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Steel vs Iron Core in Coils

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  • patrick_mcstorm
    Hello Would it be possible (and practical) to substitute a steel core such as a threaded bolt for an iron core for home brewed coils? I know the specs might
    Message 1 of 6 , Jul 9, 2014
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      Hello


      Would it be possible (and practical) to substitute a steel core such as a threaded bolt for an iron core for home brewed coils?


      I know the specs might well be different because of the different core material.


      What I'm thinking of doing is using a self-tapping screw to tap the inside of a PVC (of the proper sized tubeto make a coil tube and then adding the steel bolt to make a coil form.


      A cardboard tube of the proper size could be done the same way without the bother of a self-tapping screw- just "thread" the bolt into the tube.


      This would produce a truly home brewed adjustable coil form.


      I suspect the answer to my question is "no" because I've never heard of it being done before.


      Ideas?


      Regards


      Patrick


      p.s. NOTE: NO TYPO CHECK WAS DONE HERE; please forgive any that are present.

       

    • John Popelish
      On 07/09/2014 10:09 PM, vw_beetle_fix_it@yahoo.com ... I ve seen it done with brass screws, where inserting the screw into the coil reduces the inductance.
      Message 2 of 6 , Jul 9, 2014
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        On 07/09/2014 10:09 PM, vw_beetle_fix_it@...
        [Electronics_101] wrote:

        > Would it be possible (and practical) to substitute a steel
        > core such as a threaded bolt for an iron core for home
        > brewed coils?
        >
        > I know the specs might well be different because of the
        > different core material.
        >
        > What I'm thinking of doing is using a self-tapping screw to
        > tap the inside of a PVC (of the proper sized tube) to make a
        > coil tube and then adding the steel bolt to make a coil form.
        >
        > A cardboard tube of the proper size could be done the same
        > way without the bother of a self-tapping screw- just
        > "thread" the bolt into the tube.
        >
        > This would produce a truly home brewed adjustable coil form.
        >
        > I suspect the answer to my question is "no" because I've
        > never heard of it being done before.

        I've seen it done with brass screws, where inserting the
        screw into the coil reduces the inductance. This is because
        highly conductive materials, like brass or aluminum
        circulate eddy current in response to an AC field, and the
        field from the eddy currents cancels some of the external
        field. Unfortunately, that eddy current effect swamps out
        any flux increase effect from the ferromagnetism of iron,
        except at very low frequencies.

        This is why ferrite is used. Its ferromagnetism is usually
        a lot lower than iron, but its resistivity is very high,
        almost eliminating the eddy current problem.

        But you could epoxy a little ferrite bead (or a stack of
        several) on the end of a bolt and make a pretty good adjuster.

        The old fashioned way to make widely variable inductors was
        to wind half of the turns on a wooden ball (croquet ball),
        and the other half on a cylinder that the ball just fit in
        side of. The ball was pivoted on an axle that passed
        through the middle of the cylinder's turns and the two
        windings were wired in series. By rotating the ball, the
        inductance could be varied more than 2:1.

        These were made by early hams who built their entire radios
        from scrap.

        --
        Regards,

        John Popelish
      • patrick_mcstorm
        John Does reducing the inductance of the coil raise or lower the frequency received, assuming the tuning cap remains at the exact same setting? It looks like
        Message 3 of 6 , Jul 11, 2014
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          John


          Does reducing the inductance of the coil raise or lower the frequency received, assuming the tuning cap remains at the exact same setting? It looks like this would affect the frequency of that particular L.C. combination. Or am I looking at this wrong?

          Regards


          Patrick


          p.s. NOTE: NO TYPO CHECK WAS DONE HERE; please forgive any that are present.

        • rtstofer
          The equation for resonant frequency is still the same: f=1/(2*pi*sqrt(L*C)) So, if L goes up, sqrt(L*C) goes up (for a fixed capacitance) and, since the
          Message 4 of 6 , Jul 12, 2014
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            The equation for resonant frequency is still the same:

            f=1/(2*pi*sqrt(L*C))

             

            So, if L goes up, sqrt(L*C) goes up (for a fixed capacitance) and, since the denominator is now larger, frequency goes down.  Intuitively, you just KNOW that a 1 Henry inductor will have a lower resonant frequency than a 1 mH inductor for a given capacitor.

             

            You really should be working with LTSpice.  It's pretty easy to toss these equations around but there is nothing as helpful as stepping the parameters and watching the results.

             

            I have learned more about differential equations in the last month while playing with my analog computer than I learned in undergrad and grad school combined.  To be honest, I have no idea how I passed the courses.  I didn't really understand the material but I guess I could do the arithmetic substitutions.  Now,  I can SEE the solutions.  The equations for damped harmonic motion (RLC circuit) are a PITA to deal with analytically but watching the equations play out in real-time (well, computer time, really) gives genuine insight.

             

            LTSpice makes the solution to the RLC circuit almost trivial.  Wire it up, give it a simulation command and watch the plot.  Simple...

             

            Richard

             

             


             

          • jpopelish .
            On 7/12/14, vw_beetle_fix_it@yahoo.com [Electronics_101] ... Reducing the inductance increases the resonant frequency. Inductance and capacitance are both
            Message 5 of 6 , Jul 12, 2014
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              On 7/12/14, vw_beetle_fix_it@... [Electronics_101]
              <Electronics_101@yahoogroups.com> wrote:
              >
              > Does reducing the inductance of the coil raise or lower the frequency
              > received, assuming the tuning cap remains at the exact same setting? It
              > looks like this would affect the frequency of that particular L.C.
              > combination. Or am I looking at this wrong?

              Reducing the inductance increases the resonant frequency. Inductance
              and capacitance are both energy storage mechanisms. More energy
              storage (higher inductance or capacitance) takes more time (lower
              resonant frequency). Resonant frequence is
              1/(sqrt(L*C)).

              --
              Regards,

              John Popelish
            • rtstofer
              The definitive homebrew tuning coils were wound on cylindrical oatmeal containers. QUOTE: In 1921, factory-made radios were very expensive. Since less
              Message 6 of 6 , Jul 12, 2014
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                \\\

                 

                The definitive homebrew tuning coils were wound on cylindrical oatmeal containers.

                 

                QUOTE:

                In 1921, factory-made radios were very expensive. Since less affluent families could not afford to own one, newspapers and magazines carried articles on how to build a crystal radio with common household items. To minimize the cost, many of the plans suggested winding the tuning coil on empty pasteboard containers such as oatmeal boxes, which became a common foundation for homemade radios.

                END QUOTE

                 

                Crystal radio - Wikipedia, the free encyclopedia

                 

                Richard

                 

                 

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