Re: back-up power supply

From: Walter Roberson (roberson_at_ibd.nrc-cnrc.gc.ca)
Date: 04/01/04 

Date: 1 Apr 2004 02:12:19 GMT

In article <ca3e516b.0403301642.3079c0d4@posting.google.com>,
curious <heyimjustcurious@yahoo.com> wrote:
:I was thinking to get a back-up power supply for my PC and saw a book
:about it (at Fry's). I read a bit to know there are mainly 3 types.

That is correct, there are three main types (there's a fourth type
that gets used less often.)

:I would appreciate any suggestion on which type and in fact which
:brand/model to buy as I am pressed with time to do research on that.

You first must define which dangers you need to protect your PC against
and for how long.

The least expensive type of UPS is 'passive'. The mains electricity
flows directly through it, getting filtered a -bit-. When the power
drops, the equivilent of a selinoid acts and puts the battery into
the circuit; the battery goes back out again when the power returns.
It takes time for the battery to go online. As a generality, most
modern desktop PCs have capacitors big enough to ride over the time
required to switch in the battery... provided the UPS isn't being
overloaded, provided a bunch of other things don't go wrong, and provided
you are actually using a desktop machine with a power supply with
meaningfully big capacitors. Could be a problem for laptops, in other words.
Could be a problem if you plug your monitor and printer and speakers and
everything else into your "really only big enough for the PC itself" UPS.
[A good-sized monitor uses 2/3 of the power of the PC itself, so size
the UPS appropriately!!!]

One issue with passive UPSs is that the electricity is not flowing
through the batteries, and so the batteries tend to drain away. And if
they are allowed to drain away for significant amounts of time, then
if they are the typical lead-acid batteries then they lose their capacity
to store electricity. If you buy a cheap UPS and not much exciting
happens with the electricity source, then in 1-3 years you are almost
certain to wake up to the UPS beeping every drives-me-insane number
of seconds to tell you your batteries are dead beyond repair and must
be replaced completely, at significant cost.

Passive UPSs are usually NOT rated for significant surge filtering.
The components they use are usually far too small and too slow to
stop a big surge. The lightning just crosses the miniscule air-gap
and keeps going.

   Let me emphasize: If surge protection is a high priority in selecting
   your UPS, then do NOT get a typical 'passive' UPS!!!

The second major kind of UPS is 'line-interactive'. The electricity
does keep the batteries trickle-powered for an 'line-interactive' UPS,
and there's a single-acting convertor that helps make for cleaner power
than the 'passive' UPS case. 'Line-interactive' UPS's kick into action
-much- faster than 'passive' UPS's, but [as I recall] possibly with a
bit of a surge as they do so. I did read about 'line-interactive'
UPS's, but I must admit that it was not clear to me why one would not
just continue on to the next and better kind, unless it be for budget
reasons. But it's better to have a line-interactive than a passive,
if you can afford the line-interactive.

The third major kind of UPS is sometimes called 'double-conversion'
but more commonly called 'online' (or "true online"). They have
a double convertor -- the AC is rectified, put into the battery,
and the battery power is rectified going out again. There's always
power going through the batteries, which can allow them to last
a lot longer than the 'passive', particularily if there is smart
recharging. The surge resistance and other kinds of filtering on
online UPSs is usually orders of magnitude better than for passive UPS.
On the other hand, online UPS's are often orders of magnitude more
expensive.

If you have a 'data centre' then don't consider less than a
line-interactive facility. And make sure you allow lots of spare
supply for the expansion that is going to happen (and for the
fact that the vendors themselves often miscalculate the necessary
loads); and make sure your system is going to keep your equipment
up for as long as is going to be reasonably needed. If you are
seriously thinking in terms of kilowatts of backups, or in terms of
keeping equipment running for hours, then Research Research Research!

If you have equipment that doesn't run at 108-120V, or which doesn't
take a standard US/Canadian 15 amp three-prong plug, then check all
the specs for all of the equipment *several* times, and then have
someone else check it twice, and then read the specs backwards just in
case you missed something in other cross-checks -- because the differences
between various models can be very hard to find and can end up being
really obscure. For example, we were forced to disqualify one major
vendor just because they were unable to supply a *non*-locking 20-amp
connector in one of their distribution panels -- they just kept quoting
the *locking* 20-amp connector every time we asked, and we had to push
the issue with them firmly to find out that it wasn't just a careless typo:
that they just couldn't provide that one kind of connector we needed.

To the best of my knowledge, the three UPS manufacturers that have the
largest North American market share, are APC, Powerware, and Liebert.

APC does a -lot- of mass-market UPS's, and also has some very quality
products suitable for E911 centres, operating rooms, and such --
places where you want redundancy on your redundancy. My observations
would tend to lead me to believe that there are multiple divisions
within APC, and that the "put redundancy on redundancy" engineers
are not actively involved in planning the mass-market UPSs.

Powerware has a noticably smaller product line than APC, and,
their prices sometimes appear suspiciously inexpensive compared to APC's
prices. If you compare Powerware side by side with APC, you may end
up with a really nagging feeling that you are missing something important
as it can be very difficult to find significant differences... if they do
the same thing, there must be a catch behind them behing so much less
expensive, right? The differences -are- there, they just take time to
dig out: they are in matters such as whether the UPS has a 12 year
MTBF or a 20 year MTBF (Mean time between failure), and whether the
[hot swappable] logic boards have dual control circuits in case of failure
or just single control circuits. The fine fine details of the APC specs
in their high end systems do come out better, so if you need to
plan in the once-in-50-year scenarios then look very very hard at what
you get. But if you only need once-in-10-year quality, then powerware
might prove very interesting. And if you happen to need true online
UPS's in the fractional-kilowatt playing field (from 700 watts up) then
powerware makes nearly the only ones I could find.

Liebert... well, Liebert probably doesn't want your business. Not unless
you are building a new building or doing major retrofits. If you're
in the game of worrying about how to cool and make fire-safe the new
IBM mainframe or supercomputer you're putting in that's going to take
half a warehouse floor and punt you a few levels up in the corporate-
electricity bulk-discount rates, then you'd better talk to Liebert.
Perhaps think of them as being the equivilent of truck dealers...

Just as long as you also remember to think of the $20 UPS you see in
the local electronics store as not being much better than a toy
radio-controlled model police-car when it comes to really *protecting*
your PC against serious systemic electricity problems. 

-- 
Positrons can be described as electrons traveling backwards in time.
Certainly many Usenet arguments about the past become clearer when they
are re-interpreted as uncertainty about the future.
   -- Walter Roberson