John Popelish wrote:
wa2mze(spamless) wrote:

John Popelish wrote:


Not at all well, because you have provided no path for the inductor
current when the voltage from the transformer tires to reverse bias the
diode. So the inductor will keep the diode conducting as the voltage
reverses. This is not at all the way a choke input filter acts with a
full wave rectifier. I am quite sure you have never seen a choke input
filter in a half wave supply, for this reason.


I guess I can't recall seeing a half wave rectifier circuit using
a choke input filter, but I thought that was because half wave
circuits are usually used in low voltage circuits where a choke
input filter would not have any advantage anyway.


The advantages of a choke input filter (lower RMS transformer current
per amp of DC output, much lower high frequencies in the ripple, less
voltage sag with increase in load current, etc.) are not directly
related to the output voltage. The concept of a choke input filter is
that the current is continuous through the inductor, and so, into the
capacitor. A capacitor input filter charges the cap during brief pulses
at the line peaks, instead, producing a higher RMS transformer current
and higher harmonic ripple components, but also higher peak output voltage.

However in a full wave circuit how is there an alternate path?
The center tapped transformer simply provides two ac excitations
to two rectifiers 180 degress out of phase.


The inductor current switches from one rectifier to the other as the
transformer voltage goes through zero. At the moment the transformer
voltage is zero, the inductor draws current through both rectifiers,
producing an input voltage to the inductor 1 diode plus transformer
resistance drop below the center tap voltage.

This allows only
one rectifier to conduct at a time. True, there is a more or less
constant excitation to the choke, but there is NO reverse path as
the diodes still only allow conduction in one direction.


I suggest you follow those currents through the inductor for a full
cycle. Since the inductor is in series with the secondary, if the
secondary conducts in both directions (alternating which diode is
conducting) then the inductor must also conduct in both directions.

Either way the choke sees a DC current, not an AC one (minus the
ripple, which a half sinewave imposed on a dc current).


The diodes are turned opposite ways, so one conducts DC one way, and one
conducts DC the other way. Both those currents pass alternately through
the same inductor.
I tried a mental exercise, I redrew the voltage doubler adding another
winding to the power transformer to provide output 180 degress out of
phase and added two more diodes so I now had each capacitor feed by
both rectified phases. The result, is of course, a full wave bridge
rectifier, but with a center tap of the transformer coupled to the
junction of the two filter capacitors. This is similar to the
dual voltage power supplies so often seen in the ARRL handbooks from
the 60's and 70's for tube transmitters. I suppose a choke could
be placed in the lead from the anodes of one pair of diodes to ground,
so it would be commond to both outputs and the lead from the transformer
centertap isn't needed.