PCB coil calculation

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#1
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Hi there,

I have made some research for a pcb coil design. Before designing the pcb I took some measurements for the coil. Using a 0.25mm wire I made a winding with these results:

148mm x 100mm / 42 turns
12.8Ohm / 770uH

Now I need to design a pcb coil with the same characteristics. Using the same distances/turns (2 or 4 layers pcb) could I have the same results (~770uH)? If yes, how much must be the track's thickness in order to take the same resistance (~12.8Ohm) with the 0.25mm wire?

Thank you.

Michael.

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You will have massive proximity effects and use quite a lot of PCB real estate.

If you really want to keep similar dimensions then you will be using say 12 turns per layer. You can calculate the total length of PCB trace. The track width will depend on the copper thickness, so you are stuck with relatively wide tracks.

If your principal consideration is a coil with stable inductance, then using regular enamelled wire would be simpler.

If you really want an aerial, then you could consider larger dimensions, ferrite cores ...

David.

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David,

Quote:
If you really want an aerial, then you could consider larger dimensions, ferrite cores ...

What do you mean???

Michael.

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I assume you are either making a radio receiver or security tag identification.

David.

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SMPS?

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Hi Mike,

You don't mention if your app is air-cored or ferrite.

I looked into planar transformers a long time ago and remembered that Philips had some info on it - if its any help...

http://www.itee.uq.edu.au/~elec4400/datashts/ferrites/plandesi.pdf

Regards,

-=mike=-

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I doubt it is for SMPS, 12.8ohms resistance is pretty large, and would impose a hard limit on output current (around 350mA for a 5V output...). 770uH is also pretty massive, probably for use in a tuned crossover network or something... More common for SMPS is around 10 Ohms/mH, with inductance in the 100-200uH range. This makes for example a 100uH coil with a DCR of about 1 Ohm.

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Ah, I skipped the resistance..
That is a very atypical app for a PCB coil.

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Lets see -- 0.25mm is about .010". That is about 30 gauge wire. Unless this wire has thick plastic insulation, will will NOT be able to get PCB traces as close to each other as the windings are. That probably means more turns.

It is also not clear what your "resistance" value is. Is that DC resistance or is it reactance at some frequency? 30 gauge wire has about 330 ohms per km of wire. If that is DC resistance, it means about 38 meters of wire. There is about 0.5m per turn and 42 turns makes 21 m. Its not waaaay off, but it is not that close.

Better help can be offered if some more details are provided.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA There are some answers that have no questions.

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Guys,

There is no core. It is only a pcb antenna used for an RFID application at 125KHz.
The resistance ~12.8Ohms is the dc resistance of the cable.
The cable does not have plastic insulation, it's just a copper like those used for motor windings.
To tell you the truth I didn't measure the thickness, but it looks like 0.25mm, but I have to find the way to measure it. But I don't have a thickness meter.

Michael.

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Check this out, looks like some nice information about RFID antennas: http://ww1.microchip.com/downloads/en/AppNotes/00710c.pdf

I think i have to get that printed tomorrow!

Kim