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.

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?

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.

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

K7ITM wrote:
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

I think I understand what you are saying here, but even with
a full wave rectifier doesn't the current through the choke drop
to zero (though only for a brief instant) between the two phases
of rectification when the diodes switch roles? And since there
isn't a capacitor before the chokes the voltage at the input
to the filter would drop to zero, unlike with a capacitor input
filter.

Also, with either type of rectifier (FW or HW) is shouldn't matter
which leg the choke is placed in, as Kirkoff's law is satified
either way.

No, with sufficient load current, the choke current never goes to zero.
See the formula for the minimum choke inductance versus supply voltage
and current...the usual formula also assumes 60Hz---120Hz ripple.
Remember: V=L*di/dt. The output DC voltage is the average of the
input voltage to the choke, less any I*R drop in the choke. It MUST
be, or the current in the choke would change until it was. So the
output voltage is (nominally) 0.9* the RMS input voltage, or
0.9/sqrt(2) times the peak voltage, assuming the choke input voltage
tracks the full-wave rectified sine (in other words, the absolute value
of the sine). When the input voltage to the choke is the output
voltage, the choke current increases. When it is less, the choke
current decreases. The average choke current must be the average load
current, or electrons would accumulate somewhere. If the inductance is
large enough, and the cycles come fast enough, then the change in
current is less than enough to make the current go to zero.

It's a bit of calculating to do it for a sine wave. Just think of this
example: a square wave that sits at zero for one second, then at 2
volts for one second, then repeats. Feed it to a choke, say 2 henries.
Output of the choke to a mongo capacitor, so the output voltage
doesn't change significantly. Put a 1 amp load current on it. The
output voltage must be one volt, since that's the average of the input.
So half the time the choke has one volt across it in one direction,
and half the time it has one volt in the other direction. One volt
divided by two henries is half an amp per second. Since the average
current is one amp, the current must swing between 0.75 amps and 1.25
amps. It never goes to zero, or even close. But drop the load current
to 0.25 amps, and the choke current goes just to zero when the input is
at the end of the low period. A bit lower load current, and the choke
current would go negative (or to zero if there's a diode keeping it
going one direction only). That's why a choke input filter looses its
good regulation if the load current gets too small.

In the full wave rectifier, the choke current (if it doesn't drop to
zero) will force the diode output voltage to be one diode drop below
ground, PLUS the I*R drop in the transformer winding, when the voltage
across the outside of the secondary is zero. At that point, both
diodes will be conducting equally, assuming they are matched, and half
the choke current will come from each half of the transformer
secondary.

Hope that helps!

Cheers,
Tom
(off to Holden for a week...no internet there, so you're in John's
capable hands on this one!)

To illustrate this wi

In article , "wa2mze(spamless)"
wrote:

I think I understand what you are saying here, but even with
a full wave rectifier doesn't the current through the choke drop
to zero (though only for a brief instant) between the two phases
of rectification when the diodes switch roles? And since there
isn't a capacitor before the chokes the voltage at the input
to the filter would drop to zero, unlike with a capacitor input
filter.

Also, with either type of rectifier (FW or HW) is shouldn't matter
which leg the choke is placed in, as Kirkoff's law is satified
either way.

Spamless-

Yes, it wouldn't matter what leg the choke is in as long as it is on the
output side of the rectifier (assuming full wave).

For Half Wave, you must also consider the voltage across the choke.
Voltage is L times di/dt where di/dt is very high at the moment the diode
stops conducting. This is why a diode is often placed across a solenoid
or relay coil, to prevent a high voltage pulse across the switching
device. As a side-effect, relay drop-out is slow since current continues
flowing as the magnetic field is discharged.

With the relay analogy in mind, perhaps there could be some advantage if a
diode were placed across the choke of a choke input filter fed with a half
wave rectifier. It would be connected with cathode towards the cathode
end of the rectifier, and would allow choke current to continue flowing
during the off-portion of the rectifier's conduction cycle.

73, Fred, K4DII