Re: Help! Coil design?

On Fri, 05 Jun 2009 18:44:20 -0500, Scott Newell <newell@xxxxxxx>
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!

I understand your frustration. Coil design is still in the dark ages.

I had the boss of a nearby coil house come in and ask me how to
measure inductance at a 100kHz, his bridges only did 100Hz and 1kHz.
Not my problem.

I've got problems sorting out the design of a transformer for an off
line switcher. Nobody I've contacted seems to have a sane design of
former. Most of them wobble around on spindly pins waiting to break.

Find a good former and the core material isn't available in less than
5000 pieces. Asymetric gaps? Can't be done.

I've hated coil houses since I got badly caught out with a 2A, 100uH,
50kHz coil 20 years ago. At 40C external air temperature the units
stopped working properly after a few hours. The core of the ferrite
material was overheating and causing the inductance to crash. As I
hadn't specified the type of ferrite the coil house happily changed it
between prototypes and production.

Imagine having to specify resistors and capacitors to your local
resistor or capacitor house to get them made.
.

Relevant Pages

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