Jeff said:
I use APC products and we get hit a few times a month - Southern
California area. The ups have never failed to catch a problem. They too
always feed off the battery. So I'm not sure what 0 transfer time might
mean other than that. Perhaps it is a technical term that (neither you
nor) I understand and it's getting in your way of 100's of possible
relevant products.
-- Jeff Barnett
PS For what it's worth, all of my APC UPS are 1300 and 1500 rated in the
RS/XS series.
There are a number of different architectures for UPS devices. The
cheapest (like the one I own), are SPS type, or standby designs. They
don't have zero transfer time. The inverter on the output only runs,
when the power has failed.
An SPS connects the line A.C., directly to the load outlet, using a
relay or other switching device. If the A.C. disappears on the
input to the UPS, it takes the UPS some time to detect the loss of
input. Then, the UPS can switch over to using the internal inverter,
generating A.C. on the output. So the output is switched over to being
driver by the inverter.
During this switchover interval, there is no A.C. available on the output
of the UPS. The transfer time (perhaps 8 milliseconds), is the time that
the connected appliance, will be *without* power.
An ATX power supply, has a fairly large capacitor on its primary side. The
incoming A.C. is converted to D.C. and is stored on the capacitor. If the
A.C. disappears, the ATX supply has a "hold up time" rating, on the order
of 17 milliseconds. The hold up time will last longer, as a function of
how much less than the rated output, you are drawing from the supply.
(This is why the +5VSB output of the supply, can run for 30 seconds, after
the supply is switched off. That is the holdup time, when only +5VSB is
drawing current of an ampere or two.)
Since the ATX supply has a holdup time that is longer than the UPS, then
there is no upset to the computer. Occasionally, you'll find a computer
in an establishment equipped with many UPSes, where the computer resets
during a power failure. In that case, the assumptions don't seem to be
holding up (one of the two devices is out of spec).
Other architectures of UPSs, add other features to the UPS. For example,
the next tier of design, is line interactive. In that case, the objective
is to tune the output voltage, so as to survive mild brown out conditions.
Perhaps a 90V input, can be boosted to 110VAC, and allow a computer to
more successfully continue running. The UPS at that point, still hasn't
switched to battery. If you live in an area with frequent purposeful
brownouts, then a line-interactive is better than a vanilla SPS.
I hardly ever have brownouts here, so this is not a concern for me.
(My voltage is usually higher than it should be. High enough as to be
on the edge of being unacceptable. It has already ruined my stereo.)
Now, if we go back and discuss the SPS, the cheapest solution again,
it doesn't have "line interactive" and does not compensate for voltage.
It may have surge protection, but surge protection is intended for short
duration upsets. If the A.C. line input, has a longer term excursion from
the proper voltage, this can leak through the UPS (because it is
effectively a straight wire) and damage the equipment. My CRT monitor
was damaged in this way.
The double conversion type, is much more expensive. By converting from
AC-DC-AC, the output is filtered, so transients should not be able to
leak through. The transfer time is zero, because no transfer is taking
place. If the input AC fails, the inverter on the output is always
running, always generating heat, and always creating a constant clean
AC output signal. It would take a huge transient (nearby lightning
strike), for an upset to be transferred through the safety ground,
or via some other mechanism (induction to cabling laying on the floor).
As far as I know, there is one other architecture that has zero transfer
time, but is not a true double conversion. So if you want a true double
conversion type, you have to shop carefully. And hope the manufacturer
is honest about their tech details.
So there are some differences between UPSes, and reasons why one is
$50 and another is $1000. It isn't all window dressing, or heavier
battery packs. If I had a home theatre for example, I might be a
bit more interested in something a bit better, than the SPS type
I've got now. But for the single computer that is connected to
the current SPS type, I don't really care if it is blown up.
The SPS type UPS is only there, to handle 1 second outages.
Paul
