John Popelish wrote:
Ken Scharf wrote:

John Popelish wrote:

Wes Stewart wrote:


On Fri, 18 Nov 2005 23:06:03 -0500, John Popelish
wrote:



Ken Scharf wrote:


I was looking at some power supply circuits for
tube linears and was thinking about the full wave
voltage doubler. This is basicly two half wave
rectifiers in series. Now I could build this
circuit with a choke input filter for each half
wave rectifier of the voltage doubler, and I could
put the chokes in the lead without the rectifier.
In this case I could use one choke for both halfs
of the voltage doubler. The output should then
be about .9 * rms input voltage * 2
or 1.8 times the rms voltage of the transformer.

Has anybody ever tried this?

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Crude schematic showing transformer secondary
diodes filter choke and capacitors.



If the choke is directly in series with the transformer, it will have
to pass AC, and that won't provide normal choke input filtering
(which steadies the DC current after the rectifier), but just puts an
impedance between the transformer and the doubler.




All chokes are in series with the transformer and pass some AC
component. If they only passed DC we would need them.



I was using DC in the "unidirectional current" sense, not the "having no
AC components" sense.


Without giving this too much (likely enought) thought I think this
will fail because without loads across -each- filter cap, the critical
inductance will not be obtained.



Regardless of the loads across the caps, this inductor cannot ever
achieve critical inductance, since that is the inductance that keeps the
current reaching zero, each half cycle. In this circuit, the inductor
precedes the rectifiers, so its current must pass through zero twice per
cycle, regardless of the capacitor load.

You could also put it in series with the primary, instead, and achieve
the same effect (with the proper scaling to account for the turns
ratio).


There may be a way to incorporate an inductor into a doubler, but I
don't think this is it.



Why you all may be right, what you are failing to see is that
the choke is simply in the negative leg of the positive half
wave rectifier, and in the positive leg of the negative half
wave rectifier, and both rectifier outputs are in series.

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) |
) ---
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) |
---^^^^^^^----

This is a half wave rectifier with a choke input filter with the
choke in the negative end. Will this work?


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.

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


Now connect
this circuit in series with negative output half wave and you
notice you have two chokes in parallel. Yes you do need a
bleeder resistor or minimal load to satisify the choke current
requirement, I simply didn't draw this, the resting current
of a class AB1 linear would satisfy that.

Now what am I failing to see?


That there is a second current path through the inductor that involves
the other rectifier. So AC is applied to the inductor, instead of
unidirectional voltage.