Jack// ani said:

Just two more questions, John

1. Reactance offered by a capacitive load is always higher that a

resistive load, then why should it extract more current?

Reactance is a linear concept that applies to sine waves. To get your

mind around a capacitive input rectifier filter, you have to think in

the more general differential description of capacitance. I=C*(dv/dt)

current equals the capacitance times the time rate of change of

voltage across the capacitor.

As long as the capacitor voltage is equal to or greater than the

transformer voltage, the rectifier isolates the two. But the moment

the transformer wave rises above the capacitor voltage, the rectifier

is essentially a short circuit, and the voltage on the capacitor must

rise as fast as the transformer wave is rising, regardless of how much

current that takes. So the current into a capacitive filter is narrow

sort of half sine wave pulses that occur on the part of the

transformer voltage wave just before the peak voltage. Since the

transformer windings are heated by the RMS current, a pulse waveform

like this has a much higher RMS value than the average of the current

in those pulses. This is what the 'squared' part of the RMS does. It

is not unusual to have to double (or more) the transformer RMS current

rating relative to the DC output average current when using a

capacitor input rectifier filter to take care of this higher winding

RMS current. The exact ratio depends on how much leakage reactance

there is between primary and secondary windings that tends to spread

out the charging pulses, by sagging the waveform a bit while the cap

is charging, lowering the slope a bit. If you look at the transformer

waveform with a scope, you can see the flattened spot on the wave

where the cap is charging just before peak voltage.

2. Why do capacitor charges to peak ac voltage if placed across a

bridge rectifier? Without a cap i measured 11v, and it increased to

16v, after hooking a cap?

The diodes act as check valves, pumping the cap up all the way to the

transformer waveform peak voltage and then turning off, leaving that

voltage trapped in the capacitor. A resistor load on the rectifier

keeps it on the whole waveform so the resistor voltage is the same as

the transformer waveform (except for the inversion of one half). your

meter reads the average of the rectified transformer waveform instead

of the peak value.