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From: <bp@www.zefox.net>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: Backup power supply
Date: Sat, 16 Mar 2024 17:56:30 -0000 (UTC)
Organization: A noiseless patient Spider
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bp@www.zefox.net wrote:
> side, you could put it all downstream of the Pi power supply
> and skip line detection entirely. That's a big advantage. 
>  
On second thought, this is probably wrong. If one connects
to a Pi a 5 volt supply in parallel with a blocking diode
and a 6 volt battery, the diode is still forward biased 
and will drain the battery eventually. Some kind of switch
is needed, controlled by line detection.

Maybe a three terminal regulator set for 5 volts out powered
by a 9 (or higher) volt battery pack could be substituted. 
Still, one has to consider the idle draw of the regulator.
TI's LM340 takes 6 mA roughly. Still, that's a smallish
supply for a Pi. You'll likely want something larger. 

Apologies for the red herring!

bob prohaska