Which UPS?

[Scott Allen]

James responded to this while I was composing this message, but I'll continue.

On 14 June 2012 14:06, D. Hugh Redelmeier <hugh-pmF8o41NoarQT0dZR+AlfA at public.gmane.org> wrote:
> If the rectified DC is flat (filtered by capacitors) would it not be
> closer to 120V than 170V?  Would the filters not do some kind of
> averaging (technically: mean function)?  The RMS of the
> simply-rectified voltage would still be 120 (the quadratic mean).

No. You will get 170VDC peak with the voltage dipping slightly on each
cycle (called ripple).

I'll try to explain it simply (but may not succeed).

A capacitor has a very low resistance when accepting charge current
(and when discharging that current into a load). If you can provide
the current, it can be charged up to a given voltage almost instantly.
An AC line can provide a comparatively large amount of current.

A capacitor has very low leakage. Once charged to a given voltage it
will maintain that voltage until something (a load) pulls current from
it.

Let's look at the case where there's no load:
The AC is rectified by a diode or diode bridge and fed to a capacitor.
On the first AC cycle, the capacitor's low resistance allows it to
draw as much current as required to follow the voltage and be charged
up to the peak of 170V. The diode(s) prevent any current from flowing
out of the capacitor back into the line when the voltage drops below
170V on the downward side of the cycle. With no load on the capacitor
the voltage on it just sits at 170V. (The voltage will drop ever so
slightly due to leakage within the capacitor but will be brought back
up to 170V on the peak of the next cycle.)

Now, if we add a small load to the capacitor the voltage will drop as
current is drawn from it. However if the load is small compared to the
size of the capacitor, the voltage will only drop slowly by a small
amount over the time of an AC cycle. The capacitor will then be fully
charged back to 170V at the peak of the next AC cycle.

The ratio of the size of the capacitor to the size of the load
determines the amount of ripple voltage (how low the voltage drops
below peak). Designers will make sure the capacitor is sufficiently
large to assure that the ripple voltage will never fall below the
minimum that the load can tolerate.

Note that the current on the line will not look anything like a sine
wave. Most of the current will only flow when charging the capacitor
back up from the low voltage of the ripple to the peak voltage

-- 
Scott
--
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