and factories.

• Alarm Systems: Latching relays keep alarms active until someone resets them. This makes sure the alarm stays on even if the power flickers.

• Industrial Automation: Machines often use latching relays to hold a process in place. For example, you can start a conveyor belt and keep it running until you press stop.

• Home Automation: Smart home systems use latching relays to control fans, pumps, or heaters. You can set a device to stay on until you decide to turn it off.

• Motor Control: Latching relays help you start and stop motors safely. You can use them in garage doors, elevators, or water pumps.

Tip: Latching relays work well in circuits where you want to keep a device on or off, even if the main power supply is interrupted.

You can also use latching relays in battery-powered devices. Since they only use power when switching, they help you extend battery life. Many engineers choose latching relays for projects that need both reliability and energy savings.

Electrical Safety

When you work with latching relays, you must always think about safety. Electricity can be dangerous if you do not follow the right steps. Many safety standards and devices help protect you and your equipment.

• OSHA 1910.147 gives rules for controlling hazardous energy. You must lock out and tag out circuits before you work on them.

• NFPA 70E explains how to stay safe when working with electricity. It tells you what protective gear to wear and how to avoid shocks.

• IEC 61508 and ISO 13849 guide you in designing safe control systems. These standards help you build circuits that protect people and machines.

• Safety devices like interlock switches, emergency stops, and safety relays add extra layers of protection. You should use these in your relay circuits.

• New technologies, such as safety controllers and absence of voltage testers, help you check if a circuit is safe before you touch it.

⚡ Alert: Electrical Safety Foundation International reports show that electrical injuries dropped by more than half from 1992 to 2010. However, about five nonfatal injuries still happen every day in the U.S. from 2010 to 2020. You must always stay alert and follow safety rules.

You should always:

• Turn off power before wiring or changing your relay circuit.

• Use insulated tools and wear safety gloves.

• Label all wires and keep your work area clean.

• Test for voltage before touching any part of the circuit.

• Follow all local and national electrical codes.

By following these steps and using the right safety devices, you can build latching relay circuits that work well and keep everyone safe.

You can wire and operate a latching relay by following clear steps and using the right components. Always match the switch type to your circuit’s needs and use safe wiring methods. Experts recommend these best practices:

• Choose latching switches for steady power and momentary switches for quick actions.

• Use durable parts and feedback mechanisms to lower maintenance.

• Avoid improper switch choices to reduce safety risks and costs.

Latching relays need careful installation. When you follow safety rules, you create reliable circuits. Try building different latching relay circuits to gain hands-on experience and improve your skills.

FAQ

What is the main advantage of a latching relay?

You save energy with a latching relay. It only uses power when you change its state. The relay keeps its position even if you remove power from the coil.

Can you use a latching relay with a microcontroller?

Yes, you can. You connect the relay module to your microcontroller’s output pins. Use an opto-isolated relay module for extra protection. This setup lets you control devices safely.

How do you reset a latching relay?

You press the reset button or send a reset pulse to the relay coil. The relay then switches back to its original state. Some relays use a second coil for resetting.

Why does my latching relay not stay latched?

Check your wiring and power supply. Make sure you use the correct coil voltage. A missing flyback diode or loose wire can also cause problems. Test each part with a multimeter.

What is the difference between single-coil and dual-coil latching relays?

A single-coil relay changes state by reversing current. A dual-coil relay has separate coils for set and reset. Dual-coil relays make wiring easier because you do not need to reverse current.

Can you use a latching relay for AC and DC loads?

Yes, you can use latching relays for both AC and DC loads. Always check the relay’s datasheet for voltage and current ratings. Use the right relay for your specific load type.

How do you protect your circuit when using a latching relay?

Always add a flyback diode across the relay coil in DC circuits.
Use insulated wires and check all connections.
Test your circuit before connecting the load.

See Also

A Comprehensive Overview of Relay Symbols and Varieties

Beginner’s Guide to Understanding 8 Pin Relay Diagrams

How To Choose The Right Relays For Your Vehicle

Key Roles of Electronic Components On Circuit Boards

Understanding How Thyristors Operate In Power Electronics