[HH] UPS Tutorial & Teardown

[Peter (peabo) Olson]

On August 12, 2013 at 2:28 AM Tom Metro <tmetro+hhacking at gmail.com> wrote:
> Here's a video that describes the 3 major architectures for
> uninterruptable power supplies, then tears apart an APC rack mount UPS
> to match up the real components to the theoretical block digram:
>
> http://www.youtube.com/watch?v=Fj7e3WGUKO8&feature=em-subs_digest
>
> I went into it figuring I already knew how a UPS worked, but still
> learned some things. (AVR UPSs really just use an auto transformer and a
> couple of relays to implement voltage boosting/reduction? That's
> surprisingly granular.)

The mind-bending moment occurs towards the end, when he tries to find the
circuit which charges the backup battery.  It seemingly is not there.

Imagine an H-bridge driving the primary of a transformer that is producing AC
out when the UPS has no line input.  Cool.  Imagine the same H-bridge when line
input is available.  It is sort of backward.  Enough that the protective diodes
of the MOSFETs are conducting.  What do you have then? A bridge rectifier
supplying current back to the battery, which coincidentally has some hefty
capacitors to filter the voltage and help charge the battery.

Ingenious.  Well it is a little more complicated, but that is the basic nifty
insight :-)

peabo