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From: <bp@www.zefox.net>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: Backup power supply
Date: Sat, 16 Mar 2024 17:35:25 -0000 (UTC)
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Mike Scott <usenet.16@scottsonline.org.uk.invalid> wrote:
> 
> Seems the rechargeable bit is the issue. For my part, and purely to use 
> as a short-term standby for the Pi until mains resumes or the system can 
> shut down cleanly - I'd be happy with something with a bundle of AA 
> cells or similar. I doubt a clean switchover circuit would be difficult 
> to design, but my electronics knowledge is decades out of date :-{

Since I started mucking about with inverters the complexity of line
detection has gradually dawned on me. I started out thinking of a
simple SPDT relay with coil across the line. Line drops, relay
goes to normal contacts, job done. With enough filter caps on 
the Pi to sustain it through a few line cycles maybe that would 
work. For a larger load the caps get big, if you want (as I do)
to back up several different devices (DSL modem, router, switches)
that amounts to a custom DC supply interconnecting all devices.

If you want significant endurance (minutes or hours) capacitors
become impractical. Primary cells in series with a blocking
diode (6 volt battery with one silicon diode in series) would
get you close to 5 volts with the diode drop. Maybe that's 
your ticket. The key is finding the right battery chemistry
to give the voltage you need. Still, it's a handmade gadget.
And, whatever voltage matching device you select must not
drain the primary cells. I think a three-terminal regulator
would cause at least some trouble on that count. On the good
side, you could put it all downstream of the Pi power supply
and skip line detection entirely. That's a big advantage. 
 
When line voltage drops (and returns) it's often noisy, dropping, 
returning and dropping again. Much better if the UPS takes over 
with a hair trigger and waits for a period of quiet before retiring. 
That's where the logic gets intricate.

I puzzled over the custom DC supply versus off-the-shelf AC UPS
approach and settled on the latter. For a single Pi, that no
longer looks so attractive.  

Thanks for reading,

bob prohaska