How to create a momentary-latching relay with a dual-relay module?

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Brenden McFarling

Joined Feb 13, 2018
2
Hi,

I am working on an alarm system and part of it is that instead of resetting the alarm NAC (Notification Appliance Circuit) when the Pull Station or Smoke Detector is reset, it latches and requires a reset button to be pressed. I have a 2-in-one relay (it's two relays sharing a single coil inside one enclosure) and at one point had a circuit, but I forgot how to wire it since I just found it in storage a few months later. What I need is to create a circuit that uses one of the two relay switches to loop power back to the coil, so that when activated it loops power back and keeps the relay "self sufficient," not requiring power from the short in the IDC (Initiation Device Circuit) to keep it activated. There will be a button on the line running from the switch pin to the coil, so that when the switch is pressed it momentarily blocks the flow and stops the current. If the IDC is still activated (a device is still in alarm), it will stay activated (because the power is still going in not from the relay switch, but from the IDC short). And if the IDC is reset (not supplying current), it will stop the flow and the relay will cut power to itself. Like I said, I've successfully implemented such a circuit before, but lost the self-written instructions on how to connect it and how it operates.
 

ebeowulf17

Joined Aug 12, 2014
3,274
This article covers the basic concept of making a latching relay out of a non latching one:
http://www.reuk.co.uk/wordpress/electronics/latching-relay-circuit/

Here's the essence of it:
IMG_4496.JPG

Since you need a constant voltage source available, but need an external source to be able to trigger it (instead of the "push to make" switch on the diagram,) you may need to go a little beyond this - I'm not sure if having constant supply voltage available to this relay setup would back-feed voltage into the output of your IDC in an undesirable way. Maybe that doesn't matter, but without knowing your system, I'm just not sure.
 
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