II'm angry, I'll beat!!

This circuit is very confusing to me, I don't understand how it is
supposed to work. There seem to be two opposing forces at work in the
saturable transformer. The first is what you point out, that the two
secondary windings are connected so they "buck", so that when the
transformer is saturated by the DC in the primary, the coupling
decreases and the light dims. But at the same time when the transformer
is saturated the inductance also goes down, so even though the two
secondary windings might be not coupled as tightly, their reactance is
also lower which would tend to cause the light to become brighter. If
this is all there is to it the question would be which one of the two
effects is stronger than the other?

But maybe the windings aren't arranged as on an ordinary transformer.
What if we had E-core style laminations with the primary wound on the
center leg and one of the two secondaries wound on each outer leg. The
presence of the center leg would act as a magnetic short and greatly
reduce the coupling between the two secondaries even when the
transformer isn't saturated. When the transformer isn't saturated the
light would be dim because of the high reactance of the two secondaries
in series with the light. When the transformer becomes saturated at low
signal levels the lights would become brighter because of the lowered
reactance of the two secondary halves.

Just another shot from the hip, it would be interesting to know what the
actual disposition of the primary and secondary windings on the the
transformer core is?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

This may not shed light, since the drawing quality is poor, but there is a
connection detail for the dimmer.

I still don't understand the need for 2 caps of such different values and
voltage ratings.

Dave Burson

Dave Burson wrote:

I still don't understand the need for 2 caps of such different values and
voltage ratings.

Has to do with the caps themselves. The large cap is for the 60 cycles
(actually 50 to 133 cycles); while the smaller cap is for higher
frequencies. I didn't see the power supply schematic - but dimes to
donuts it's full wave- so the ripple frequency is double the AC line
(110 to 120). That's likely to be phase-shifted a bit before reaching
this circuit. The lamp runs on line frequency - and in combination with
the ripple (riding on the B+) it'd be easy to generate some rather
complex waveforms - with some pretty high harmonics. The large cap
"eats" the lower frequncies - however - it's construction limits it's
usefulness at higher frequencies - so there is the smaller one to deal
with those. Look at most any power supply - you'll find smaller value
caps by-passing the main filters.

The voltage is insignificant (within reason). I'm sure the 25V was
overkill (likely the highest voltage across the primary was 10V); but
25V was "common" back then (often found as the output tube cathode
bypass cap). Since a .1 is seldom seen in lower than 150V - that value
was probably stocked on the shelf as well. Even today - most .1 - even
in solid state stuff - are seldom less than 50V. Just a matter of what
was already on hand (big quantities of a common value are cheaper than a
few "special" values even if those special values could be smaller).


best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

"John Byrns" wrote in message
...

Interesting stuff... snipped for brevity.

I'm a neophyte to this circuit, but let me posit a thought.

Perhaps the fact the secondarys being wired in bucking fashion has not as
much to do with the function of the circuit itself as to assist in the
prevention of the AC filament voltage from being introduced into the plate
supply for the 1st. detector and 1st. IF (otherwise it seems to me it would
make a rather effective modulation transformer). Also, it seems to me that
I've seen other radios (if not the specific circuitry therein) that had
tuning lamps that dimmed when tuned on station. I can think of a couple
reasons for this. First and foremost, more light is needed when tuning
across the dial than when on station. Also, this would tend to cause the
dial lamp to last a considerably longer time than normal, since when on
station it would be running at a fraction of it's normal voltage.

On the other hand, when DC saturates the secondary, wouldn't that tend to
make the primary look like a direct short?

Brenda Ann wrote:

Perhaps the fact the secondarys being wired in bucking fashion has not as
much to do with the function of the circuit itself as to assist in the
prevention of the AC filament voltage from being introduced into the plate
supply for the 1st. detector and 1st.

No, the phasing of the two winding to be bucking IS the purpose of the
circuit, and how it works.


IF (otherwise it seems to me it would
make a rather effective modulation transformer).

Still would - that's what those capacitors across the primary are for.


On the other hand, when DC saturates the secondary, wouldn't that tend to
make the primary look like a direct short?

There is no dc in the secondary - only the primary (the primary is to
the right in this circuit). Remember TRANSFORMERS by nature are designed
to operate on AC; DC only "messes them up". In this case - this circuit
is intentionally designed to take advantage of that.

Let's walk through it one more time - but this time secondaries first -
then primary.

First - let's think about the two (secondary) windings as a primary and
secondary - after all - what windings are "called" has to due with their
use, nothing more. If you hooked AC directly to one winding and a bulb
directly to the other - the AC would couple from one winding to the
other and light the bulb (leaving aside current density, etc. for the
moment). If the two windings are 1:1 ratio - 6.3V applied to one would
show up as 6.3V on the other. You could wire the bulb either way (i.e.
"turn it around") and the current would flow through either the same
phase as the primary (ignoring simple inductance) - or 180 degrees "out
of phase". Point being - the two windings are the same - both oriented
on the same core - and form a 1:1 ratio between them. As long as the
core works as a transformer - the voltage couple between them will be
(ignoring losses) 1:1 - the only "variable" would be the phasing - as
determined by how the windings are hooked up.

OK - now lets wire the two windings as shown in the schematic: Both
windings on the same core; both having "equal effect" (1:1). Now when
current (attempts) to pass through one winding - it "couples" to the
other winding - which then generates an equal (but because of phasing)
but opposite voltage - which tends to cancel (buck) the voltage applied
to the first winding. Think of it as two batteries. If two batteries
are wired "nose to nose" with a bulb in series with them - what happens
to the bulb? Nothing. The two voltage "buck" each other - and (provided
the batteries have an equal charge) equilibrium is reached - no current
flows. Same thing with our two windings - WHEN the transformer's
ability to couple is un-imparied.

This "wild card" then - is what makes the circuit useful. This
particular "transformer" is a special kind which unlike the "usual"
transformer (which has modifications to help it "ignore" DC in the
windings) - but rather is designed to indeed easily saturate the core
when (sufficient) DC passes through one of it's windings. As the core of
a transformer approaches saturation - it's ability to couple AC between
the windings starts to fail; to the point that a fully saturated core
couples virtually nothing.

So - in this circuit - when the AGC has the RF / IF stages biased way
down (on station) the B+ current draw is low - which passing through the
primary (or control winding if you prefer) has little effect on the
transformer's ability to couple AC between the other windings - in this
case causing one winding to "buck" the other - and the bulb is dim.
When the AGC falls - biasing up the gain (current) of the RF & IF stages
- the current in the primary (or control) winding increases - pushing
the core towards saturation - and the two windings in series with the
bulb loose their coupling, reducing the induced bucking emf - and the
bulb brightens.

As you noted - when there is significant coupling between the two bulb
windings - that will also couple to the primary (or control) winding.
That's where the two capacitors come into play - they bypass any ripple
impressed on that winding back down to the B+ rail - which of course has
it's own filtering to ground.

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

Randy or Sherry Guttery wrote:
Brenda Ann wrote:

Perhaps the fact the secondarys being wired in bucking fashion has not
as much to do with the function of the circuit itself as to assist in
the prevention of the AC filament voltage from being introduced into
the plate supply for the 1st. detector and 1st.

No, the phasing of the two winding to be bucking IS the purpose of the
circuit, and how it works.


IF (otherwise it seems to me it would make a rather effective
modulation transformer).

Still would - that's what those capacitors across the primary are for.


On the other hand, when DC saturates the secondary, wouldn't that tend
to make the primary look like a direct short?

There is no dc in the secondary - only the primary (the primary is to
the right in this circuit). Remember TRANSFORMERS by nature are designed
to operate on AC; DC only "messes them up". In this case - this circuit
is intentionally designed to take advantage of that.

Let's walk through it one more time - but this time secondaries first -
then primary.

First - let's think about the two (secondary) windings as a primary and
secondary - after all - what windings are "called" has to due with their
use, nothing more. If you hooked AC directly to one winding and a bulb
directly to the other - the AC would couple from one winding to the
other and light the bulb (leaving aside current density, etc. for the
moment). If the two windings are 1:1 ratio - 6.3V applied to one would
show up as 6.3V on the other. You could wire the bulb either way (i.e.
"turn it around") and the current would flow through either the same
phase as the primary (ignoring simple inductance) - or 180 degrees "out
of phase". Point being - the two windings are the same - both oriented
on the same core - and form a 1:1 ratio between them. As long as the
core works as a transformer - the voltage couple between them will be
(ignoring losses) 1:1 - the only "variable" would be the phasing - as
determined by how the windings are hooked up.

OK - now lets wire the two windings as shown in the schematic: Both
windings on the same core; both having "equal effect" (1:1). Now when
current (attempts) to pass through one winding - it "couples" to the
other winding - which then generates an equal (but because of phasing)
but opposite voltage - which tends to cancel (buck) the voltage applied
to the first winding. Think of it as two batteries. If two batteries
are wired "nose to nose" with a bulb in series with them - what happens
to the bulb? Nothing. The two voltage "buck" each other - and (provided
the batteries have an equal charge) equilibrium is reached - no current
flows. Same thing with our two windings - WHEN the transformer's
ability to couple is un-imparied.

This "wild card" then - is what makes the circuit useful. This
particular "transformer" is a special kind which unlike the "usual"
transformer (which has modifications to help it "ignore" DC in the
windings) - but rather is designed to indeed easily saturate the core
when (sufficient) DC passes through one of it's windings. As the core of
a transformer approaches saturation - it's ability to couple AC between
the windings starts to fail; to the point that a fully saturated core
couples virtually nothing.

So - in this circuit - when the AGC has the RF / IF stages biased way
down (on station) the B+ current draw is low - which passing through the
primary (or control winding if you prefer) has little effect on the
transformer's ability to couple AC between the other windings - in this
case causing one winding to "buck" the other - and the bulb is dim. When
the AGC falls - biasing up the gain (current) of the RF & IF stages -
the current in the primary (or control) winding increases - pushing the
core towards saturation - and the two windings in series with the bulb
loose their coupling, reducing the induced bucking emf - and the bulb
brightens.

As you noted - when there is significant coupling between the two bulb
windings - that will also couple to the primary (or control) winding.
That's where the two capacitors come into play - they bypass any ripple
impressed on that winding back down to the B+ rail - which of course has
it's own filtering to ground.

best regards...
So, what is the design method for a transformer that saturates easily? Ken

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