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  • Combining Inductors

    I'm combining two coils in series to reach a target inductance as part of a low pass woofer circuit. Once connected, an inductance meter showed that I am right on target. Do I still need to be concerned with distancing the inductors or placing one at a right angle when the goal is to combine them? Are there other potential interactions that could cause a problem?

    Thanks for your comments.

  • #2
    Re: Combining Inductors

    Yes, their orientation and proximity will affect their combined inductance due to mutual coupling.
    Craig

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    • #3
      Re: Combining Inductors

      This is a good question. My answer is: I don't think so. I have not performed any testing to prove or disprove my point, but my reasoning is as follows:

      The total combined inductance is placed in the exact same electrical location in the circuit. It is when they are placed in 2 different electrical locations that we have to be concerned about crosstalk between the 2 different branches in the circuit. Therefore in this case the inductors may be placed behind or next to each other.

      However it is important that your measurement of the total inductance is done with the inductors in the same location as they will be once mounted on the circuit board as the total inductance will vary depending upon the relative location between the inductors. Once the location is set, the total amount of inductance will not vary.

      If measured under those conditions and the total inductance is acceptable, you should be good to go.

      I have not seen any information or testing done contradicting my thesis.
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      • #4
        Re: Combining Inductors

        Originally posted by PWR RYD View Post
        Yes, their orientation and proximity will affect their combined inductance due to mutual coupling.
        Yes, I am aware that close proximity can affect their combined inductance which is why I verified the measurement after. The sum is within 2% of the target. Are there any other issues here?

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        • #5
          Re: Combining Inductors

          Originally posted by thekorvers View Post
          This is a good question. My answer is: I don't think so. If measured under those conditions and the total inductance is acceptable, you should be good to go. I have not seen any information or testing done contradicting my thesis.
          That is what I am thinking as well; just wanted to know what others think. I believe cross-talk is another concern, but that should only come into play when the inductor is in close proximity to an inductor from another circuit. In this case, the two combined inductors should be acting as one, at least in theory.

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          • #6
            Re: Combining Inductors

            Originally posted by fernandov View Post
            That is what I am thinking as well; just wanted to know what others think. I believe cross-talk is another concern, but that should only come into play when the inductor is in close proximity to an inductor from another circuit. In this case, the two combined inductors should be acting as one, at least in theory.
            If you've measured to within 2%, you're good. Just keep other inductors away.

            Now, try stacking them on top of each other with their centers aligned and see how much it changes, just as an experiment.
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            • #7
              Re: Combining Inductors

              If you can measure inductance, you can reach your target inductance by taking the wire from your smaller inductor and wrapping on to the larger inductor. It should take less wire in total.
              John H

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              • #8
                Re: Combining Inductors

                If you've measured to within 2%, you're good. Just keep other inductors away.

                Now, try stacking them on top of each other with their centers aligned and see how much it changes, just as an experiment.
                Interesting question...
                I was playing with 2 hefty inductors the other day and noticed when moving them together (like stacked donuts) you could feel the physical resistance, like magnets.
                Perhaps its best to be prudent and avoid any possible interaction by using the "tire rolling over a donut" orientation.


                The other thing you should check is the total resistance. If they're small gauge it could easily add up to over 1 ohm, which could have an impact.
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                • #9
                  Re: Combining Inductors

                  Iron cores get mad at each other if they get close. (Ask me...) They can get hotter than a $2 pistol, so don't do that! "I got blisters on me fingers!"

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                  • #10
                    Re: Combining Inductors

                    Originally posted by Whitneyville1 View Post
                    Iron cores get mad at each other if they get close. (Ask me...) They can get hotter than a $2 pistol, so don't do that! "I got blisters on me fingers!"
                    You got blisters on your fingers from improperly mounted inductors? Seriously? Who was stupid enough to do that in the presence of current potential high enough to cause burns - and more importantly, who was idiotic enough to ensure that happened - and even MORE importantly - who was stupid enough to get burned?
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                    • #11
                      Re: Combining Inductors

                      Originally posted by johnnyrichards View Post
                      You got blisters on your fingers from improperly mounted inductors? Seriously? Who was stupid enough to do that in the presence of current potential high enough to cause burns - and more importantly, who was idiotic enough to ensure that happened - and even MORE importantly - who was stupid enough to get burned?
                      I'm pretty sure he was just quoting John Lennon, and probably just exaggerating for emphasis...

                      I was thinking about this last night....
                      if they were they same physical dimension and you were able to stack them in a way that the windings were the same direction and continue from the bottom of one to the top of another...
                      I wonder if you could get away with it since its almost like one with a discontinuity in the middle

                      In any event I'm temporarily patching different ones together for testing, but I keep them 6-8 inches apart. (I'm not worried about economizing space on the final layout)
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                      • #12
                        Re: Combining Inductors

                        How ever they are laying when measured is how you'll have to have them when utilizing them. If you have a way to measure then you can prove this to yourself by stacking/moving them/inverting them, it all effects the mH out come.
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                        • #13
                          Re: Combining Inductors

                          If you've measured to within 2%, you're good. Just keep other inductors away.

                          Now, try stacking them on top of each other with their centers aligned and see how much it changes, just as an experiment.
                          Yeah, I would think the ideal alignment would be to have them aligned on-axis with their windings in the same direction.
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                          • #14
                            Re: Combining Inductors

                            Originally posted by john trials View Post
                            Yeah, I would think the ideal alignment would be to have them aligned on-axis with their windings in the same direction.
                            This is the defacto guide...
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                            • #15
                              Re: Combining Inductors

                              Originally posted by jhollander View Post
                              This is the defacto guide...
                              Nice!
                              But while it nicely demonstrates the relative effect, one thing I'm missing is the actual measurements of the 2 individual coils.
                              Are they .56 by label or is that their actual measurement on this rig?

                              If they are actual .56 than the theoretical combination would be 1.12 and many of the measurements shown are more than that.
                              8" apart they are >1.14
                              4" > 1.13
                              (not such a big dif)

                              photo # 5 / pdf 6 (the generally acceptable "tire rolling over donut")
                              > 1.14
                              (same as 8" apart, so I guess this is ideal)

                              But whats wrong about photo7/ pdf#8 "never do this "?
                              stacked coils (non-inverted) are > 1.31
                              That's an increase of .2
                              - with the price of copper, why not do it (at least in this case, and as long as you can measure)?

                              same for photo6 / pdf4 "very bad" > 1.17
                              still an increase.
                              Free lunch?
                              OR are there unpalatable side effects?
                              Last edited by jeff_free69; 11-25-2013, 03:10 PM.
                              BEER: Proof that God loves us and wants us to be happy

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