Double-pole Relays in Latching Applications

Problem: When a customer calls into Technical Support and starts to ask me questions about our latching relays, I know the question that they are usually working up to. “I see you have latching relays with 12 Vac/dc and 24 Vac/dc coils, but I do not see any with a higher coil voltage. Do you have any?”

Solution: It is true that our RIB® latching relays only come with 12 or 24 Vac/dc coils, but any double-pole relay can be made into a latching relay with a couple switches and some simple wiring. This can be achieved by following the diagram below.

How does it work?

In the diagram above, the Normally Open (N/O) momentary switch would act like a Start button and the Normally Closed (N/C) momentary switch would act like a Stop button. The load being controlled by the relay would be wired through the N/O or N/C contact of the second pole (depending on the application). When the N/O switch is pressed, the constant power is applied to the RIB® coil. That causes the N/O contact from the first pole of the relay to close in parallel with the pushbutton. Now when the pushbutton is released, the coil still has a path for power and will remain energized.

The relay will remain energized until either the N/C momentary switch is pressed, breaking the coil circuit, or the constant power being applied is lost. If either of those events occur, the relay coil will lose power, and the relay contacts will return to their normal state. The above latching application would work without the N/C momentary switch present as well, but the only way to de-energize the relay in that setup would be to remove power.

When designing a control that implements the above, make sure that the momentary switches and the RIB® device chosen are rated for the continuous power applied, and that the relay contacts are rated to handle the load being controlled. Models RIB2401D and the RIB01P are DPDT relays that could be utilized in this setup, but we offer so many more options. Call us and find out.

About the Author

Henry Smith is a 34-year-old engineer at Functional Devices, Inc. He has a BSEET obtained from Purdue University in 2014 and a lifelong interest in electronics. As an engineer at Functional Devices, he gets to provide Technical Support to our customers, from distributors to specifying engineers and installers.

Henry enjoys providing tech support, as it allows our company to assist at every level of our product’s lifecycle and exposes us to interesting and unique applications. While not every technical question is unique, even answering a simple question or providing someone with the information in a timely manner can go a long way to helping him or her meet a deadline.