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:
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