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[TCML] Current Along Secondary Length
(changed the subject line to better serve the subject)
I modeled the demo coil of Javatc with and without a top load plotting
the current and voltage along the length of the coil) and graphed the
data in Excel. Uses a 12/60 NST size with a 18.8nF cap.
The first case is a coil without a top load. Note how the current
changes to almost nil at the top of the coil from the initial base current.
The second case is a coil with a relatively large toroid installed. Note
how the current changes, but only slightly.
The third case is a coil with a small toroid installed. Current doesn't
go to zero, but is does change significantly.
In all cases, Ibase=(Vcap(pk) x sqrt(Les/Lp))/Sec Reactance. The
calculated base current is only a near representation of the highest
base current expected (it's not a value to size coil wire with). If you
were to use a measured Ibase current, the current variation would not
change, only the actual current scale.
The graphs make it easy to see how the top load geometry and size
affects the distributed capacitance and how this changes the
distribution of current along the length of the coil. If your wondering
why base currents change with the same coil, cap, and transformer,
realize the change in currents due to the distribution of capacitance
affects the inductance from turn to turn. This is why I use the
inductance at frequency (Les) within the base current equation.
Does it really make sense to change wire size along the coil? Most coils
will fall between case 2 and 3, and the highest percentage of the coils
will be nearer to case 2 where there is only a slight change. You gain
nothing by winding the lower portion of the coil with larger wire and
you actually lose out on inductance overall. If you must splice due to
"what is on hand", then fine. But from a performance standpoint, I don't
see a need to design for this.
I think Ed is correct. According to models, the larger the top load is
relative to the coil size, the "less" the current changes from top to
bottom. But if a bare coil, it is max to zero (or near). Also, the
linearity of the distributed current along the length of the coil is
dependent on the h/d (more linear for low h/d coils).
For the normal h/d of 4 or 5, it's not quite linear, but curved even
with bare coils. Throw a big toroid on the coil and the current is far
less changing from bottom to top because the toroid is very well
adapted at forcing itself (it's C imposed on the inter C of the
windings) and changing the overall distributed capacitance of the coil
due to the toroid geometry. The current as a result becomes less
changing from bottom to top (but there is still a difference). So no,
current is not the same from end to end from a technical stand point.
And it can never be the same unless you throw turn to turn C out of
If you make a statement as "the top current is smaller than the bottom
current", then you would be correct. The problem is the person stating
this usually does not know "how much smaller". In rare cases, it can
be a lot. In most TC cases, it's not a big change. If I were to give a
"number", I would say the current on average coils is about 25% less
at the top than it is at the base. But, that's just truly rough
guesstimate of what I would call average coils and average tops to
Ed Phillips wrote:
It's a long piece of wire with capacitance between turns and
capacitance of each turn to the surroundings. Current distribution
depends on how much top load capacitance is on the coil. With no top
loading the current will be lower at the top; if there are no
streamers the current should be essentially zero at the top.
Distributed capacitance is the culprit.
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