How to build a flat inductor?

[ro0ter]

Hello all,

I`m struggling to build a flat inductor for one of my projects that should harvest power from a RFID field.
I use a very thin copper wire (0.2mm) and I don`t really have any clue on how these coils are actually built..

A colleague showed me the inductor inside an access card, but I can`t really extract it since there`s plastic poured allover it..

How are these inductors made? Anyone has any clue?

How can someone make a diy, in-house flat inductor? (not necessarrily 1 turn per level, maybe 2~3, but the end result should be under 0.5mm... I`ll epoxy it later on.

Thank you all!

[Ian.M]

You either need a dismantleable  former that constrains the wire to a single layer as you wind it, (e.g. two rigid disks with nonstick surfaces, bolted together at their centers, with a washer between them slightly thicker than the wire diameter, one of the disks slotted or drilled for the tail at the center end of the winding), and apply a suitable 'dope' to the wire as you wind it,  or for a one-off you could try laying down the coil on thin double sided sticky tape, with a large washer or similar as a former for the first turn, remove the washer and coat the coil with epoxy and a layer of fine glasscloth or kraft paper for strength, and let cure under pressure,  then strip off the double sided tape, solvent clean the coil surface and epoxy coat and reinforce it as you did the other side.

[RoGeorge]

An adapter for 3D printers can do flat coils:  https://hackaday.com/2018/06/16/delta-printer-morphs-into-cnc-flat-coil-winder/

[chemelec]

You could design it as a PCB, than Etch it.

[SiliconWizard]

Some manufacturers make flat wire coils. It's a dead cause to make one yourself unless you have very specific reasons to IMO.
Still, you can use this as a design helper: http://www.tesla-institute.com/!app/sim/fscic.php

A lot of RFID cards or tags use flat coils etched on PCB as traces. You can use this for approximate values: http://www.circuits.dk/calculator_planar_coil_inductor.htm
Be aware that those are meant to harvest very little power.

You said you want to "harvest power" but didn't give any figures. The solution to use depends on the power you want to harvest and target size.

[Fred27]

Most RFID cards can be dissolved in acetone. You could harvest one that way. There are also keyfobs, etc. that might make a good donor.

There are a few devices that are intended to use for RFID energy harvesting and card emulation - e.g. TI's RF430CL330H or ST's M24SR. I'd take a look at some of their reference designs if you want to go for a PCB trace antenna.

[Benta]

The RFID access cards I've seen all use etched flat (PCB-like) coils.

[T3sl4co1l]

Heard about a mod involving a CNC vinyl cutter: the free-rotating blade was swapped out for a wire guide and bearing.  The bed, covered with double-stick tape.  So, wire sticks to bed, CNC path traces the coil, and winding proceeds.  Cover the top with tape, peel it up, and there's your coil.  (Think it was on Hackaday?)

You can do this by hand as well, but it may be too tedious to bother with.  In that case, the other method mentioned (a conventional bobbin windup, but with the "cheeks" only one wire-width apart, thus giving a single layer pancake winding) will be best.  Build the fixture in such a way that you can remove the sides progressively, so you can tape down the winding as you go, without it unraveling.

Tim

[tszaboo]

The 13 MHz cards will be flat with small inductor, the 125KHz cards will be fat with large inductor. Those ones you can pop open with a screwdriver sometimes. Or order some standard parts:
https://katalog.we-online.com/en/pbs/browse/power_magnetics/wireless_power_transmission

[cdev]

I've used copper tape and graph paper in the same size as its width to successfully make rectangular spirals using copper tape. Or better yet, mock it up in a drawing program, and then print it out and use that as your template. Use a piece of totally copper-less pcb in the same thickness to mock up the circuit to test if its resonant in the right ballpark.

When you are sure it works close enough to just need some tweaking, etch it and test, then modify accordingly. Batches of FR4 are slightly different. Keep that in mind if size and VF etc is important. There is a lab at Georgia tech that has a TON of RFID related papers online, I would spend some quality time reading their papers because this antenna stuff is really what they specialize in.

 You can run your coil around the outer edge of a portion of your device working inward with a via in the center for the return part of the connection.