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Help! Coil design?

S

Scott Newell

Jan 1, 1970
0
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!
 
S

Scott Newell

Jan 1, 1970
0
Tim said:
Yes you do. Think of the series resonant frequency as the absolute,
positive, maximum limit to the useful frequency range of the coil. You
probably want to stay well below it if you're using the coil to set the
frequency of operation.

Of course, but I didn't think that the strays in a 40mm coil would be
anywhere near significant at such a low frequency (125kHz). Maybe I
should check into that a bit. (Looks like SRF will be above 1-2 MHz,
according to "multilay.exe".)

Because you're the customer, and they see you as the source of the
specifications that they should build to. They're used to building
coils, testing them out on an LCR meter, and shipping the ones that pass.

I guess I thought I could get by giving them some electrical specs and a
rough physical box to fit in, and they'd work out the exact dimensions
and turns to best suit their winding setup.

(Maybe I'll tell them to try 96 turns of 34 gauge, 39mm ID, 0.15mm
high.)

Have you tried saying "Look, I'm no expert, and there's a lot of stuff I
don't think is critical. Why don't you write _all_ the specs, and I'll
buy to them".

That's pretty much what I said from the start. I can tune my circuit
to fit the coil, and I'm adding resistance to bring the Q down to 10-15
in circuit at 125 kHz, so I didn't think it would matter if it came out
to 700, 750, or 800 uH. Or if the coil Q was 40 or 60. (Consistent
would be good, of course!)

Then be ready for frustration all around when your circuit doesn't work
and you decide you need the coil changed...

The circuit is pretty forgiving--all the random wound coils I've done
by hand have worked fine so far.
 
S

Scott Newell

Jan 1, 1970
0
alien8er said:
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.

I was assuming a multilayer coil, but again, I'm not sure that we care.
We're going to epoxy the coil/antenna to our housing and then fill
said housing with potting compound. As long as it's not too large,
the exact size and shape are not physically critical to us.

This is going in an RFID read head, not a tag. (The tags I've sectioned
contain multilayer coils as well, but with much finer wire than I
would use for my application!)
 
J

john jardine

Jan 1, 1970
0
Scott Newell said:
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!

Change your supplier. They're requesting far too much information. Which
leads one too suspect they don't really know what they're doing.
I "specified"! a custom transformer Thursday morning as (quote)

"Nominal 240Vac~ primary with two low voltage secondary windings.
Transformer is suggested at 30VA rating
Secondary #1 is Nominal 6.6V ac at 2Amp ac~
Secondary #2 is nominal 12.7V at 2amp ac~
(Yes. I know the VA things don't add up to "30" but sod it :)"

I receive the prototype on Monday morning and will expect it to work, having
had zero technical questions from it's designer.
Suppliers should have the experience to fill in the basic stuff themselves.
 
R

Raveninghorde

Jan 1, 1970
0
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.
 
J

Jim Thompson

Jan 1, 1970
0
They can read those damn things from 3ft!

The Tag Chip I originally designed around 7 years ago, and have
updated every few years since, now has a range exceeding 30' ;-)

It pays to hire a pro ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food
 
S

Scott Newell

Jan 1, 1970
0
Benj said:
Yeah, you are in trouble bunky! My coil calculator show for the
following:

OD = 1.5"
ID = 1.1"
thickness = .2" (makes a square cross-section)

Wire = 34 Ga.
Ohms = 66.8 ohms
Inductance = 25 millihenrys
turns per layer = 27
layers = 28
total turns 756

Yeah, that's not even close to what I was asking for.

I finally just started over and gave them physical specs,
rather than electrical specs (100 turns of 34g on a 36mm
form, 2.1mm high). They asked to make an adjustment or
two based on their hardware, and wound up some samples.

Got the samples yesterday and everything seems to work ok.
It was nice to see that my hand wound coils were close
enough that I didn't even have to retune the circuit for
the machine wound coils.

I *will* be giving them a handful of suggestions on
their order/quotation process to help other avoid the
trap I fell into.

Thanks everyone!
newell
 
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