Transfer of charge wirelessly via inductive coupling

Hello everybody! I am not very good at understanding some of the advanced topics of electricity, and I think the only way I'll learn is by doing it and just experimenting. My goal here is to efficiently transfer power through coils by using resonance.

Electromagnetic coils I understand, if a magnet is moving near a wire it creates electricity. I've also seen all the videos showing how resonance is all about timing, and others simply telling us what coil to use and how to make it. What I was thinking is that maybe I could use this function: Arduino - Tone to make the arduino act as an oscillator, but I don't know how to apply this to a project.

Basically, can someone guide me towards using an oscillator with a coil, and calculating how a secondary could receive the power from the primary.

Thank you for reading, sorry for the stupidity. I simply do not understand these things! :slight_smile:

What is your goal here?
How much power do you want to transfer?
What will be connected to the secondary coil.
Your questions are too vague to really comment on.

Let take a little orientation on what already exsists in general:

Power transfer by induction is used in a transformer. Here the coils have VERY close magnetic coupling (by the iron core). This is the only way to achieve good efficiency and low voltage drop.

If you want to transfer information only without respect to power or maybe limit the power to fractions of a watt you can layout a coil covering the whole room andhave another coil as the inductive pickup. This is used for listening devices for hearing impaired in theaters for example.

The third way is to modulate a higher frequency carrier to boost range. In this case its a radio.

What you describe sounds a lot like what Tesla tried...
There are some physical obstacles to that. Mostly that air has a very low permeabilty. This means that you have to use a very large coil and/or very high current to get a high enough flux density. Large coils means high cost and high current means high losses because of P=I^2*R

mauried:
What is your goal here?
How much power do you want to transfer?
What will be connected to the secondary coil.
Your questions are too vague to really comment on.

Oh, sorry. The amount of power is approximately 5V, to power a simple LED.

A simpler way to explain my question I think is how can resonance be implemented? I have tried a lot of things that I really never expected to work, like a big coil and a small coil with same number of turns, put a 9V battery into the bigger and just hope the smaller will get something. I really couldn't get even that to work.

So, how can I achieve wireless power transfer of around 5V simply by the primary's magnetic field, and the secondary picking it up and get some power out of the secondary. Later I would like to try out using resonance and see how much farther I can get it.

Thanks! :slight_smile:

nilton61:
Let take a little orientation on what already exsists in general:

Power transfer by induction is used in a transformer. Here the coils have VERY close magnetic coupling (by the iron core). This is the only way to achieve good efficiency and low voltage drop.

If you want to transfer information only without respect to power or maybe limit the power to fractions of a watt you can layout a coil covering the whole room andhave another coil as the inductive pickup. This is used for listening devices for hearing impaired in theaters for example.

The third way is to modulate a higher frequency carrier to boost range. In this case its a radio.

What you describe sounds a lot like what Tesla tried...
There are some physical obstacles to that. Mostly that air has a very low permeabilty. This means that you have to use a very large coil and/or very high current to get a high enough flux density. Large coils means high cost and high current means high losses because of P=I^2*R

How could I do what Tesla tried on a VERY small scale? As in, a few inches? I've seen some demonstrations of people doing this at around 13 in max on average. Is there any reason besides air that this is the case? I read somewhere that Tesla successfully did this 45 whole miles somewhere in Colorado, I don't intend to try getting past maybe 4 inches.

Thanks for reading :slight_smile:

Without extensive knowledge of eletromagnetic field theory? With a lot of luck and lateral thinking i guess Not to mention long hours and hard work and the ability to endure repeated dissapointment…
One of the most accesible texts on subject is: Introduction to Electric circuits by H.W. Jackson and P.A.White isbn 0-13-477142-7. Originally from 1959, mine is from 1989. I dont know if it’s still in print.

Some basics: e=N*d(fi)/dt, meaning that the momentary value of the induced voltage is the number of turns in the coil multiplied by the rate of change of the magnetic field density. This would be quite easy to accomplish. The bad news is that in the moment you let this voltage generate any secondary current this current will generate its own magnetic field with such a direction that i weakens the original magnetic field generated by the primary coil. And as i stated before, air has low permeability so this weakening of the field does not result in any significant increase in the current of the primary winding as it would in a transformer. This means that the current in the primary winding must be highenough to start with. The effect of the problem is that you get a very large current depending voltage drop in the secondary. This can partly be compensated by connecting a capacitor in parallell to the coil, hence the resonance.

If I was to build something like this i would start with coils maybe 100mm in diameter. Use a H-bridge to drive the primary with some hundred mA. my choice of frequency would be some 10’th of kHz. In addition to that you will have to apply som voltage regulation on the secondary. This could give you a few inches.

I have tried a lot of things that I really never expected to work, like a big coil and a small coil with same number of turns, put a 9V battery into the bigger and just hope the smaller will get something. I really couldn't get even that to work.

What were you expecting? You only get power picked up from an alternating magnetic field. Putting a battery through a coil you get a steady field. The only thing you will see in the second coil is a small spike as you applied the power. We're you looking with an oscilloscope? You would not see it otherwise.
You need to drive one coil with AC and then you pick up AC on the second.

Sorry but this is not advanced electricity it is very basic stuff.

Do you mind how big the coils are? If you want to transfer power wirelessly over 4 inches, and you are happy for the coils to be a foot or more in diameter each, then it should not be difficult. Here are some suggestions:

  • Use 50 to 100 turns of wire for each of the coils

  • For the transmitter, use a mosfet driver chip such as MCP1407. Connect the output of the MCP1407 through a capacitor to one end of the transmitting coil, and connect the other end to ground. Power the MCP1407 from about 10V to 12V. Choose the capacitance so that the capacitor + coil resonates at the frequency you are using. Use a few 10s of kHz as nilton61 suggested.

  • Connect the receiving coil either directly to the LED (in which case, don’t place the coils too close together!), or use a series resistor. For slightly better efficiency, you could use a bridge rectifier made with Schottky diodes. Also connect a capacitor in parallel with the receiving coil, again to resonate at the frequency in use.

  • Generate the frequency from timer 1. Here is some code I use to generate a fixed frequency on a Uno:

// Generate a square wave of a given frequency on the OCR1A pin

#define REQUIRED_FREQUENCY  (38000)
#define REQUIRED_DIVISOR ((F_CPU/REQUIRED_FREQUENCY)/2)

#if (REQUIRED_DIVISOR < 65536)
# define PRESCALER  (1)
# define PRESCALER_BITS  (1)
#elif (REQUIRED_DIVISOR < 8 * 65536)
# define PRESCALER  (8)
# define PRESCALER_BITS  (2)
#elif (REQUIRED_DIVISOR < 64 * 65536)
# define PRESCALER  (64)
# define PRESCALER_BITS  (3)
#elif (REQUIRED_DIVISOR < 256 * 65536)
# define PRESCALER  (256)
# define PRESCALER_BITS  (4)
#elif (REQUIRED_DIVISOR < 1024 * 65536)
# define PRESCALER  (1024)
# define PRESCALER_BITS  (5)
#else
# error Bad frequency
#endif

# define TOP        (((REQUIRED_DIVISOR + (PRESCALER/2))/PRESCALER) - 1)

void setup()
{
  pinMode(9, OUTPUT);
  digitalWrite(9, LOW);
  TCCR1A = 0;
  TCCR1B = (1 << WGM12) | PRESCALER_BITS;    // turn on
  TCCR1C = 0;
  OCR1AH = (TOP >> 8);
  OCR1AL = (TOP & 0xFF);
}

void on()
{
  TCNT1H = 0;
  TCNT1L = 0;  
  TCCR1A = (1 << COM1A0);
}

void off()
{
  TCCR1A = 0;
}

void loop()
{
  // Generate a burst 2ms long, then wait 10ms before generating the next one
  on();
  delay(2);  
  off();
  delay(10);
}

If you don’t have an oscilloscope to help you determine what capacitance you need for resonance, then you can calculate the coil inductance (google “inductance of a flat circular coil”).

nj_scioly:
Basically, can someone guide me towards using an oscillator with a coil, and calculating how a secondary could receive the power from the primary.

There's nothing stopping you from investigating if you're interested, but the problem of wireless electromagnetic power transmission is one that has had a lot of money invested in it in the past few years and still hasn't been solved well enough to make a viable product. The problem requires you to understand how magnetic fields are generated and propagated and how to direct and focus them. If you're trying to do it by connecting a DC power source to a coil this suggests to me that you are several years of education away from understanding this well enough to get anywhere.

If you're willing to consider a less ambitious project then you could just take the base and charging coil from a rechargeable electric toothbrush, connect the coil output to a boost converter and see over what sort of distance you can transfer enough power to operate an LED. I think it'll fall well short of 4" but you never know 'til you try.

If you decide to take your investigation further than that then you don't need an Arduino - you need an electronics lab.

PeterH:

nj_scioly:
Basically, can someone guide me towards using an oscillator with a coil, and calculating how a secondary could receive the power from the primary.

There's nothing stopping you from investigating if you're interested, but the problem of wireless electromagnetic power transmission is one that has had a lot of money invested in it in the past few years and still hasn't been solved well enough to make a viable product. The problem requires you to understand how magnetic fields are generated and propagated and how to direct and focus them. If you're trying to do it by connecting a DC power source to a coil this suggests to me that you are several years of education away from understanding this well enough to get anywhere.

If you're willing to consider a less ambitious project then you could just take the base and charging coil from a rechargeable electric toothbrush, connect the coil output to a boost converter and see over what sort of distance you can transfer enough power to operate an LED. I think it'll fall well short of 4" but you never know 'til you try.

If you decide to take your investigation further than that then you don't need an Arduino - you need an electronics lab.

Hi, thanks for the reply. I am pretty far away from understanding electronics, this is just a thing I've never understood when I was younger. Now that I actually have some time and parts to play with, I thought I'd give this a shot again. Still need to do more research again.

Anyways, I was wondering if I could just pulse it with tone(), and try something like this:

Except with a smaller coil. I was wondering if anyone knows how to calculate the capacitor needed?

Anyways, I was wondering if I could just pulse it with tone(), and try something like this:

Well no. That requires 150 KHz and that is way to high to heard unless younare a bat and so tone does not go that high.

Except with a smaller coil. I was wondering if anyone knows how to calculate the capacitor needed?

Simple if you know the inductance L of the coil. The formulare is given by this jingle:-

One over two pi root LC gives you the resenont frequency.

But then you probbly don't know L

There's nothing stopping you from investigating if you're interested, but the problem of wireless electromagnetic power transmission is one that has had a lot of money invested in it in the past few years and still hasn't been solved well enough to make a viable product.

Murata Develops Wireless iPad Charger – TechCrunch dated 2011

Check above links, viable product.......Sorry PeterH.

However larger power levels and significant distances is probably still on the drawing board/tablet

Tom...... :slight_smile:

Ive repaired a number of Braun electric toothbrushes which use a close coupled inductive loop in the base of the toothbrush to charge the internal AA battery.
The oscillator in the charging base runs at 50 Khz and consumes 1.3 watts.
The power delivered to the battery charging cct in the toothbrush is 25 mw.
The coils are extremely close , around 3 - 4 mm spacing.
1.3 watts in for 23 mw out isnt very good , but in an application like this , the efficieny doesnt matter.
Trying to scale the idea up though , is very hard .

TomGeorge:
Check above links, viable product.......Sorry PeterH.

Check above links, does not transmit power over a 4" gap......Sorry TomGeorge.

A couple of organisations have been developing ways to transmit low power wirelessly over distances from a few inches to a few feet and working solutions have been set up in carefully controlled situations but it's an extremely difficult problem to solve and the most promising approaches seem to use radar-like directed and focussed transmission of EM radiation with a lot of clever feedback to maintain the connection. It's possible but it's very hard and there are no commercially viable solutions available yet. The 'modified toothbrush charger with a boost converter' idea is the most promising one for DIY implementation but is still only likely to give you a range of fractions of an inch.

Grumpy_Mike:

Anyways, I was wondering if I could just pulse it with tone(), and try something like this:

Well no. That requires 150 KHz and that is way to high to heard unless younare a bat and so tone does not go that high.

Except with a smaller coil. I was wondering if anyone knows how to calculate the capacitor needed?

Simple if you know the inductance L of the coil. The formulare is given by this jingle:-

One over two pi root LC gives you the resenont frequency.

But then you probbly don't know L

L is inductance of the inductor, I'll use the above calculator to find that. Your formula is to find our what frequency I need to pulse the coil with right? Does anyone know of a good square signal generator that is not very expensive?

I think I'm starting to understand a little more of how this all works, thank you everyone! Hopefully soon I'll be able to get a small experiment down.

Thanks for the formula by the way.

What I meant was that for your coil you do not know the value of L for the coil you have.
You will have to measure it.

Grumpy_Mike:
One over two pi root LC gives you the resenont frequency.

Oh, I forgot to ask, how can I create a secondary coil tuned to the frequency obtained by that formula?
Does the size need to remain constant, do I need to have the same number of winds, etc.

I know that the capacitor needs to remain constant, just not the specifics of the coil.

Grumpy_Mike:
What I meant was that for your coil you do not know the value of L for the coil you have.
You will have to measure it.

Ah, already hot glued it, I will build a larger coil and actually calculate what I need to get it right. Thank you very much Mike! :slight_smile:

Edit: Oh, and do you know of any cheap signal generators?

No the capacitor can be anything, so long as the product of inductor and capacitor fit that formular.
So get a meter out and measure the inductor.

and do you know of any cheap signal generators

What about using an Arduino?

nj_scioly:
Does anyone know of a good square signal generator that is not very expensive?

Yes, it's called an Arduino.