Power failure detection

So I've been diving into power failure detection and I though I would share in case anyone had a comment or better idea. My plan is to use a relay like this one used in HVAC applications. The coil is 120v and will be tied in parallel with the inputs of my 24v power supply. The contacts are NO Double Pole Single Throw. My thinking is that on the loss of 120v power the contacts will open, one set of contacts will trigger an input on the control board (a Duet 3) to activate a shutdown config, and the other will open the hot end heater cartridge circuit. The bed heater and enclosure heater will run off of 120v, so the only other major draw on the 24v PSU will be the hot end heater.

I've been reading that the duet has power failure detection, but I would rather try to detect it sooner if I can. The only big issue I see is that the contacts are not rated for a DC load, but I believe the loads are small enough for this not to matter. The heater is a 30W, so I guess at most it would be an amp or two and the contacts are rated for 25. Realistically, I could also use a STPS relay and just have the firmware handle the heater.

The relay will take a while to respond to the loss of power before the contacts open, so I don't think this scheme will work well. There are electronic circuits that will detect loss of power more quickly. Typically they use opto isolators. There is a limit to how fast you can detect loss of power anyway, because the mains voltage drops to zero twice every cycle and you have to wait a little while to see whether it is coming back up again.

RepRapFirmware already turns all heaters off as soon as power failure is detected.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks dc42. I was thinking that might be the case with a relay. I may be understanding this wrong, but the data sheet seems to spec the relay response time to 30ms, and 60hz AC would need something like 16ms to measure at least 2 voltage drops? I may have to order one and do some actual tests under a load to see how long a 24v psu stays running, as I'm sure that varies by machine, and see if the loss of a few milliseconds is even a big deal.

If you are trying to detect loss of mains power earlier than the Duet detects that the VIN voltage is falling, then I think 30ms is too long. It should be possible to detect loss of mains power in 5ms or less.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Fair enough, maybe I'll just leave room for a capacitor when I lay out the electronics if it becomes an issue. I'd like to stay away from a ups if I can. Thanks for you help.