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Dual field coil - White's TDI

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  • #16
    Originally posted by satdaveuk View Post
    Hi Ism
    Glad you did post because i had same problems with my dual field coil , it was rendered useless, looks good but crap, didnt realize at the time of construction that inductance balance between inner and outer coil was critical.
    The minipulse 3 also has a type of ground balance which they call range, the coil made it so spasmodic with adjustments, just cant get a stable even keel, so gave up on that one, the Next one shall go for critcal inductance measurments to balance them up, which which is easy.
    Your total inductance of around 680uh for the TDI is what is there abouts require for the MP3, funny enough thats what my 6" mono coil is which works well for the size, although did run to 3.5 ohms.
    Whats the total resistance of your factory coil?, what gauge/type wire? the MP3 is 2.7-3ohms using .5 swg
    Regards
    Dave
    Dave,
    My total inductance (If I remember correctly) was 330uh even though the individual coils added up to 300uh. There is some increase when the two coils are added together. There may be some information on the original TDI DF coil. I have my TDI boxed up right now but I will try to get you some info in the next couple days.
    Ran

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    • #17
      Originally posted by Reg View Post
      Ism,

      Whoa, did you say you had to throw out the 18" coil?

      If you tried to make a dual field and the larger coil winding is much bigger than the 12" factory coil, then most likely the coil will not work at the minimum delay. I have no idea if this was your problem, but it very well could have been or at least was part of the problem.

      The TDI is right on the ragged edge of the maximum size that can be used when you use the factory 12" coil Go bigger on coil size and it is very easy to have the coil do strange things. It doesn't matter if you are building a plain mono or a dual field either.

      One can cheat and lower the inductance some to get a larger coil to work, but the average coil builder will usually end up with a coil that simply needs more delay to work right.

      A simple way to tell if the coil is going to work right is to turn off the GB and do an air test. With the GB off, the only signal processed is the main target signal which is the one that will be affected by the larger size. So, with the GB off, a coil that won't work at the delay used will act muted or have very little sensitivity. Tweak the delay up and suddenly the coil will act totally different.

      Reg
      Reg, thanks for your reply but if you remember we had a discussion about this. tried upping the delay but the results were the same, the coil was useless in GEB mode even with the delay at 25us. It worked fine with GEB off at 12us delay. I made a similar 8" DF coil with mismatched inner and outer coils and it performed in a similar way. I was able to dial it in however the GEB was way off.

      I could adjust some low conductors to become high conductors. While some low conductors will become high when GEB is changed, the range was way off the normal scale. With the 18" there was no way. A nickle was a high conductor no matter what setting I used. Now my 18" with balanced coils worked fine but I had used a donut style coil housing and it was terribly front heavy so I never finished it.

      I tried everything I could think of to remedy the problem but nothing worked. Now I know I am just a novice but I built 2 DF coils that worked fine. Its when I went to imbalanced coils (because the wire size I used was too big for the inner coil to fit it in the housing), I ended up with problems. One other possibility is coil resistance wasn't enough because of the larger wire size but that didn't affect my mono coils. I tried individual damping resistors but damping both coils as a set worked better.

      I don't know what else to say...It is my experience and I wish I could have isolated why the coils performed so strangely. The fact that I got 2 resonant frequencies also baffled me. I posted the question here but didn't get any answers. That was a long time ago and I concluded that I was seeing resonance of each coil or maybe a harmonic from the imbalance. This is just my wild guess and certainly not the conclusion of an expert.

      I haven't built a coil since but may order another housing from Bill and give it a go with balanced coils (smaller wire size)to see how it all works. My failure was I potted the last 18" coil after I tested for working delay speed, but never did a thorough test in GEB mode. My bad!

      Ran

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      • #18
        Erm hold up... I thought the Dual Field coil were SERIES wound but you are saying they are PARALLEL wound?

        I made one in series mode and it worked OK.

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        • #19
          Ism,

          You need to have all the parameters of the factory coil: resistance, inductance, capacity and self resonance... Good to know that the materials used to build the original coil.

          I have a good look at the coil response on the oscilloscope. Without such a comparison will be difficult to build a good coil..

          Regards
          Taktyk

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          • #20
            No the DF coils are series but the inductance is more than the sum of the individual coils.

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