[Amps] Tapping a plate tank coil

John Popelish jpopelish at rica.net
Sat Jan 7 00:17:19 EST 2006


Bill Turner wrote:
> I know it is common and accepted practice to short out the turns on 
> an air-core plate tank coil for bandswitching, but is it ok for 
> powdered iron core toroids in that situation?

It is probably a bad idea.  The turns in a spaced turn air cored 
inductor are only loosely coupled, so shorting out turns just detours 
flux around those turns.  Since flux is fringing all over the place, 
anyway, this is a minor redistribution.  Once the bulk of the flux is 
circulating in a high permeability core, the turns are much more 
tightly coupled (which jacks up the inductance and makes it more 
closely proportion to turns squared) but it is like having a 
transformer with a short circuited secondary if you short out some 
turns.  The inductance plunges dramatically as the flux is forced out 
of the core to go around the shorted turns.  It is as if a big air gap 
(the size of the section with the shorted turns) has been cut out of 
the core, and the shorted turns have high current circulating in them, 
soaking up energy.

> My understanding is that the turns on an air-core inductor are only 
> loosely coupled to each other and no harm results, but in a toroid 
> (or any iron core inductor) the turns are all tightly coupled to each 
> other and placing a short between taps will cause excessive losses.
> 
> My Command HF-2500 amp uses a large toroid for the 160/80/40 portion 
> of the tank coil, and the taps are shorted out for each band. In the 
> last five years I have burned up two of those toroids and I'm 
> wondering if that might be the cause. I do a lot of RTTY contesting 
> at the full legal limit, but the amp is supposedly designed to take 
> it. On the high bands, it does.

I think you are on the right track in your understanding.

Since the inductance varies so much more per turn with windings on a 
core, you can make modest adjustments by tapping down a few turns and 
leave the extra turns unconnected, though this will produce extra 
voltage on those turns and multiply the effective inter winding 
capacitance which is not what you want when you are trying to move the 
resonance up.  It quickly gets counter productive.

The ultimate solution is to have a separate core and winding optimized 
for each band, but then the switching gets twice as complicated.

Another possibility (that I have not seen) is to use a single gapped 
core structure with the turns you will be shorting out bridging the 
large gap, where the flux is already fringing, so that shorting the 
turns just increases the effective size if the gap, as the flux is 
forced to avoid passing through the shorted turns.


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