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From: "P E Schoen" <p...@peschoen.com>
Newsgroups: sci.electronics.design
Subject: Re: UPS design (APC Back-UPS 650)
Date: Sat, 31 Mar 2012 18:30:05 -0400
Organization: A noiseless patient Spider
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"Don Y"  wrote in message news:jl7r49$as2$1@speranza.aioe.org...

> Yes.  It's not even a "stepped sine wave" -- think of the primary
> as a center tapped coil with FETs tugging alternately on one
> side, then the other.

> The "difference" between various VA models is the number of
> FETs paralleled for each "side" of the coil.

I suppose a normal load would clean up the waveform to some extent. The=20
"ringing" was at a very low frequency and probably a result of surge=20
suppression components on the 120 VAC side.

> I don't think the serial port does anything besides passing
> *levels* to the monitoring PC.  I.e., the "data" isn't the sort
> of "ASCII text" that you might otherwise believe.

I figured that it simply uses one of the RTS, CTS, DTR, or DSR lines as =
a=20
power fail indicator, and the PC software does all the rest. It probably =

doesn't have the ability to read the battery level or load or anything =
like=20
that. Just a signal to save open files and perform an orderly shut-down =
of=20
the computer.

> These are typically "consumer kit" -- even the models marketed
> for SOHO use.  I inherit probably two a month from folks (usually
> 1000-1500 VA models) suffering from bad batteries ("Gee, it
> costs as much to replace the batteries as I paid for the darn
> thing to begin with!"  or "Yikes!  I just replaced the batteries
> a year or two ago -- and we haven't had any power outages
> in that time...") or blown FETs (I have three XS/RS 1200's
> with that problem, now).

> For most folks, UPS's *sound* like a great idea!  Likewise,
> various "(data) backup devices".  In practice, the novelty
> usually wears off pretty quick.  :>  I have 6 deployed here,
> currently, and they tend to be more of "outlet/receptacle
> multipliers" than real UPS's.  I suspect they have alarmed
> more times from battery failure than from actual power
> outages.  But, our power tends to be pretty reliable
> (below grade service seems to make a difference -- esp
> in the warm/dry environment).  So, the most
> annoying thing is brief "flickers" when something, somewhere
> might be switching to cover a changing distribution load
> (most of my machines will survive a 1 second "outage"
> without even requiring a UPS).

> I previously had a 5KW unit that proved impractical to use
> (the size of a dishwasher).  And, dangerous to work on (the
> *120* VDC battery).  Routing the conditioned power to various
> loads meant a long "extension cord" (or, a dedicated circuit!).

> Some of the more "professional" units offer a better mix of
> features, output waveform quality, etc.  (I was offered a
> large unit -- the battery bank being effectively a 40U rack
> -- but was smart enough to realize I could never afford
> to keep replacing the slew of batteries it needed!)

> I've also seen smaller units used as DOOR STOPS to take
> advantage of their size and weight (the power cord serves
> as a convenient "handle" to drag it to where you want/need it!)

That's probably all it's good for, IMHO. The surge protection components =

might be useful, but more conveniently located in a power strip. Seems a =

shame to throw away a functioning unit, but I use a laptop for all my=20
computing needs, and it has its own UPS in a fully charged battery that=20
allows about an hour of continued use after a power failure (and the =
screen=20
provides a nice emergency light so I can find my flashlight).

I went to the Timonium Hamfest today, hoping to find some inexpensive =
SLA=20
batteries, but didn't find any. I have a 17 A-H battery (date code =
1999?) I=20
bought there at least 5 years ago. I had it in an old garden tractor for =

several years, and I was amazed that it still had juice after being =
outside=20
and only recharged once a year or so. It still seems to be going strong.

My project is a small electric tractor that has a three phase motor, and =
I=20
plan to use a couple of 12V batteries and inverters to generate about =
300=20
VDC which I'll use with a 2 HP VF motor control. I know the 17 A-H SLA =
is=20
really too small, but a pair of them should allow maybe 10 minutes run =
time=20
at 1 HP average. The best price I found on-line was $33 for 18 A-H SLA,=20
while I can get a regular lead-acid garden tractor starting battery for=20
about $30 with 200 CCA (probably 20-30 A-H), and a 100 A-H deep cycle=20
battery for $85.

Thanks,

Paul=20