Which UPS?

[D. Hugh Redelmeier]

| From: Scott Allen <mlxxxp-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org>

| It's not a similar DC voltage. The AC input supply rectifies 120VAC to
| 170VDC. The DC input supply (that you're questioning my reasoning
| about) requires 48VDC.

Something that may be confusing to the casual reader:

Normally, when one says 5V DC, one assumes that it is 5V all the time
(within, say, 5%).  Rectified AC is not like that at all.

The 170VDC you are mentioning is really the function abs(sin(t)) and
bounces between zero and 170 volts.

Note that the original 120V AC actually goes up to 170 volts too!
That's because "120V AC" means that the root means square of the
voltage is 120V but the peak is 170V
	120V * sqrt(2) = 169.7... V


Moving on to guesswork (informed by insufficient knowledge)...

I *assume* that 48V DC that was being referred to is "flat".  So
converting rectified 120V AC to this 48V DC is similar to what I think
a modern PC power supply does:

	Under circuit control, feed the input power intermittently
	into a capacitor, in such a way that the capacitor voltage
	remains close to and above the target voltage.  That output
	goes to a regulator to chop it down to that actual target.

The input to the power supply has a wide latitude.  Different kinds
of inputs may affect efficiency.

Anyway, going through a 48V DC intermediate would seem to double
inefficiency: two levels of power supply.  But I'm no expert and the
devil is in the details.  I think 48V DC is just a telecom convention
dating back to the days before transistors.  Is there anything
technical to recommend it now?

| > What is it about that referenced DC supply
| > that makes it so inefficient?

That is a question I'm mildly interested in.  I'd be more interested
if I thought I could do anything with the information :-)

Current good PC power supplies are about 80% efficient or a little
better.  What is the source of inefficiency?  How do vendors work to
improve efficiency?

I think that each power semiconductor that the power passes though drops 
the voltage a bit, eating the corresponding power (voltage drop * 
current).

The regulator eats any over-voltage to produce the nice fixed output 
voltage.  So the closer the input is to the desired output, the less needs 
to be eaten.

| From: Scott Allen <mlxxxp-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org>

| I have five of this type of UPS here at home:
| - APC Personal Powercell 170W
| - APC Back-UPS ES 350
| - APC Back-UPS ES 500
| - APC Smart-UPS 700
| - Belkin 750VA

Note that each of these is rated/named by power output.  Why isn't
energy capacity mentioned?  When I read blurbs (not spec sheets) about
these UPS units, they give no KWh or equivalent rating.  Why is that?

I recently bought a replacement battery for an old UPS and was
surprised to find that it is used in a bunch of different UPS models,
including APC BackUPS 200, 250, 280, 300, 400, 500, 700 CS 350, 500 (I
could not find a good list).  Since I think that only one is used per
unit, I think that each of these has the same energy capacity!

My unit is a Back-UPS ES 500.  The replacement battery is called RBC2.  It 
was $38 at NCIX a month ago but now seems to be 44.25.  That's cheaper 
than a new UPS.
--
The Toronto Linux Users Group.      Meetings: http://gtalug.org/
TLUG requests: Linux topics, No HTML, wrap text below 80 columns
How to UNSUBSCRIBE: http://gtalug.org/wiki/Mailing_lists