Monday, December 30, 2013

Building a coil for the ID-3LA

Partially to put my thoughts in order and also for others who might be trying this…

Here’s a list of steps that I performed to build a coil for the ID-3LA which a bought from Sparkfun (here). I found a data sheet online here.  This really is pretty easy to do and since my EE skills are really old and rusty, fun.

Decide on what wire to use. I built two: one using 30AWG magnet wire from my local Radio Shack and also 26AWG magnet wire that I got from my local Electonic Supply. Why 26AWG? According to the Microchip AN678 application (here), anything above “larger” than 26AWG (e.g. smaller numbers but bigger wire), has a lot loss do to skin effect. See the section titled “Wire type and ohmic losses.” I was hoping to get more distance out of the 26AWG coil.

Calculate coil radius and number of turns.  I’ve seen a variety of calculations and suggestions for starting points (including in the ID-3LA data sheet). I built my first coil 30AWG coil using the information in the ID-3LA data sheet in section 12. 6cm radius and 71 turns. I built my second coil with 26AWG and used the calculator at www.circuits.dk for the same size and the inductance from the ID-3LA data sheet. Based on measuring my first coil, width was about 2mm and height was about 2.5mm.

Output from circuits.dk

Build the coil. I measured 6cm for a center point and used finishing nails to make a "form" (see picture). Originally I used some trim coil to "smooth out" the circle, but the wire kept slipping under the trim coil. I decided it didn’t help that much so I gave up. I’m sure there’s a way to make a more tightly packed coil, but I don’t know how much you gain. I basically wound and counted the required number of turns. When I reached the required number of turns, I stopped and taped the coil with electrical tape. I then wound two turns more to allow removal of loops in the tuning process. I taped the whole coil, with the turns that could be easily removed, in a different color electrical tape.
Rig to wind coil
Insert into circuit. To remove the insulation off the magnet wire, I used a lighter and then cleaned with a bit of steel wool. Seemed to work acceptably though I’ve seen other suggestions. My magnet wire did not appear to have “solderable” insulation. For the 30AWG wire, I attached some 22AWG solid conductor wire. To do this, I tinned the 22AWG wire and tried to tin the 30AWG magnet wire (which didn’t work to well). Then I wrapped the 30AWG magnet wire around the tinned 22AWG wire and heated. Seemed to work but there’s got to be a better way. For the 26AWG wire, I inserted the magnet wire directly into the breadboard.
Completed circuit
Tune and test.   I hooked my scope (a DSO Nano) up across the leads of the coil and started measuring. You can start to see the limitations of a $90 oscilloscope. ;) One thing I noted was a sine wave at ~360 Hz (picture). I didn’t investigate this. When I set the time division to 1 microsecond, the 125khz signal shows up clearly.

360Hz Harmonic

12khz Signal

Note:  I tried measuring with Vrms with my Fluke 115.  It gave me a Vrms of 1.5V and a frequency of 32kHz.  Maybe it's my wall-wart power supply?

With the 30AWG wire, my Vpp was 30.8V. I was able to read a tag (used a token hooked to an Arduino) but the range was very low. I remove a coil at a time and was able to increase the range up to Vpp of 109V — and at that voltage, couldn’t read the coil. I didn’t investigate this but theories would be Q was too high so that the influence of the token really had a negative effect or maybe this was more voltage than the ID-3LA could handle. I added one coil back and got a Vpp of 76V. With this, I got a read distance of about 2 inches.
30mm 125khz token from Trossen Robostics
With the 26AWG wire, I was able to get a Vpp of 77.4V and a read range of 6.5".  Is this about the max that can be gotten out of the ID-3LA?

Note:  Some have suggested tuning by measuring the current consumption of the ID-3LA.  I didn't try that.

For curiosities sake, I decided to order and directly measure the inductance of the 26AWG coil with a Sinometer UT58D.  It gave me an inductance of 1.332mH, an Rdc of 3.9 ohms, and a capacitance of -37.4 micro F.  I was pleasantly surprised that the inductance closely matched about what I wanted.

The bottom line though was about 6.5" of read distance.  I'm hoping to get about 16".  So, my next project is to do a little research on how to increase the read distance.  Supposedly increasing the radius of the coil can increase the distance.  But before I go to the extra effort of winding another coil, I'm going to do some calculations to see if this would actually provide benefits.  I suspect, and others have noted, sooner or later its about power.  If that's the case, I'll need to build my own circuit.

Open questions:
Why does the Fluke read values so differently?
Should I have fine-tuned with a capacitor?
Why did the ID-3LA fail to read when I tuned Vpp up over 100V?
What really is the best way to strip the insulation off of the magnet wire?

Next step:  Try to remember enough emag to perform some calculations and investigate the feasibility of increasing the coil size.


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