Lately, I have been messing around with inductors and wireless power transfer. I have studied couple online tutorials on RLC circuits. I thought I have basic knowledge and good ground to actually build something, but sadly no... I am trying to couple two air core inductors, I tried lots of combinations so I will only show the latest one

Rx and Tx coils, Both have value of 1.6uH

Tx 1.6uH with 900pF cap in series. Rx 1.6uH with 900pF in a parallel combination, later I added 100ohm resistor in parallel to lower damping factor. Resonant frequency isenter image description here 4.19410MHz.

I drive mosfet with a square wave from a function generator at the resonant frequency. And I get nice sine wave between cap and inductor. I am using peak detector circuit to measure voltage drop when I get two coils close. But voltage drop never occurs, so I am pretty sure coupling doesn't happen. Anyone know what the problem is? I used different types of inductors for Rx as well, drum core, resistor shape ones with different inductance and cap values, but with no luck so far.

Maybe the problem is related to Q factor of Rx coil. I am not sure if I understand Q factor correctly but according to online calculator dumping factor of receiving coil is 0.21 so it is underdamped and its Q is rather high, but the bandwidth is pretty high too, almost 10MHz, not sure how that affects receiving coil.

EDIT: The reason I chose single layer coil is that I want to design flat double sided PCB coil later with a spiral shape. That's why I am experimenting with one layer spiral coil design. Sorry for not inclouding this information from the beginning

https://i.stack.imgur.com/mkI6N.jpg coils picture.

https://i.stack.imgur.com/JlIAM.jpg circuit schematic.enter image description hereenter image description here

  • \$\begingroup\$ Edited in the images for you, despite out of focus and not cropped, we can see what it is. Upside down ground symbols though, I'll let it slip for now but I can't answer for everyone else ;-) There is a built in schematic tool here which you can use. \$\endgroup\$
    – winny
    May 22, 2017 at 20:25
  • \$\begingroup\$ The way you made your inductors results in only very weak coupling between the two. The radius (that's where the H-field is !) of the coils is very small and the receiving coil should "catch" as much of thet field as possible. Look at how for example Qi based systems are build, google: "qi wireless charging coils" and look at "images". Note how these coils are flat, have thick wires and some magnetic material. The circuit can never deliver deliver more than a snippet of power, you need a proper push-pull stage. \$\endgroup\$ May 22, 2017 at 20:26
  • \$\begingroup\$ For the purposes of learning and experimentation, try building your inductors on ferrite rods. \$\endgroup\$
    – sstobbe
    May 22, 2017 at 20:41

1 Answer 1


Your coils are all wrong. You need a much bigger radius and you need to couple them face to face like this: -

enter image description here

That is how to couple magnetic fields. The closer the two coils are the more the coupling. Here's the theory: -

enter image description here

You can use parallel or series tuned coils for the transmit coil but parallel tuning works most effectively on the receiver on light loads.


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