Good afternoon, I am a high school student and in my scientific fair project, I would like to do an experiment about wireless energy.

I want to implement the resonant inductive coupling.

According to what I have read and researched:

The advantage of the resonant inductive coupling, compared to the inductive coupling, is that, when the receiver and transmitter coils are on the same frequency, the loss of energy is much less.

In addition, the diagram is: (the image is from wikipedia post about resonant inductive coupling)

enter image description here

And this, is other diagram: ( the image is from this pdf)

enter image description here

You will understand, that I do not have much knowledge of this, for which I want to know if what I understand of the diagram, is correct. If not, I need to know how it works.

According to what I have read and researched:

The resonant circuits are the coils, which are made of insulated copper.

The coil on the left has a "resonant antenna" or "resonant coil" on its left, which is a circle of isolated copper wire, i think that is the D in the second diagram, because is connected to to what is providing the energy, a battery or something like that. This "resonant coil" is connected to an electronic oscillator, what I do not understand for what it serves in this. And this electronic oscillator is connected to what provides power (battery or something like that).

And on the right side, it is the same, but instead of an electronic oscillator, there is a rectifier, which, as I have been investigating, I must use it to transform the alternating current into direct current (since this current is the one used by electronic objects).

So, my doubts arise and they are:

What is the oscillator for in this experiment? Why are two copper circles used, instead of directly the resonant coils? Is my understanding of the diagram correct?

In addition, there are some symbols that escape me, I think one is the resistance and this is to control the voltage, but the other 2 do not know what they are, I have been looking for, but no symbols appear like those.

I would greatly appreciate your help, I do not have anyone else to ask, my teachers are not too good.


closed as too broad by Andy aka, Neil_UK, winny, MCG, Dmitry Grigoryev Jun 1 '18 at 11:44


  • 2
    \$\begingroup\$ There is a lot you will have to learn to get to the bottom of this properly and therefore your question has to be regarded as too broad for this type of site. Just explaining why an oscillator is needed and faraday's law of induction is enough without going into why resonance is used (not the reason you assume) and what inter-coil distance advantages this leverages. There is a lot of theory that you are probably not well-equipped to understand too. Ask a simpler question. Start from a simpler angle. \$\endgroup\$ – Andy aka May 30 '18 at 18:01


Remember a setup with two coils is like a transformer with space in between. An inductor with a steady field can't transmit energy to another inductor. To create a voltage change in a loop of wire the field either
1) Needs to change over time
2) the loop needs to be moving across a magnetic field that has a gradient

In both of these situations the magnetic field is changing, and the voltage across the loop changes with the magnetic field. If the magnetic field does not change, then no voltage change and no energy transfer.


The first box means a sine wave generator for creating the oscillations in the first coil. The rest are simply loops.

Look at a Rectifier circuit to understand why you need to convert the voltage back to DC. You don't have to have a rectifier if you need AC current for your load.

  • \$\begingroup\$ Thank you very much for your answer, I really need the "sine wave generator"? \$\endgroup\$ – Mattiu May 30 '18 at 21:40
  • \$\begingroup\$ And how is that of a gradient field? , could you explain it to me in simpler words, please \$\endgroup\$ – Mattiu May 30 '18 at 21:40
  • \$\begingroup\$ Instead of asking for an explanation, look it up (it's a skill you need), then ask if you still can't figure it out. I'll explain it anyway. A gradient is simply a slope between two points. If your standing on a hill, the gradient would be the slope. If your standing on flat, there is no gradient. Gradients can be with anything, if you have a hot spot and a cold spot on a material there is a temperature gradient (a slope of temperature). If you measured all the points between the hot spot and the cold spot, they would decrease, and you would be measuring the gradient. Same goes for magnets. \$\endgroup\$ – laptop2d May 30 '18 at 22:40

The principle of this type of charging is mainly based on two of four Maxwell Equations. Namely:

Ampère's circuital law

When a current flows through a piece of wire, it generates a magnetic field that circles around that piece of wire like a tornado with the wire in the middle (nature chose to make that magnetic field around the wire to go clockwise if you look along the current apparently). Now if you turn the wire with current into a loop-shape, the magnetic field tornado will turn full donut, and all the magnetic fields in the middle will all go in the same direction, making it stronger. If you make multiple loops, its strength gets multiplied even more!

This is how magnetic fields are generated. Ever made an electromagnet? Exactly the same: you turn a wire around a piece of metal, and all the small magnetic fields of the current through the wire magnetize the piece of metal with a push of the button.

Wikipedia image

This image shows the currents and magnetic lines as they are generated by a constant current. It is a cross section of a few loops of wire, and the magnetic field lines that go through those loops. The \$\bigodot\$ represent the pointy side of an arrow, and it means that current there is pointed at you. The \$\bigotimes\$ represent the backside of an arrow, and that's where the current is running away from you. You can imagine the loops by connecting the \$\bigodot\$ and \$\bigotimes\$ to be connected on the front and back-side of your screen in a circle. The arrows are then just the magnetic field lines, as simple as that.

Faraday's law of induction / Maxwell-Faraday Equation

The opposite turns out to work as well: If you force a magnetic field through a loop of wire, you will generate... uh... nothing actually.

That is, if you keep the magnetic field strength constant anyway. Once you start changing the magnetic field, you see that a current starts to flow, that circles again like some kind of tornado confined in the looped wire. So actually, the situation is rather similar to the last image, except that the current is proportional to the rate of change of the magnetic field.

Putting it together

Because Faraday's law requires a constantly changing magnetic field, we need our source of magnetic fields to be constantly changing as well: this is what the oscillator does. It generate a constantly changing, oscillating current inside the magnetic field generator. An endless repeat of magnetic fields pointing left and right, will lead to currents positive, then negative (the other way), at the same rate of the oscillator.

The resonators are used kind of like batteries for the oscillating magnetic field. They take a while to get started, but they also take a while to die out. Once you can get one started, they kind of stay connected together, which can extend the range of the magnetic field lines.

A second concern is this:

OK, we made magnetic field lines, and we know how to turn them back into current, so how do we actually make sure that all our magnetic field lines are captured by the receiving end? Any magnetic field lines that do not go through both these loops, will not be able to generate current, and so will unfortunately be quite useless. Well, for a good transfer of energy, we need the loops to be close to each other. That's why it'll probably never be a long distance thing.

Then we have our last (but not least) concern:

Charging most devices requires a constantly positive current, not a current that changes from positive to negative all the time. Fortunately, we can build something called a rectifier, who's job is just that: rectify the current, and make it go only in one direction: positive. And then it can be used to charge your smartphone... WIRELESS!

I hope it helps with your project!

  • \$\begingroup\$ Thank you very much for your reply. The only thing that I still do not clarify, is why do I need two wire circles, why, I do not do it directly with the two coils, and without those wire circles? I have seen that the "inductive coupling" diagram is without those two wire circles. But, the "RESONANT inductive coupling" is with those two circles, so what do they do? \$\endgroup\$ – Mattiu May 30 '18 at 21:46
  • \$\begingroup\$ You don't need the resonators to make it work. They can resonate with the oscillating magnetic field and they store that oscillation energy, a bit like a small battery. You can also use them to change the magnetic field lines to have a better "connection". \$\endgroup\$ – Sven B May 30 '18 at 21:54
  • \$\begingroup\$ So, those wire circles, are they to change the lines of the magnetic field? and how is that better "connection", it means that there will be less loss of energy? \$\endgroup\$ – Mattiu May 30 '18 at 22:05
  • \$\begingroup\$ Well in order to have good coupling, you want the coils to be as close as possible to each other. Then no magnetic field lines can escape, and all energy will be transfered. Or you can also add these resonators in between the two to extend the magnetic field lines, but then you'll still need the resonators to be close to each other though to catch all magnetic field lines. \$\endgroup\$ – Sven B May 30 '18 at 22:21
  • \$\begingroup\$ My explanation wasn't quite right there, so I edited it. \$\endgroup\$ – Sven B May 30 '18 at 22:29
