This guide will provide information to assist the reader in making an RFID reader coil.
A good reader is very dependent on a good coil.
A RFID reader generates a clock frequency, e.g. 125 KHz, then for proper LC resonance to occur the coil inductance and series/parallel capacitor must be balanced for said frequency.
When the inductance and capacitor is well balanced at the said frequency then the voltage over the coil will be much higher than normal.
For my DIY kits, the Arduino generates a 125 KHz frequency, and the capacitor value is 1nF.
The only element that can be adjusted is the coil diameter, wire thickness and amount of turns. The easiest to manipulate is the amount of turns.
The coil wire included in my kits is already 120 turns of 32awg wire with a coil diameter of 55mm. Another layer of tape is used at layer 100 as a place holder, just in case the wire jumps loose or if you lose your count.
To properly tune a coil for your board do the following:
- Board must be fully constructed and operational.
- Attach the crimp terminals each to about 5cm of wire.
- Remove the enamel from both coil ends; use a sharp blade to scrape it off.
- Connect the coil wire to the two wires with the crimp terminals.
- Attach the terminals to the board, it does not matter which one you connect to which side.
- Switch on the board.
- Use a multimeter, in ac voltage mode, measure to voltage across the coil terminals.
- as a start, you should measure about 100 V AC. Carefully unwind one winding at a time and re-measure.
- The voltage should increase and you reach the best point and then start to decrease again. You might see voltages of 180 V AC.
- Once you have the correct amount, in my experience it is about 108 turns. Remove the terminal wires again, remove the excess coil wires and neatly connect the terminal wires, this time consider that the terminal wires will form part of the coil windings, so cater for that as well.
- Use narrow strips of insulation tape and tightly secure the coil wires. Making the coil “solid” also increase the voltage over it.
- on average my coils measure about 250 V AC, but I have also managed to get coils in the 400 V AC range.
Hope this guide helps in making a good working coil. If you have access to an oscilloscope, that will me even better, but you should be able to manage with only a multimeter.
Remember, the higher you get the voltage, the better your reader will perform, but should work ok with anything above 180 V AC.