OT: UPS for FreeBSD

Darren Pilgrim list_freebsd at bluerosetech.com
Mon Dec 1 05:16:26 UTC 2014

On 11/30/2014 1:03 PM, RW wrote:
> On Sun, 30 Nov 2014 11:31:31 -0800 Darren Pilgrim wrote:
>> An unloaded, ideal, single-phase FWR produces Vdc = 0.900Vrms.  The
>> factor is 2sqrt(2)/pi, to be precise.  A half-wave rectifier produces
>> half that.
> That's the mean voltage, which is of limited relevance.

The mean voltage is the output DC voltage.

>>   From where did you get your figures?
> He's referring to a rectifier charging a smoothing capacitor; the
> voltage will rise to sqrt(2)*Vrms.

No, the voltage across the capacitor will rise to 0.900*Vrms.  Look at 
it this way:

The output of the diodes is a time-varying (AC) signal with a DC offset. 
  The cap provides a low-impedance path for the AC signal, turning the 
voltage cycle into a current cycle that accumulates a charge in the cap. 
  Once the cap is fully charged, it fully negates the AC component and 
you're left with just the DC offset of 0.900*Vrms.

> He's added on an unnecessary  factor
> of two - possibly because he's mixing it up with a bridge-rectifier in a
> centre-tap configuration.

A bridge rectifier doesn't use a center-tapped transformer.  You 
wouldn't see 2*Vpeak in a two-diode rectifier either.  The voltage 
between the center tap and either diode tap is at most Vpeak.

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