INTL 3:770/1 3:770/3   
   REPLYADDR bp@www.zefox.net   
   REPLYTO 3:770/3.0 UUCP   
   MSGID:  609bfbf3   
   REPLY:  58c2c92d   
   PID: SoupGate-Win32 v1.05   
   Mike Scott  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 to design, but my electronics knowledge is decades out of date :-{   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   
      
   --- SoupGate-Win32 v1.05   
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