Which UPS?

Lennart Sorensen lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org
Thu Jun 14 19:04:11 UTC 2012


On Thu, Jun 14, 2012 at 03:02:10PM -0400, Scott Allen wrote:
> 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

The utility would hate such a design.  Only drawing power at the peak
of the AC wave does not make them happy.

-- 
Len Sorensen
--
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





More information about the Legacy mailing list