In sci.electronics.design wrote:
The parts arrived. I tested the 1 farad/RLY-635
circuit and it yields 4 minutes,45 seconds. + or -
measurement error.

Thanks for posting the results! As you noted, the original poster has
moved on, but I might use something like this in a UPS I'm designing.

Basically, I want to switch from AC to battery at the slightest hint of
a problem, but once AC comes back, I want it to have stayed on for some
amount of time before I switch from battery to AC. I'm not too worried
about the exact time, just as long as it meets a minimum. There will be
a big cap in there someplace to handle the switching glitches.

This is for holding up a cable modem and VoIP box during power outages.
I know I could just buy a commercial UPS and plug in the wall-warts, but
it seems wasteful to me to go from 12 V DC to 120 V AC back to 12 V DC.
Besides, for what I pay for a UPS, I can buy a bigger battery and a
couple of support parts that will keep me up much longer than the UPS
will.

Followups to s.e.design.

Matt Roberds