[time-nuts] Minicircuits 10% discount in December
Richard (Rick) Karlquist
richard at karlquist.com
Fri Nov 28 16:59:53 EST 2014
On 11/28/2014 1:04 PM, Bob Camp wrote:
> If you do need to run substantial current through a choke core, the larger binocular cores with a half turn through them are a better choice.
> Still useless for 20A (or even 2A) though …
The binocular cores come in several hole sizes.
All other things being equal, current handling
capacity is directly proportional to hole size.
One thing to watch out for with putting DC thru
binocular cores happens in push pull RF power amplifiers.
The output transformer is usually a binocular
core on steroids, or its equivalent constructed
with beads or sleeves, etc, threaded over a single
"turn" made from brass tubes connected together
at the end away from the transistors.
In cheap (illegal) CB amplifiers, you will frequently see
+13.6 VDC connected to the junction of the brass
tubes, as if it were a center tap. It actually isn't
a center tap in terms of core saturation, and the DC
currents to the transistors are unmitigated in terms of
magnetizing the core. Although the cores are larger,
so are the currents, and these amplifiers just live with
the degradation including the magnetization. This
occurs because each core sees only a half-turn. If
you replace the tubes with a 2 turn wire primary, then
the problem goes away, but of course then the amplifier
would never work as high as 27 MHz, which is does normally
only by resonating stray PC board trace inductance with
peaking capacitors on the transformer. This forms a
two stage step up structure. If you "improve" the layout
to get rid of the trace inductance, the amplifier no longer
works! See Motorola AN-762.
Rick Karlquist N6RK
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