Which UPS?

[James Knott]

D. Hugh Redelmeier wrote:
> | I think both James and I were assuming that the 170VDC was also
> | "flat", being filtered with capacitors so that it was a constant
> | 170VDC with a little bit of ripple voltage (but unregulated).
>
> I think it is an interesting point.  Thanks.  Remember: I don't really
> know much about this.
>
> 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).
>
> (<http://en.wikipedia.org/wiki/Root_mean_square>  explains RMS.)
>
> My intuition: you can put a diode between the raw rectified voltage
> and the capacitor.  But if you keep the capacitor at 170V, there are
> only two instants in the cycle where current can flow into the
> capacitor.  If the capacitor is at 120V, current can flow in in two large
> portions of the cycle.  Without any diode, I imagine the charge in the
> capacitor being some kind of average.
>
> I emphasize again: I don't know what I'm talking about and would
> welcome corrections.

It's not quite that simple.  Depending on load, you may have a lot or a 
small amount of ripple, depending on how fast the capacitor is 
discharged.  If you were to look at it with a scope, you'd see the 
waveform peak and then discharge to somepoint, before charging again.  
Without any load, voltage would settle at the peak input voltage  Also, 
with switching power supplies, filtering that first stage is not that 
critical, as the variable output is easily handled by the following stages.
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