[Noisebridge-discuss] Fwd: [BerkTIPGlobal] Fwd: Re: [HH] UPS Tutorial & Teardown

giovanni_re john_re at fastmail.us
Tue Aug 13 18:11:42 UTC 2013


----- Original message -----
From: Tom Metro <tmetro+hhacking at gmail.com>
To: hardwarehacking at blu.org
Subject: Re: [HH] UPS Tutorial & Teardown
Date: Tue, 13 Aug 2013 01:45:16 -0400

Peter (peabo) Olson wrote:
> 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.

Yes, but I didn't want to spoil the big reveal. :-)


> 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.

It seems a bit overly convenient that the protection diodes happen to
have adequate current rating to work as a charging power supply. But I
guess the specs align with the requirements. (Or they're using custom
MOSFETs in their H-bridge.)


> Well it is a little more complicated...

Yeah, I would have like to see him take the next step and figure out how
they keep the forward and reverse currents isolated on the DC side, and
how they regulate the charge current.


This perhaps explains why the control electronics in most UPSs behave
strangely (random blinking lights and unresponsive buttons) when the
batteries fail. They lack an AC power supply, and must depend on the
battery voltage to run the electronics, so if a failing battery is
drawing a lot of current from the charge circuit, the electronics see an
under voltage. (Not a completely satisfying explanation, because
butteries often fail (have no capacity) without drawing high charge
current.)

 -Tom

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