This is an overview of how to build a latching relay circuit from a regular non-latching relay with the use of a few additional components. There are many other methods of latch circuits using digital logic, PLC’s, or the use of microcontrollers.
Fig. 1 BOM
- Relay with a N.O. contact
- Voltage source matching the relay coil voltage
- N.O. switch (this can also be a float switch or similar N.O. component)
- N.C. switch (this can also be a float switch or similar N.C. component)
- A load with voltage equal to or below source voltage with use of a resistor
- Any other components not listed needed for circuit operation or safety such as a fuse or circuit breaker
Fig. 1
Latching relay circuit with the load using the same power supply
Schematic built using Scheme-it
Fig. 1 Operating Principal
Once the N.O. switch is pressed, it supplies source voltage to the load and relay coil. The closing of the relay contacts maintains the voltage to both the load and the relay coil until the N.C. switch is pressed.
Fig. 2 BOM
- Double pole relay with (2) N.O. contacts
- Voltage source matching the relay coil voltage
- Additional voltage source matching the load voltage
- N.O. switch (this can also be a float switch or similar N.O. component)
- N.C. switch (this can also be a float switch or similar N.C. component)
- A load with voltage equal to or below the additional voltage source with use of a resistor
- Any other components not listed needed for circuit operation or safety such as a fuse or circuit breaker
Fig. 2
Latching relay circuit with the load using a separate power supply
Schematic built using Scheme-it
Fig. 2 Operating Principal
Operating the same as Fig. 1, but the use of a relay with a double pole and (2) N.O. contacts allow for latching ON of a load using a separate voltage source. This is used when the load voltage is different than the source voltage.
Tech Tip: You can make a fun trip wire alarm using a long breakaway wire in place of the N.C. switch and a buzzer or light on the N.C. relay contacts.
Tech Tip: Once latched, the N.C. switch or a cycle of the power supply will reset the latch.
Tech Tip: The relay in the schematics have a built-in suppression diode to counter the reverse EMF voltages from the collapsing of the coil field. If the relay you are using does not have one or you want extra protection for your circuit, you can add your own suppression diode in parallel but reversed-biased to the coil, or you can add a diode in series to a DC load among other methods.
Tech Tip: These are fairly generic circuits and can be modified to fit a variety of uses such as float switches in manufacturing, sump pumps etc. The power supplies, switches, and relay must be able to handle the voltage and current of the load.
DigiKey Products
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Related Topics
The MOSFET Active Clamp: The Case Against a Relay’s Parallel Flyback Diode
Basics of Switches and Relays: An Overview of Poles and Throws
Electromechanical vs. Solid State Relay Characteristics
Initial state of Latching Relay is changed before use
Relay Normally Open vs Normally Closed Contacts
Introduction to the 3-Wire Start-Stop Circuit
Unlatching a Latching Relay
Tips for Relay Operation
Latching relay