Re: Help! Coil design?

Tim Wescott wrote:

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?

The DIY coil house is best if only a few are needed...
.

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