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?

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

Crude schematic showing transformer secondary
diodes filter choke and capacitors.

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?

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

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.

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

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?

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

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.

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.

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

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?

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

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.

So, why do you want to use a choke-input filter in the first place?
AFAIK, they are most useful in giving you better output voltage
regulation under varying load than a capacitor input filter. They have
the added advantage that you can get more DC _power_ from a given
transformer by using a choke input filter, because although the output
voltage is lower, the RMS transformer winding current is lowered even
more.

BUT--the voltage regulation advantage is lost if you try this with a
half-wave rectifier circuit, because you cannot maintain constant
enough current in the choke. To get the voltage regulation, the
current in the choke must not drop to zero at any time in the cycle,
and that's not going to happen while maintaining reasonable output
voltage in a half-wave circuit. (There's some limited help if you put
a "catch diode" to keep the voltage across the choke from swinging too
far negative, but that's not enough to get the advantage of the
full-wave circuit.)

In addition, as John says, in the circuit as drawn, the choke is simply
in series with the transformer secondary, so you must reverse the
current in it between half-cycles to get conduction on both
half-cycles. It will not behave anything even close to the way that a
full-wave rectifier feeding a choke input filter will.

Suggest you try a simple Spice (e.g. the free LTSpice from the Linear
Techonolgy website) simulation of this and the normal full-wave
circuit, and look at the huge differences. Note especially what
happens when you vary the DC load on the output.

Cheers,
Tom

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?

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

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.

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

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

In article , 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.

Ken-

This doesn't make sense to me. My recollection of the choke-input filter,
is that it can only be used following a full-wave rectifier. You are
suggesting they be used prior to the rectifier, which is not where a
"filter" is normally placed. Instead, the choke would act as a series
impedance to the AC source.

It seems to me that you can't separate the capacitors from the rectifiers,
or you wouldn't have doubler action. Therefore, capacitor-input is the
only filtering that makes sense for this circuit. Of course you might use
the choke in a Pi configuration between the output and another filter
capacitor.

If you have any success with this approach, it will be from extra voltage
generated by the choke's collapsing magnetic field. This is similar to
how switching regulators work, but without any active regulation.

73, Fred, K4DII

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?

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

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.

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

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.

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.

Fred McKenzie wrote:
In article , 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.


Ken-

This doesn't make sense to me. My recollection of the choke-input filter,
is that it can only be used following a full-wave rectifier. You are
suggesting they be used prior to the rectifier, which is not where a
"filter" is normally placed. Instead, the choke would act as a series
impedance to the AC source.

It seems to me that you can't separate the capacitors from the rectifiers,
or you wouldn't have doubler action. Therefore, capacitor-input is the
only filtering that makes sense for this circuit. Of course you might use
the choke in a Pi configuration between the output and another filter
capacitor.

If you have any success with this approach, it will be from extra voltage
generated by the choke's collapsing magnetic field. This is similar to
how switching regulators work, but without any active regulation.

73, Fred, K4DII

I played around with choke input filtering for this circuit with Spice
and got "continuous inductor current" if I used two highly coupled
inductors, one after each rectifier, and another pair of diodes from
the input side of the chokes to the capacitor common point. However,
this "continuous current" switches back and forth between the two
coupled inductors on alternating half cycles so each end of the
capacitor pair sees current as a half cycle approximately square wave
pulse. So each capacitor charges and discharges with a quite
triangular voltage ripple. But the sum of the two capacitor voltages
is a very pure DC, compared to the no choke version, since the ripples
cancel quite well. However, this reduces the output voltage to only
half of the no choke version, so you might as well have made a full
wave supply, instead of a doubler configuration.

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?

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

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.

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

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.