OT: UPS for FreeBSD
rwmaillists at googlemail.com
Mon Dec 1 15:28:52 UTC 2014
On Sun, 30 Nov 2014 21:16:13 -0800
Darren Pilgrim wrote:
> 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.
Defining the DC component of a pure rectified sine wave as the output DC
voltage isn't really useful, unless the design smooths to that value.
> >> 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
When you connect a smoothing capacitor it charges to the
peak rectified voltage; when the rectified voltage move off-peak, the
capacitor can't discharge through the diodes.
Most linear regulators operate with a smoothing capacitor kept close
to the peak voltage to minimise the headroom needed by the regulator,
which in turn reduces the heat generated. This is the most substantial
reason why a square wave is problematic, it can't have both the correct
RMS and peak voltage.
In practice a computer power-supply would have active circuitry
between the rectifier and the capacitor for power-factor correction. In
that case the voltage on the capacitor is arbitrary.
> > 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.
Bridge rectifiers are commonly used with a grounded centre-tap to
provide positive and negative rails. In that case the smoothed voltage
on the bridge output is ~ 2*sqrt(2)*Vrms, but only because Vrms is
specified per secondary winding.
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