Introduction: I Need Your Help. IPT (Inductive Power Transfer)

Picture of I Need Your Help. IPT (Inductive Power Transfer)

THIS INSTRUCTABLE IS NOT YET COMPLETE. I have been trying to build a wireless charger but have come across a problem. I have documented what I have done and learnt so far. Hopefully from this you can see my errors and help me out. Please read the last page to see what I think the problem is and suggest any ways to get around this. THANKS

The Project
I am trying to build a circuit that will allow gadgets that are usually charged by USB to be charged wirelessly. As an example I am reverse engineering an A4tech battery-less mouse. However it is too great a challenge for me and I am seeking help from you. I thought it would be better for me to turn this into a group effort than to ditch the project. I will give a detailed description of what I have built and learnt and hopefully you can tell me where I went wrong.

Step 1: Background Information

Picture of Background Information

Transferring power wirelessly is reasonably simple. If you think about it, all transformers are wireless. But we want something that's truly wireless. Like the Splashpad (see picture). It is pretty much a transformer with an air core.
The mouse I was talking about (see other picture) in the introduction is exactly the same as the Splashpad it uses induction to transfer power across an air gap. This is the same technology as RFID; in fact it uses this to communicate with the pad. To make our own wireless device we need to know more about induction.

Step 2: Basic Theory

Picture of Basic Theory

We need two coils. A primary and a secondary. The primary will be connected to an AC power source that creates an alternating magnetic field spreading out into space to infinity; however as we get further away from the primary coil the strength of the field dramatically decreases. Now, if we hold our secondary coil anywhere in that space an alternating current will be induced. Unfortunately this current is so low that it is too low to do anything useful or should I say too low to even measure. Gigantically low, so low it might as well be nothing. I think you get the point. So the question is; how do we Increase the power that is transferred to the secondary coil?
It is called inductive coupling of tuned circuits (Here is some more detailed information, more tesla coil orientated but more or less the same principles.
We need an AC power source for the primary, any frequency would do, but to make it more efficient we need a high frequency. To explain this think of two solenoids. If we hook up the primary coil to a DC supply. Initially the current will be zero, the current will increase at a decreasing rate due to a magnetic field that is created to oppose the change, once the two reach equilibrium the current reaches a steady maximum (can be calculated by I=V/R) and the magnetic field also becomes constant. This changing magnetic field induces a current in the secondary which produces a magnetic field in the opposite direction. This is just a small blip of current that lasts just for a short period of time. We have just transferred a small amount of power. Now if we did this over and over again we would induce more tiny blips and hence more power. This is why a higher frequency would transfer more power.

The battery-less mouse has a frequency range of about 119kHz to 135kHz, which is what we will use; it is probably a legal frequency???

*The third image I grabbed from a lecture slide, the lecture had no name on it. If someone objects to me taking this slide, please let me know and I'll remove it.

Step 3: Resonator

Picture of Resonator

So we have a high frequency power source, now we need a primary coil, this coil shouldn't be too large so that it is more like a pad than a brick. It doesn't really matter too much but we want something like the mouse/Splashpad.
To decrease the resistance/Impedance of the primary circuit we need a coil in series with a capacitor. So the final circuit will look something like the circuit on the left in the image below. The capacitor and inductor make something called a resonator. If the capacitor was fully charged and then connected in parallel with an inductor (right-hand side of image) we would get an alternating current flowing through the two components. This is because the capacitor discharges through the inductor, a magnetic field is created. When the capacitor is fully discharged the current stops, the energy of the magnetic field is converted back into electricity; this recharges the capacitor. This cycle then repeats many times. This type of oscillation is called resonance if the reactance's (see next sentence) of the inductor and capacitor are equal. Reactance is the equivalent of resistance in a DC circuit.

Let's take a look at my high school notes book.

Step 4: AC in Resistors

Picture of AC in Resistors

The current and voltage are in phase

Step 5: AC in Capacitors and Inductors

Picture of AC in Capacitors and Inductors

The current and voltage are out of phase
The resistance is not easily calculated as AC resistance is affected by frequency, We therefore use a different physical quantity called reactance, it still has the same unit as resistance. The ohm.

It is clear that frequency affects the resistance and hence the current. See second image

Step 6: LCR (Inductor,Capacitor,Resistor) Resonant Circuit

Picture of LCR (Inductor,Capacitor,Resistor) Resonant Circuit

This is slightly different from the LC circuit mentioned above, but the same principles apply.
The graph pretty much explains how the resistance of a resonant circuit decreases if the reactance's of the capacitor and inductor are equal. The frequency that this occurs at is called the resonant frequency.
(see picture)

Note: the circle diagrams with the arrows are phasor diagrams that represent the magnitude of the quantity at a moment in time. In the resonant circuit above the reactances of the capacitor and inductor are equal and opposite having the effect of cancelling eachother out.

The Whole picture
This theory helps us understand that if the primary and secondary circuits in our wireless circuit are at resonance we increase the effieciency. No power is lost in the inductor and capacitor.

Step 7: The Circuit (AC/square Wave Generator)

Picture of The Circuit (AC/square Wave Generator)

We need to construct a circuit that can produce alternating current at a desired frequency. My first attempt was using a Multivibrator ( However that didn't work unless it had a separate power supply, whenever I attached the load, the change in current due to a parallel circuit would affect the rate of the multivibrator. It also isn't very accurate because it relies on the accuracy of the values of the capacitors and resistors. Take a look at the page anyway because it is quite interesting to know that the multivibrator can make a flip flop.
Another way would have been to use a comparator as a square wave generator ( The maths looked complicated so I soon discarded that idea.
In the end I decided to use a 555 Timer. We have probably all heard of this universal device. As far as I know it is quite an accurate timing device and is rather simple to use. We are going to use it in its Astable configuration. Below is a small excerpt from a webpage ( with everything you need to know about it.

Step 8: Choosing Resistor Values for 555 Timer

Picture of Choosing Resistor Values for 555 Timer

We need to choose resistor values that will give us a frequency of about 120kHz. I happen to have a 181K capacitor.

Lets use the formula from the previous webpage to calculate the value of R2 (in the picture below).
(see second image)
This gives us a result of 32407.41. The closest resistor value I have is 33kOhm, which has a value of 32.5kOhm when measured with a multimeter. We will use this more accurate measurement to work out the true frequency of our circuit using the following formula
(see fifth image)
Our true frequency is 119658.12 Hz

The value of R1 has to be about one tenth of R2 to make the mark and space time approximately equal! So, 3.3kOhm

Step 9: Square Wave Generator

Picture of Square Wave Generator

Now we need something that takes this signal and turns it into AC using a DC source. I have come up with various circuits. Below is the original circuit I designed. I first tested this in combination with the multivibrator but it affected the rate of the multivibrator. I thought it didn't work because of this and decided to look at google patents for another idea. Check these out, there's some interesting stuff.

Step 10: AC Generator

Picture of AC Generator

With a bit of research I found many varieties of DC to AC inverters/converters. Many were complicated and so I went back to the drawing board. The circuit below is what I came up with. It uses a voltage divider to create dual voltage power supply. In my case +3V and -3V with a neutral centre tap. How does it work? (see diagram)

We have alternate current through the resistor by the centre tap.

I have tested this and it does work at low frequencies for testing.

Step 11: Primary Coil

Picture of Primary Coil

All we have to do now is decide on the coil shape and size and the capacitor that will create a resonant circuit. For testing I have used primary/secondary coils that are of similar size so that they can be wound on the same pipe, just for simplicity. There are many online calculators for calculating inductance (See the external links for some of the calculators I looked at). I decided to use a program called MiscEl (, its freeware!).

First choose a capacitor and calculate its reactance. Then calculate the required inductance you need to create a resonant circuit. I have used the values I used in my circuit.
(see image)
MiscEl can work backwards. Enter the inductance and the diameter of the coil. It will then give you the number of windings needed to produce that inductance. Make this coil and attach it to the capacitor in series. Ok thats the primary circuit complete. As you may have noticed, all my work has been done on a breadboard. That's because it doesn't work yet. When I get this thing going I'll make all the printed circuit boards and modify this instructable with all the correct values and dimensions. Your help is appreciated.

Step 12: The Secondary Circuit

Picture of The Secondary Circuit

As you would have guessed. This circuit is also at the resonant frequency. Using the frequency of the primary circuit we will calculate the value of the capacitor and the value of the inductor. The secondary coil cannot be too large as it needs to eventually fit onto one of our electronic gadgets. Therefore we want a small number of turns. When I originally thought of doing this I was going to use a flat spiral coil. I will do that as soon as I get this working. The final secondary circuit will be all on one circuit board, for ease of production.

Anyway. I have a variable capacitor that I should use in conjunction with a fixed capacitor to allow us to fine tune our circuit. I have a variable radio capacitor with a capacitance of up to 220pf when the two internal capacitances are connected. We do this by connecting the two outside leads (A &O;) of the variable capacitor. Like in the picture below.
We also attach a capacitor in parallel with the variable capacitor to increase the capacitance. I used a green cap with the markings of 104J (100000pf). Using this information we can once again calculate the reactance's and hence the dimensions of the coil using MiscEl.
(see second image)

Step 13: Finally, the Problem

Picture of Finally, the Problem

We can turn on the circuit and see whether we induce a voltage on the secondary coil. Now unfortunately it doesn't work. I have analyzed the mouse and its circuit to find out what the difference is between my circuit and theirs. One significant difference is the voltage across the primary. I measured a voltage of 8V. The voltage on mine was about 0.4 volts. But how is this possible. A low resistance coil with a high voltage. If we apply ohms law we would find that the current would have to be huge. But it's a USB powered device and is restricted by the maximum current the computer can supply. And this current is very low.

This is where I'm stuck. How can I increase the voltage across the primary coil?
Below are some photos of the A4Tech mouse. The secondary circuit of the mouse is exactly like mine, minus the rectifier which I will add later. The primary is similar too, a capacitor in series with the coil. A coil, which has very little turns and a low resistance.

Please, if you are good with electronics and physics. Could you suggest some ways that would make this thing work and point out any errors I have made?

Step 14: Update 1

Picture of Update 1

Below is a short video clip of what I have built so far . All the plans for the circuitry with the values you see in the video clip are below.

For some reason I cant get the video to play. The link below is to the video above

Step 15: References So Far

This file contains click on links with descriptions.

This file was created on a mac with opera. If it doesn't open try opening it with a text program like notepad or textedit

Step 16: A Part IV Project Report on 'An Inductively Coupled Universal Battery Charger'

I know its long, but if you're interested in the power transfer part its definately worth reading.

Thank you to Cerincok who brought this to my attention!

Step 17: Microcontroller Info

If you're interested in using a microcontroller to generate the signal.

See PDF's


tayyary (author)2016-01-22

send me wirless power transfer project circuit diagram input ac 230v 50 HZ and secondary coil side 12v 40 khz or other valid circuit daigaram on this mail or look project category...

MukeshM11 (author)2015-10-24

Please send me wirless power transfer project circuit diagram input ac 230v 50 HZ and secondary coil side 12v 40 khz or other valid circuit daigaram on this mail

lawa2010 (author)2014-01-18

Hi , i want to build wireless transfer coil antenna to get 12 v and 5 v , how do i calculate the number of winding of the coil and the diameter and thickness of the coil , whats the right formula to be calculate to get the right coil please.thanks

kamalkannan12 (author)lawa20102014-10-18

I've the same doubt..

odij (author)kamalkannan122014-12-03

So do I :p

TiagoS1 (author)2014-09-12

Provided you have matching tuned LC circuits, you need to excite the primary coil with an alternated supply. If the magnetic field don't alternate you can't pick up the power on the secondary coil. Only some weak spikes caused by the induction feedback... From what I saw, I think that your problem is on the excitation circuit. A simple 555 can't do it. Also, the problem is not about using sine or square waves... Just alternate it from negative to positive and you will have a good surprise :-)

Try using a small micro controller (e.g. arduino is easy) and an h bridge to drive a coil with a square alternated signal. You can do this with a 555 and some gates and the fets... But will require more know how.

Have fun :-)

LesB (author)2014-02-21

Regarding your question about voltage across the primary, I assume you mean a.c. voltage. The drop across the primary is proportional to the impedance of the primary at the signal frequency. This will depend partly on the reflected impedance from the secondary. And, I couldn't find the refrence to your operating frequency, but check the specs on your meter to see if it measures at that frequency.

One solution If your portable device can be kept in one position during power transfer and it does not need to be movable during transfer like the mouse, then you might want to consider taking a ferrite core and saw it in half (probably need a carbide blade for this). On one half wind your primary, on the other half wind your secodary. Mount the halves of this transformer in their respective devices so that you can re-contact the two halves by placing one device on top of the other.

This way, using a ferrite core, you would have better coupling and you could operate at a lower frequency than with an air core transformer.

nerd7473 (author)2013-10-31

interesting I dont know how I could be of help private message me if you have any thoughts on this

49percentGood (author)2013-07-21

I think you would be better off using a Crystal Oscillator instead of a 555 based timer. For one, you'll be generating a much much higher frequency than a 555 could. They're limited to about 500Khz depending on manufacturer. Check out
Alan Yate's site has a very nice outline for this entire project. Highly recommend checking his stuff out. About higher frequency, the higher your frequency the less inductance you'll need.

Inquisitive30 (author)2013-07-16

I'm just new to the electronics world so if anyone could answer me. I have a 1k and 10 ohms resistor, and I need to have a default of 1 minute on 555 timer, did I have the right value? I would appreciate any reply. thank you

Inquisitive30 (author)2013-07-16

I'm just new to the electronics world so if anyone could answer me. I have a 1k and 10 ohms resistor, and I need to have a default of 1 minute on 555 timer, did I have the right value? I would appreciate any reply. thank you

aaronmurray (author)2013-03-14

Great post, thanks for all the accompanying info. Is it possible to dim the secondary by way of reducing the voltage through the primary or does that mess up the tuning of teh coils?

akshaya ladwa (author)2013-01-07

i need to charge my mobile by wireless system using wifi radiations & signals

davelanda (author)2012-11-04

very well...

MadScientist101 (author)2011-05-05

how many microfarads is 181K ?

180 pF
.00018 uF
here's a handy link:

awmayhall (author)2011-01-08

So I tried using your circuits in a circuit simulator. I couldn't get them to work properly.

ARJOON (author)2010-08-16

i need to connect one of the circuits to a transformer without a centre tap how can i do it

Aiden1015 (author)2010-06-03

 I think your problem is you're having 2 different resonating frequencies. the primary may be at one, but the secondary is at another, thus a low, almost 0 voltage. the easiest thing would be to play with inductor lengths. or figure out which capacitors on the mouse, with the inductor, create the resonator, figure out the capacitor value and calculate the needed inductance for your secondary.

cerincok (author)2007-12-30

From your video, the circuit seems to be working greatly. But less power are transferred. What voltage did you get at your secondary coil?.

puffin_juice (author)cerincok2008-01-01

I dont think my meter can measure such low voltages at such high frequencies. I got a reading of 0.015mV while the LED was glowing.

BinnX (author)puffin_juice2010-04-01

Can you tell me which transistors you used.... its no. and what is your wire gauge for primary secondary coil.

cerincok (author)puffin_juice2008-01-01

Yep...i think you're right. Maybe you should use an oscilloscope to measure the exact voltage value..

faham (author)2010-02-18

Your problem is simple: you have an impedance mismatch. This means that your primary coil is not well matche to the signal source, and to make it work you will need to match the signal source impedance to the coil impedance. Also you can consider the coil and the parallel cap as an LC resnance circuit acting as once circuit. You need a transistor driver and voltage multiplier, do not worry too much about the current now but please lift up the voltage as of now so you can get more transfer of power. Also remeber that the USB is limited in power and the manufacturer have made accurate calculations on how much power to transfer vs matching the impedance. What I would do is the following: The antenna coil must be driven by higher voltage higher current signal source first, since the USB cant provide higher current lets focus on higher voltage. Now charge up this high voltage in a cap and drive it to the coil via a MOSFET transistor with low ON rDS resistance and proper gate charge. I bet once you do that you should be able to then measure much higher voltage than what you reported. The secret to solving your problem is impedance matching... Please let me know how it goes and call me if you have a problem. Good luck... By the way I worked with high voltage security circuits and TV and power transfer circuits and I can help you as my time allows me. 

Gjdj3 (author)2010-01-18

I don't know if you still check this instructable, or if you're even an active member of the community, but I think I may have found the problem.

You say throughout the instructable that you want a high frequency. For an inductive coupling application, however, high frequency is not a good thing. To fix this you could try adding a capacitor in series with the coupled signals, as sort of a filter. You could also try lowering the frequency.

arhodes18 (author)2010-01-12

 Can anybody explain the part of the schematic he added in step 10 I can't for the life of me understand it!!

hms1018 (author)2009-12-22

Check out this person's instructable!     It's pretty cool!

framistan (author)2009-12-17

It is possible your circuit is using more milliamps than your charging circuit is delivering...  Ohms law works only when your source of AMPERAGE is UNLIMITED...  In the real world, you are limited by what your SOURCE can deliver.  For example, if you place two tiny 9volt batteries in series, you will have 18 volts... connect this to your automobile in place of your car battery.  Ohms law says it should work... but you know there are not enough AMPERES AVAILABLE to do that job.  The same is true on little circuits that only draw milliamperes.  If the current source is too low, then amperes will not be delivered and THEN you see the voltage sag down low as result.

DarStarr (author)2008-07-08

Dont power it off the pc. Grab a more powerful adapter that plugs straight into the wall.

Masteroffencing (author)DarStarr2008-07-29

i agree with darstar u should just get a wall mount for it

Yeah I also agree. I might try to start my own project for my black berry. I will post my figures as soon as i start.

viking_r (author)2008-10-28

HI, I am working on the same project.(for my final year). right now i am working on the primary coil and acc. to my calc. i need to transfer .24 henry though it. for that purpose i need a 32awg wire(copper). my question is do i have to use copper windg. or can i use aluminium wndg also??? i think puffin_juice is usin al wire in video. for rest of the circuit i am currently using function generator.... one more thing is it possible for you guys to post the full circuit diag.(or the one used in the video by puffin_juce) thanx

puffin_juice (author)viking_r2008-10-29

It doesn't make a difference, any conductor that is carrying current will produce a magnetic field.

frikkie (author)puffin_juice2008-11-12

the only difference with copper and aluminium wire will be resistance with heat i believe

Oorspronklikheid (author)frikkie2009-12-10

not really when magnetizing  any element and demagnetizing it again you get a energy loss called hysteresis loss. basically it takes energy to magnetize an element and energy again to demagnetize it again. copper has a very low hysteresis loss compared to other metals, in other words it doesn't stay strongly magnetized.

The aluminium wire, (or the lower resistence wire) will draw a higher current, this will mean that a larger magnetic field is induced. if you cooled the primary circuit, you would notice an improvement (although alot of energy would be wasted on the refrigeration.

viking_r (author)puffin_juice2008-11-02

is it possible for u to send primary circuit diagram?? if yes pls send it to Thanx

killersquirel11 (author)2009-12-10

Also, with what little I know of electronics...
Would increasing the voltage and number of coils on the primary also provide a marked improvement?

EvilMika (author)2009-11-24

I'm currently working on a similar project.  Have you looked into using a FET Driver instead of a 555 timer?  This will lead you to much higher voltages and current, thus a stronger magnetic field. 

Also, how did you wind such nice coils?

killersquirel11 (author)EvilMika2009-12-09

4 Nails and a wooden board
4 pegs and a pegboard
4 pencils and two friends
A very well-trained cat

...basically anything that has the ability to have four rectangularly spaced supports ought to do the trick

breadpig (author)2009-12-01


Anyone has infomation on how to determine the mutual inductance or coupling factor? And also the equation to determine the inductance of the coils?

hope if anyone can help me :(

EvilMika (author)breadpig2009-12-03

Breadpig, this is simple an air-core transformer so M should follow as such:
M=N1*N2*Phi=K * (L1*L2)^.5
where N1 is the # of turns on the primary, N2 is the # of turns on the secondary, and Phi is the permeance of air.

From this you can extract K, the coupling factor,
K= (N1*N2*Phi) / (L1*L2)^.5

As for determining the inductance of the coils, if you do not have a multimeter or some other device to measure this, you can sweep the frequency on a frequency generator looking at the voltage across a parallel cap and your coil.  The peak voltage is then related to 2*pi*f = 1/(L*C)^.5

paulm (author)2009-11-18

 Make sure you are perfectly aligned with the primary coil, at an angle of 90 degrees, you will get no power.

also, where is the power regulation for the mouse?

freeh18 (author)2009-11-16

 why are you using square wave?
why not sine wave ?

robotkid249 (author)2009-07-26


What is you simply had an wall wart that supplied 9VAC after it conevrted the '120 VAC. Then run a transistor so the 555 timer, turns the Wall Wart on andoff at 120 kHz, providing you with the 120 kHz square wave, AC signal at9VAC

edfel01 (author)2009-06-16

wow u put alot of work into this good job even if its not done yet

headcipher (author)2009-05-16

Have you tried using a much lower frequency? I know it may not be exactly the same thing, but I am a utility locator, and I use inductive antennas daily to apply a alternating current to a pipe or wire. I do not pretend to understand the physics, but I can tell you the results I see. When using induction I have various transmitters that project signals at these frequencies: 9.82khz, 33khz, 65.5khz, 83khz, and 200khz. My most advanced couple of recievers also measure the milliamps applied to the target for identification purposes I. E. the signal with the highest rating is the target. The lower the frequency the higher the amp reading, but on the otherside of the coin the lower frequency has less penetration. If you ever have an opportunity to use one of these pieces of equipment, turn it on on a plate of any kind of metal and the lower the frequency you use the more likely you will hear the metal vibrate.

TheMaskedAvenger (author)2008-08-23

I recently read about an MIT experiment that took this kind of thing to the "next level." It involved using something called "evanescent waves." Here's the article:
Does anyone here know more about this? It seems to me that a magnetic field, unless it was really, really strong, would not be able to do this kind of thing.

well, TECHNICALLY it's not really a magnetic field. it's really an electromagnetic field, AKA radio/gamma/microwaves/light.

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