Re: Help! Coil design?

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; OR
Number of turns = (squareroot(inductance*(9*radius + 10*length)) / radius.
Pick your wire for reasonable fit, as #0000 will no work here and #80 is too resistive..
.

Relevant Pages

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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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