Steve Nosko wrote:
Some real brain fodder here. John P. Your Spice model with the coupled
inductors seems to take a divergent turn and I am not sure about this
"coupled" part. Did you try a single inductor.

Yes. First I tried a single inductor in the common leg of the
transformer. I used a 1 Hy inductor and two 220 uF capacitors and a
100 ohm load resistor across the doubler, with a +-10 volt peak sine
source as a transformer secondary. The inductor current settled to a
+24 mA to -24 mA sine wave. The output voltage across the load
resistor was about .825 volts with a 120 Hz ripple that swung from
about .77 to .88 volts. Not much of a doubler. Almost all the
secondary voltage is dropped across the inductor.

I also tried several different configurations with two separate
inductors, one between each diode and capacitor. Only the coupled
inductor did anything like a choke input filter. And such coupled
chokes exist. See type 2-2690 and 2-2691:
http://www.stancor.com/pdfs/pg56.pdf

On to my original thoughts. This really takes me back and requires serious
thought. One thing to keep in mind. An inductor (by virtue of the
magnetic field cutting its own turns) tries to keep whatever current is
flowing, flowing. An inductor will make the voltage across it "do whatever
it takes" to keep that current flowing-- and allow this current to decay
(some say discharge) in what can be considered a normal manner.

The relation between voltage and current is V=L*(dI/dt). The only way
the current can change is if the inductor has voltage across it.

The diode
on the relay coil is a good example. The voltage can rise very high without
the diode, but using this model, you can figure out what the inductor
voltage does when the normally conducting device turns off.

Yes. The current ramps down as determined by the drop across the
inductance. In this case, that is a diode drop added to the resistive
drop of the coil.

That said... I had never studied choke input filters to such a degree...
However, its action must allow the filter cap to charge for a longer time,
thus keeping the average diode current lower...

Make that "the peak diode current lower". The average diode current
has to be equal to the average DC output current, regardless of the
filter.

Does the current through the inductor drop to zero in the normal choke input
filter?

If the inductance is below the critical value, it certainly does. But
most choke input filters are designed to produce continuous (but
varying current) throughout the cycle at minimum current load. But
all choke input filters will go into interrupted current operation at
some minimum load current.

If not, a close look at the current path in the full-wave circuit will show
where the current goes at the cross-over points. Brain full - can't figure
out now. I'd have to model it in Spice and watch things

Very interesting thingh. If the current does drop to zero, then it seems
the single choke would work.

I guess that depends on what you mean by "works". It cannot ever work
as a normal (continuous current) choke input filter.

Then, reading some of the latter posts, I too, wonder why the desire for
choke input.

It has advantages for lower transformer heating and low line harmonic
currents and improved DC voltage regulation (compared to a capacitor
input filter) with changing load currents (as long as the minimum is
above that which maintains continuous current) and low output line
harmonics above the second. If any or all of those are important to
you, it may justify the high weight and cost of an inductor.