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Beginner ... ferrite core coil for crystal set

Discussion in 'General Electronics Discussion' started by ecollen, Sep 28, 2015.

  1. ecollen

    ecollen

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    Jul 31, 2015
    Hi, I’m a beginner, and I would like to know if anyone could please explain to me how to make this ferrite core coil:

    www.scitoyscatalog.com/product/FERRITECOIL.html

    The core can move independently inside the windings, and there are two windings (4 leads). I think the independent movement of the core is achieved by doing the main winding on paper wound around the core. However the second winding stumps me. Does it have something to do with the ridge that can be seen along the top of the main winding? How long is it? Is it separate from the main winding?

    Any assistance and suggestions would be much appreciated.

    I need the ferrite core for this little crystal set which I’m planning to build:

    http://sci-toys.com/scitoys/scitoys/radio/closeup_two_coil_loop_crystal_radio.jpg

    Thank you,
    Errol
     
  2. Old Steve

    Old Steve

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    Jul 23, 2015
    I think the separate strip of cardboard is just there as a means of anchoring the wire ends.

    Ordinarily, the second winding would be wound over the top of the first, or sometimes beside it. And there would be no direct electrical connection between the coils.
    Do you have any instructions, or do you just want to work from the photo?
    If from the photo alone, there's no way of knowing how many turns are in either winding.

    I'd recommend that, at $3.47, you simply buy a pre-made coil.
    Be wary of postage price, though, and double-check before buying, because he lists the weight at 1 pound, ~500g.
    Do you have a diode to suit? It needs to be a germanium diode, like a 1N34A.
     
    Last edited: Sep 28, 2015
  3. ecollen

    ecollen

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    Jul 31, 2015
    Hi Old Steve, thank you very much for your reply.

    Thanks, the strip of cardboard makes sense.

    Would it be okay to put the second winding directly on top of the first with only the enamel of the wire as insulation?

    There are instructions on the site (http://sci-toys.com/scitoys/scitoys/radio/ten_minute_radio.html ) for building the set, but unfortunately not for the coil. One is supposed to buy the coil from them, and I would normally be quite happy to do so, but regrettably (as you suspected) the postage is the problem. They want to charge me $35 postage (and that’s only the cheapest option). Okay, it’s coming from the US and I’m in South Africa (quite a long way I know), but even so it’s quite excessive. So I’ll have to make do and see how far I get on my own.

    Yes, the number of windings is one of the problems. How many would you suggest I try?

    I can’t find a 1N34A diode, but I did manage to get another germanium diode from one of the suppliers out here, the 1N60P, and it worked okay in a previous crystal set, so I expect that will be okay.

    Thanks,

    Errol
     
  4. Old Steve

    Old Steve

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    Jul 23, 2015
    Errol, the second winding will be fine on top of the first, without additional insulation.

    I suspected that they might want a ridiculous amount for postage.

    And I'm sorry, but I have no idea how many turns to suggest for either winding. I've never built a crystal radio.
     
  5. Minder

    Minder

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    Apr 24, 2015
    Using modern magnet wire the insulation is good enough to wind directly on top if needed.
    Also unless you are aiming for a particular RF spectrum, the amount of LW-MW signals out there are plentiful compared to 80 years ago when the cats whisker radio was becoming popular.
    I would also suggest stringing a aerial and if possible attach to an earth ground.
    I would have though you could have got ferrite very cheap on ebay, also there are sellers of small coils of magnet wire.
    M.
     
  6. ecollen

    ecollen

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    Jul 31, 2015
    Hi Old Steve, thanks - one problem solved! I'll put a second winding on top of the first and not worry about that.

    I'll look through some other plans and so on and try to get an idea on the kind of windings that would work. Luckily I think there's quite a wide range that will still give some kind of result.

    Thanks,
    Cheers,
    Errol
     
    Old Steve likes this.
  7. ecollen

    ecollen

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    Jul 31, 2015
    Hi Minder

    Thank you very much for your reply. Thanks, now I don't have to worry if I wind one layer on top of the other. But, since I see it's just two separate windings, I'm wondering whether it makes any difference whether they're on top of one another or side by side. As far as I can judge that ferrite coil is pretty small - only about 2" - so perhaps they just put one on top of the other to save space. Perhaps I'll be okay to wind them side by side.

    No, I'm not aiming for any particular frequency at present. I just want it to pick up something. Antenna and earth, yes, thanks. I've got them from my previous crystal set, so that's no problem.

    Yes, I have seen ferrite rods on eBay. I've also found one local supplier that supplies them, but only in one size. I will therefore try to get some from eBay, but unfortunately I have only found the rods so far, not with the coil/s, so I guess I'll still have to do the windings myself.

    Thanks,
    Cheers,
    Errol
     
  8. Alec_t

    Alec_t

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    Jul 7, 2015
    This article might help with the coil-winding; at least to give some ball-park figures for number of turns.
     
  9. Old Steve

    Old Steve

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    Jul 23, 2015
  10. ecollen

    ecollen

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    Jul 31, 2015
    Hi Alec
    Many thanks. The article will be most helpful. A quick glance at it showed me that. But I'm going to read it through in detail now, and then I'll have a much better idea of how to go about the winding.
    Thanks,
    Errol
     
  11. ecollen

    ecollen

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    Jul 31, 2015
    Hi Steve
    Thank you for going to the trouble of finding this for me. He certainly has exactly the components I'm looking for. I'm still a bit concerned that the postage could be a problem, but I'll find out exactly how much it will be and take it from there.
    Thanks,
    Errol
     
    Old Steve likes this.
  12. Colin Mitchell

    Colin Mitchell

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    You can see a very large article on Talking Electronics website
    Crystal Set - The NEW

    and it includes a frame aerial that is just as good as a ferrite rod but is much easier to make.
    It also shows how to make an adjustable "tuning capacitor" out of tin foil so you don't have to buy an expensive item.
    But it is much better to add a few stages of amplification as a crystal set requires a long aerial and a water pipe as an earth to get any sort of reception.
    And you also need a crystal earpiece, which is expensive to buy.
     
  13. ecollen

    ecollen

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    Jul 31, 2015
    Hi Colin
    Thank you very much for your reply. This is indeed an interesting article. The frame aerial looks as if it will make life a lot easier, and I will definitely try it. If I can get the bits for the ferrite rod aerial together I would like to try it too - just to see how it works.
    I like the idea of the homemade tuning capacitor and other items very much. There's a lot of fun in trying to make things instead of having to buy them.
    Thanks for the link.
    Cheers,
    Errol
     
  14. Minder

    Minder

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    Apr 24, 2015
    As a kid we did not have access to Ferrite whatsoever, we just used a coil, we also used the first solid state device used in electronics, a lump of Galena! (cats whisker).
    M.
     
  15. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
  16. ecollen

    ecollen

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    Jul 31, 2015
    Yes, that's what we did in those days too. But a diode is a lot easier nowadays, and it's nice to try out different coils. The idea of a ferrite core sliding up and down inside the coil is a new one for me, and I'd like to see if I can get it to work
     
  17. ecollen

    ecollen

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    Jul 31, 2015
    Hi Colin, oh boy, thanks for letting me know that. Now I don't have to bother about a second winding. I'll still try my hand at the single winding around the ferrite core later though, when I find a core, because I've never tried that before.
    That article of yours is great. I haven't got very far with it yet (busy day), but it's going to be a central part of my exploring.
    Thanks,
    Errol
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    But then it's not a Crystal Set. Since its inception the allure of the Crystal Radio has been the fact that it doesn't require a power supply.

    Chris
     
    Minder likes this.
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