At 29-03-2004 12:56, [EMAIL PROTECTED] wrote:
In a message dated 2004-03-28 03:33:51 PM Eastern Standard Time,
[EMAIL PROTECTED] writes:

>......The coil is about 3.25 X 4.25", 700uHy and takes 60 turns of mag wire. Can I do this with traces?........

You can. EM does it in their proximity reader reference design, but use a lower inductance.
They do 32 turns and swap layers every 4 turns.


I did something almost exactly like this a while back. Better do some calculations before you do those 4/4 design rules. I was very disappointed in the Q of the resulting coil; I needed a fairly high Q.

But with prox readers, the Q has to be limited for bandwith reasons. Depending on the type of datarate and modulation used you can end up with a maximum Q of 1.5.......... Q's used with 2kb/s and ASK are in the range 10-25.


Should have done the calculations myself before layout, because it behaves as theory would dictate. I used 10/10 rules. With 1 oz copper (1.4 mils thickness), the cross-sectional area of a 10 mil trace is only 14 sq mils - roughly equivalent to #37 wire, which is about 2' per ohm. The coil you describe is about 60' long, so you've got at least 30 ohms with 10/10 rules. If you go to 4/4, the cross section drops to 5.6 sq mils, which puts you up around 90 to 100 ohms for 60' of coil. Now you need to bring the rest of the circuit into the picture to compute Q, if that's relevant, but you can at least say that you'll lose 1 volt of your drive for every 10 mA of current through that coil. If I were doing it over again, I'd figure out a way to get some iron inside the coil and reduce the number of turns, and if possible use wider traces and thicker copper.

For proximity cards you will need to maximise the current, after all that's what powers the card. Some good computations can be found in Microchip's AN678.



Leo Potjewijd hardware designer Integrated Engineering B.V.

[EMAIL PROTECTED]
+31 20 4620700



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