Re: [TCML] primary tap location

[Jim Lux <jimlux@xxxxxxxxxxxxx>]

On 3/11/13 4:40 AM, Phil Tuck wrote:
> Mark,
> A primary in theory behaves like a variac, and the inner free end would
> develop a higher voltage than the tap point at alternate cycles. How much
> this would be, would depend on the number of free turns and also how
> effective the transformer action actually is of course.
> Voltage issues aside though, it would make tap adjustment awkward, both
> physically & the fact you're trying to make small inductance adjustments in
> the midst of lots of small diameter turns.
> The question some are wondering is why bother anyway :))

The coupling between turns of a air core pancake coil isn't all that 
wonderful, so the autotransformer effect might be small.

Since the inductance formula accounts for that you might be able to 
estimate the effect of it by calculating L@tap and L@end
 and then figure that Vend = Vtap * sqrt(L@end/L@tap)

Phil's comment about it being a tough place to work for adjusting the 
tap is well taken.

However, as for why.. I think it might look prettier.  The connection to 
the outermost tap, which is what someone sees can be permanent.


I tried some numbers..

1/4" tubing, 1/4" spacing (i.e. turns are 1/2" on center)

6" inside diameter, 15.5" OD, 10 turns	30.3 uH
7" id,  9 turns 27.9 uH
8"      8 turns 25 uH

Using my speculative square root of inductance ratio, I get Vend = 
1.1*Vtap  for the 10 turns, tapped at 8 scenario.

In a highly coupled autotransformer (e.g. with a core), you'd expect 
proportional to turns.. that is Vend = 1.25*Vtap


is it an issue?  maybe.. Especially in an extreme case.. say 10 turns, 
tapped at turn 5.  I get an inductance ratio of 2.14 for a voltage ratio 
of about 1.5.




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