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!

-- 
newell

http://www.rac.ca/tca/RF_Coil_Design.html

might help 

On Fri, 05 Jun 2009 18:44:20 -0500, 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? 

Well, they're probably thinking of using a multi-layer coil, and they 
want to know how far in they can go from the max ID.  Either that or 
they're not listening.

> "What is the series resonant frequency?"  I don't know that I
> care? 

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.

> "What is the Q?" Isn't that set by the inducatance, operating
> frequency, and the DCR? 

And the coil size, and the winding arrangement, and the kind of wire 
(plain, Litz, braided, etc.)

> Why are you asking me?

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 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!

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".

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

-- 
http://www.wescottdesign.com

On Jun 5, 4:44=A0pm, Scott Newell <new...@cei.net> wrote:
> I'm having a terrible time getting an air core coil wound
> for my RFID project. =A0The 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. =A0Can 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. =A0Exact DC resistance
> doesn't matter too much, but I suggested 10-12 ohms or so
> based on some coils I wound by hand. =A0Exact wire gauge doesn't
> matter either, so I suggested 34 gauge or so, again, based
> on my hand wound samples. =A0I told them the coil thickness
> didn't matter much either, but a max of 5 mm or so seemed
> reasonable. =A0The 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? =A0For the last
> week I've had to take calls daily with question after
> question..."can the ID be 10mm?" =A0Probably not, if you expect
> to make the OD ~40mm, but how would I know? =A0"What is the series
> resonant frequency?" =A0I don't know that I care? =A0"What is the Q?"
> Isn't that set by the inducatance, operating frequency, and
> the DCR? =A0Why are you asking me?
>
> I haven't felt this ignorant in at least a month. =A0
> Should I have iterated through with the approximate
> coil formulas and just told them ID, thickness, gauge,
> and number of turns? =A0Am I asking the impossible? =A0Will
> I end up winding 500 coils my hand on a toilet paper tube
> form next week?
>
> ARGH!
>
> --
> newell

I'd ask for their standard coil form sizes as a starting point. I
can't say I've seen a 40mm form. That's nearly two inches. The again,
125KHz is more like audio frequency than RF. Maybe you need to find a
company that makes components for speaker crossovers. Ah, but those
use overlapping wires. This is quite the pickle you got yourself into.
[RFID project?]

I suspect their are all sorts of pitfalls if you don't use one of
their standard core sizes. Can their machine handle the size, blah
blah blah.

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..

Tim Wescott wrote:
> 
> > "What is the series resonant frequency?"  I don't know that I
> > care?
> 
> 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.


-- 
newell

On Jun 5, 6:49=A0pm, Robert Baer <robertb...@localnet.com> wrote:
> Scott Newell wrote:
> > I'm having a terrible time getting an air core coil wound
> > for my RFID project. =A0The 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. =A0Can 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. =A0Exact DC resistance
> > doesn't matter too much, but I suggested 10-12 ohms or so
> > based on some coils I wound by hand. =A0Exact wire gauge doesn't
> > matter either, so I suggested 34 gauge or so, again, based
> > on my hand wound samples. =A0I told them the coil thickness
> > didn't matter much either, but a max of 5 mm or so seemed
> > reasonable. =A0The 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? =A0For the last
> > week I've had to take calls daily with question after
> > question..."can the ID be 10mm?" =A0Probably not, if you expect
> > to make the OD ~40mm, but how would I know? =A0"What is the series
> > resonant frequency?" =A0I don't know that I care? =A0"What is the Q?"
> > Isn't that set by the inducatance, operating frequency, and
> > the DCR? =A0Why are you asking me?
>
> > I haven't felt this ignorant in at least a month. =A0
> > Should I have iterated through with the approximate
> > coil formulas and just told them ID, thickness, gauge,
> > and number of turns? =A0Am I asking the impossible? =A0Will
> > I end up winding 500 coils my hand on a toilet paper tube
> > form next week?
>
> > ARGH!
>
> =A0 =A0Well, 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.
> =A0 =A0With N=3D125, (dia/len) is about 2.7; more turns decreases that ra=
tio.
> =A0 =A0The simple formula is L =3D (radius*number of turns) * (radius*num=
ber
> 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

alien8er wrote:
> 
>   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!)

-- 
newell

On Fri, 05 Jun 2009 21:17:49 -0500, Scott Newell wrote:

> Tim Wescott wrote:
>> 
>> > "What is the series resonant frequency?"  I don't know that I care?
>> 
>> 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.

I have had wildly varying success with asking a vendor to go outside of 
their usual ordering parameters and deliver something that works to the 
specifications that I care about.  Some people just aren't comfortable 
outside of their little rut.  Some people don't realize that there's a 
whole big world outside of their little rut.  Some people, when presented 
with this sort of problem, go "cool!  you want this and this and this, 
and if you'll bend on this then I can save you money (etc., etc., etc.)".

Unfortunately, I don't know of any way to find out if a particular place 
is one kind or another without giving them a whirl.

Have you tried a different coil house?

-- 
http://www.wescottdesign.com

"Scott Newell" <n...@cei.net> wrote in message 
news:4...@cei.net...
> 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!
>
> -- 
> newell

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.