Will Dyson on 9 Jun 2009 15:46:09 -0700

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Re: [PLUG] [Slightly OT] UPS recommendations

On Mon, Jun 8, 2009 at 11:58 PM, Paul L. Snyder<plsnyder@drexel.edu> wrote:
> Just tripped the circuit breaker...the second time this summer that the
> combined load of the printer and the A/C have pushed it over the edge.


> Any suggestions for a where to start when shopping for a UPS?

Aside from the basic considerations of inverter power (the VA rating)
and battery capacity, there are a few qualitative differences between
UPS models.

One difference is if the UPS is a "standby" or "online interactive"
type. This refers to how the inverter is hooked to the output.

In the standby type, the mains waveform is normally passed directly to
the output (well, hopefully through the surge suppressor circuit). The
UPS monitors the mains waveform and switches the output over to
batteries + inverter if things go outside its "normal" parameters.
This switchover takes a few milliseconds, in which the output is not

In the online interactive type, the inverter is always wired into the
output, and can be used to supplement the mains power in a brown-out
or reduce an over-voltage. Because this uses less energy from the
battery than fully powering the output, this type can have much
tighter tolerances for when to kick in. It also means that there is
much less of a gap when going to battery power.

While the online interactive type is nice for expensive and delicate
equipment, it is mostly wasted on a computer's PSU. The PSU has its
own internal capacitors that can ride out the standby's switch to
battery power. It also tolerates under and over-voltage pretty darn

The other thing that more expensive UPSs have is a serial port
interface that allows you to read and set values in the UPS's
firmware. Things like the threshold voltages for going to battery,
current battery charge, power draw history, you name it. This is
generally termed a "Smart UPS", although some units without this label
have a serial port with some basic functions.

Oh, and replaceable batteries. Lead-acid gel cells have a finite
lifetime, and the charging circuits in most UPSs are not very gentle.
This is generally a feature of all but the smallest UPSs. Ones without
it are really thow-away units.

Of these three items, I think replaceable batteries is the only thing
worth spending extra cash on.

I like the Trip-Lite brand of UPSs, but I've owned a number of APC
units as well.

Will Dyson
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