Re: Help! Coil design?

On Jun 5, 6:49 pm, Robert Baer <robertb...@xxxxxxxxxxxx> wrote:
Scott Newell wrote:
I'm having a terrible time getting an air core coil wound
for my RFID project.  The big problem is that I simply don't
know how to specify a coil, and the engineers at the magnetics
company can't seem to help me fill in the blanks.  Can anyone
shed some light on how I should procede?

I've told them I'd like a 750uH coil with a max OD of
40mm to fit our physical constaints.  Exact DC resistance
doesn't matter too much, but I suggested 10-12 ohms or so
based on some coils I wound by hand.  Exact wire gauge doesn't
matter either, so I suggested 34 gauge or so, again, based
on my hand wound samples.  I told them the coil thickness
didn't matter much either, but a max of 5 mm or so seemed
reasonable.  The coil is to be operated at about 125 kHz.

I'd like to maximize the OD without exceeding the 40 mm spec,
as it is acting as an antenna.

(Sketchy specs, I know, but I had to start somewhere.)

So, what did I forget and what do I do wrong?  For the last
week I've had to take calls daily with question after
question..."can the ID be 10mm?"  Probably not, if you expect
to make the OD ~40mm, but how would I know?  "What is the series
resonant frequency?"  I don't know that I care?  "What is the Q?"
Isn't that set by the inducatance, operating frequency, and
the DCR?  Why are you asking me?

I haven't felt this ignorant in at least a month.  
Should I have iterated through with the approximate
coil formulas and just told them ID, thickness, gauge,
and number of turns?  Am I asking the impossible?  Will
I end up winding 500 coils my hand on a toilet paper tube
form next week?

ARGH!

   Well, i started with the assumption tha a single-layer coil would do
what you want, and used the chart in the Allied Radio Shack Electronics
Data Handbook 5th Ed. 1970 pg 36.
   With N=125, (dia/len) is about 2.7; more turns decreases that ratio.
   The simple formula is L = (radius*number of turns) * (radius*number
of turns) / (9*radius + 10*length) where dimension is in inches;

That's what I recall from one of the ancient ARRL handbooks. Well,
the numerator was written r squared n squared, but still.

Anybody know where that equation came from?

Also, OP didn't mention whether a flat or solenoidal coil was
preferred; I note that industry standards lean heavily to flat
spirals, but that may mostly due to packaging considerations. Speak
up, Scott.


Mark L. Fergerson
.

Relevant Pages

  • Re: Help! Coil design?
    ... know how to specify a coil, and the engineers at the magnetics ... Exact wire gauge doesn't ... matter either, so I suggested 34 gauge or so, again, based ... is about 2.7; more turns decreases that ratio. ...
    (sci.electronics.design)
  • Help! Coil design?
    ... I'm having a terrible time getting an air core coil wound ... Exact wire gauge doesn't ... matter either, so I suggested 34 gauge or so, again, based ...
    (sci.electronics.design)
  • Re: why is distortion bad for speakers
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  • Re: Help! Coil design?
    ... know how to specify a coil, and the engineers at the magnetics ... Exact wire gauge doesn't ... matter either, so I suggested 34 gauge or so, again, based ...
    (sci.electronics.design)
  • Re: Help! Coil design?
    ... know how to specify a coil, and the engineers at the magnetics ... Exact wire gauge doesn't ... matter either, so I suggested 34 gauge or so, again, based ...
    (sci.electronics.design)
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