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Old 22nd May 2006, 02:51 AM   #1
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Join Date: Dec 2003
Location: Montreal, Canada
Default building air-core crossover autotransformer

Hi,

I am trying to create an air-core autotransformer equivalent to the iron-core auto transformer found in the crossover network of my vintage Tannoy loudspeakers.

The original step-up iron-core autotransformer has an inductance in the vicinity of 10 mh. with four taps in addition to the two extreme ends of the single coil autotransformer. The different taps allow one to select different voltage levels to the tweeter ranging across plus/minus 3 db.

I have purchased an air-core inductor of equivalent inductance.

This coil consists of 480 turns of 18 guage wire wound around a 1-3/4 diameter inch air core.

In theory, I should be able to assign one end of this coil as ground and look for various positions on the coil so that if I electrically attach different taps, then I should get output voltages at those taps that are identical to the voltage readings at the different taps on the original iron-core autotransformer.

It's just a question of finding the particular turns of wire in the coil that delivers the proper results.

But before I can do that, I must decide whether the ground should be attached to the inner most turn of wire in this coil or the outer most turn.

In the original iron core autotransformer, all turns of the single continuous wire wrap around the iron core so that no matter which segment of the wire serves as the input (primary) to the transformer, the magnetic field which is induced into the iron core by the primary loops will radiates back to all the loops of the same wire which correspond to the output (secondary) of the transformer.

My guess is that with my air-core inductor, I must assign my ground to the outer most loop of the the wire so that the loops forming the secondary section will be inside, just as in the case of the Tesla coil with 2 independant coils where the secondary coil is concentric within the outer primary coil. In my air-core autotransformer, the primary and secondary coils are basicly one continuous coil.

I do not know exactly how the lines of flux radiate around a wire carrying current, but my intuition tells me that more energy will be transferred between the different turns of wire in the coil if the output turns (secondary) are contained within the input turns (primary) of the coil.

Any opinions?
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