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Old 1st Sep 2015, 2:25 pm   #1
vu2nan-nandu
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Default Crystal Radio Output Transformer

The large size of the power transformer, used as an output transformer for my Loudspeaking Crystal Radio, was good enough reason to attempt homebrewing a relatively small one.

A search through the junk box yielded a 47mm diameter, Siemens N22 pot core set.
https://www.vintage-radio.net/forum/...1&d=1441113729

Winding was for an autotransformer, having 500 turns (the maximum number that could be accomodated) of 30 SWG enamelled copper wire, with output taps at 50 and 100 turns.
https://www.vintage-radio.net/forum/...1&d=1441113829

The 50-turn tap was to match the existing 3 Ω speaker and the100-turn one a provision for speakers of higher impedance.

A small PCB, also from the junk box, provided termination points for the winding.
https://www.vintage-radio.net/forum/...1&d=1441113869

A couple of white plastic cosmetic jars were reworked to form the enclosure, giving the transformer a presentable appearance.
https://www.vintage-radio.net/forum/...1&d=1441113902

Tests indicated that, for its size, the output transformer's performance is comparable, though not equal, to that of the power transformer.
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Old 2nd Sep 2015, 3:49 pm   #2
Colin Ames
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Default Re: Crystal Radio Output Transformer

Nice work!
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Old 2nd Sep 2015, 7:23 pm   #3
greenstar
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Default Re: Crystal Radio Output Transformer

That's an interesting idea. I wonder if it can be adapted with different cores (are the pot cores the same thing as ferrite rods, and could one be wound on a ferrite aerial?). 500 turns isn't a lot. Eg for low impedance 'phones on a crystal set. Clearly the low number of turns doesn't impede the performance of your set - I wonder if more will be needed for a sensitive design, as i should think 2KOhms at least would be optimum.
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Old 2nd Sep 2015, 8:57 pm   #4
G6Tanuki
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Default Re: Crystal Radio Output Transformer

In the past, when wanting to use low-impedance 'phones with a crystal-set I've used one of the little 'LT44' push-pull driver transformers.

Primary to the diode detector, then use half or all of the secondary to drive the 'phones - experiment with half- or full-secondary to get the best volume.
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Old 5th Sep 2015, 6:57 am   #5
vu2nan-nandu
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Default Re: Crystal Radio Output Transformer

Yes, Tanuki, crystal radio is fun because of its endless variants!

Quote:
Originally Posted by Colin Ames View Post
Nice work!
Thanks, Colin!

Quote:
Originally Posted by greenstar View Post
That's an interesting idea. I wonder if it can be adapted with different cores (are the pot cores the same thing as ferrite rods, and could one be wound on a ferrite aerial?). 500 turns isn't a lot. Eg for low impedance 'phones on a crystal set. Clearly the low number of turns doesn't impede the performance of your set - I wonder if more will be needed for a sensitive design, as i should think 2KOhms at least would be optimum.
Hi Greenstar,

The open magnetic path of a ferrite rod would increase the losses and result in an inefficient transformer. A ferrite E/I core would make a better replacement for the pot core.

More turns with the same wire would increase the copper loss. Using thicker wire to counter that would result in a bigger transformer!

Regards,
Nandu.
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Old 7th Sep 2015, 10:05 pm   #6
greenstar
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Default Re: Crystal Radio Output Transformer

Thanks Nandu - a great deal of lateral thinking needed for crystal sets!
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Old 15th Sep 2015, 7:19 pm   #7
vu2nan-nandu
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Default Re: Crystal Radio Output Transformer

Anytime, Greenstar!

Regards,

Nandu.
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