I have a DVR that I do not want to reset when AC Mains goes down. The DVR is normally connected to a 12vDC Switching PS through a relay that maintains this connection while the AC Mains are active. If the AC Mains go down, then the relay coil loses power and the DVR 12v input line is automatically switched to 12 Battery supply via the relay (NC contacts). Is there a way to maintain the 12vDC during the time that the relay contacts switch from NO to NC. DVR current requirements are about 2A at 240vAC.
What's a DVR? My suggestion is to adjust the Switching Power Supply, to be a smart Battery Charger, and simply run the system on a battery under charge, all of the time. If it is a lead acid battery, simply imitate an automotive system. Throw away the relay.