OT: UPS for FreeBSD

[Perry Hutchison]

Erich Dollansky <erichsfreebsdlist at alogt.com> wrote:
> As switching voltage regulators use a few hundred volts,
> it is a bit difficult to match this with a battery.

Depends on how many cells one is willing to string together
in series.  IIUC the battery in a Toyota Prius supplies a
few hundred volts.

However, there is no intrinsic reason why a switching regulator
_has to_ take a few hundred volts as input.  Those under discussion
here are set up that way because that is what is easily and cheaply
available:

120VAC ==> full-wave rectifier ==> ~300 VDC ==> switching regulator
                                                       |   |
                                              12VDC <--+   |
                                                           |
                                               5VDC <------+

(That is typical for North America; in Europe the AC would be
~240V and the DC therefore ~600V.)

To integrate a UPS with the PSU, one would instead build something
along the lines of:

          multitapped
120VAC ==> step-down ==> 6VAC ==> full-wave ==> ~15VDC ==> battery1
          transformer             rectifier
                  |
                  +---> 3VAC ==> full-wave ==> ~7.5VDC ==> battery2
                                 rectifier

battery1 ==> regulator ==> 12VDC

battery2 ==> regulator ==> 5VDC

IOW the computer runs off the batteries, which are charged by the
stepped-down but unregulated (or minimally-regulated) line voltage.

There is nothing the least bit new about this -- the telephone
industry was doing it in the mid 20th century.  They installed
enough battery capacity for their system to run on battery power
alone for however long it would take to get their standby generators
started up (or for at least a few days in smaller offices that did
not have standby generators).