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Hi and welcome to my first instructable!
I'm going to show you how to make your own low-power wireless charging circuits that will let you pass electricity through the air (or any other non-metallic medium) over short distances. This is suitable for wireless battery and capacitor charging and powering of very small un-buffered circuits (such as a single LED).
Please make sure to check out the last page as there are tons of references and other sources I managed to gleam from the internet and other instructables. Also note that I spent a GREAT deal of time experimenting and researching to get this right. I'm an electrical engineer, and even still it took quite a while to get my head around some of the technical challenges. As such this is for experienced hobbyists only, unfortunately it's not easy to do although I tried to make it as simple as possible. It doesn't take a lot of skill, just a lot of tinkering to get it to work right.
Now there shouldn't be current patents on any of this (Tesla, Colpitts, Cockcroft, and Walton all made this stuff yeaaaaars ago), but I would look into it first if you wish to sell anything using this design.
If you want the circuit then just skip ahead to step 2 and ignore the theory part :).
I'm going to show you how to make your own low-power wireless charging circuits that will let you pass electricity through the air (or any other non-metallic medium) over short distances. This is suitable for wireless battery and capacitor charging and powering of very small un-buffered circuits (such as a single LED).
Please make sure to check out the last page as there are tons of references and other sources I managed to gleam from the internet and other instructables. Also note that I spent a GREAT deal of time experimenting and researching to get this right. I'm an electrical engineer, and even still it took quite a while to get my head around some of the technical challenges. As such this is for experienced hobbyists only, unfortunately it's not easy to do although I tried to make it as simple as possible. It doesn't take a lot of skill, just a lot of tinkering to get it to work right.
Now there shouldn't be current patents on any of this (Tesla, Colpitts, Cockcroft, and Walton all made this stuff yeaaaaars ago), but I would look into it first if you wish to sell anything using this design.
If you want the circuit then just skip ahead to step 2 and ignore the theory part :).
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Signing UpStep 1Theory of Operation
The short story: this is a Cockcroft-Walton generator hanging off a resonant transformer. If you don't mind wasting a couple minutes with detailed theory then charge ahead intrepid reader! Otherwise skip to the next step.
The long story, well, it's not much longer. Take a coil, make it resonate at a particular frequency using a capacitor, then place it near a similarly tuned coil and use the oscillating magnetic field of the first to cause the second to resonate. Use a clever AC to DC converter and voila, you have a method of wireless energy transfer.
After some sleuthing on the internet, I went about devising the first part, an oscillator. Various homebrew methods have been used (see: Wireless Power Instructable) but weren't very good or just temporary solutions. I used the suggestion on wikipedia of using a Colpitts oscillator. This is a decent solution because it's dead simple to build and, most importantly, it's a current oscillator and not a voltage oscillator. As current through an inductor is what generates the magnetic field, this is what will drive both coils.
The second part is fairly easy to understand, that being the two coils. Although they don't have to be the same physical size, they do need to resonate at the same frequency. The combination of number of turns and diameter determine the inductance, and some capacitors were added to obtain the correct oscillating frequency. It gets tricky when you get into the details however (and they get very, very detailed, so I won't put the majority down here) as you need to select the diameter of wire to go with the amount of current going through your coil, which will determine the amount of resistance in the coil, which will impact the viability of your oscillator. To make it somewhat easy, go with 24AWG enamled magnet wire.
You now get to pick a some-what arbitrary frequency for your circuit. This I decided to go with 80KHz, it happened to be a nice middle ground between easiness and efficiency. Then you pick a capacitor value that's commonly available, I picked 150nF. This took a while to select because you need to get an inductance that is within the realm of being hand made. Using the equation:
frequency = 1/( 2 * pi * sqrt(inductance * capacitance / 2) ) (from Colpitts oscillator)
we use the capacitor value to try to get the inductance in and around 20uH to 70uH. Air-core inductors around those values are easy to make. I used a value of 53uH.
From here you need to use this handy inductor calculator to try to figure out what diameter and number of turns are needed. I used values of ~22 turns at 6cm diameter, with an arbitrary length around 4-5x the wire thickness for the secondary, and ~13 turns at ~15cm diameter for the primary. These values will be your STARTING POINT ONLY. You have to experiment to get it right (covered in the next couple steps).
Note that you are using the same inductance and capacitance for both the resonating coils, this is so it's easy to tune. Don't go crazy with different inductances and capacitances or else you won't get it to work.
OK, the last part of this picture is the AC to DC converter. This is what will shape the received AC into something we can use to charge a capacitor or a battery at a usable voltage. I used a CW generator here to great effect; it allowed me to tune the slave coil to produce exactly the right voltage without going over the charging voltage. I determined (through experimentation) that a two stage generator would be enough, and that will generally be fine when trying to generate ~5V. For the capacitors I arbitrarily chose 2.2uF caps, and for the diodes I chose a nice Schottky diode array with a very low 0.38V forward voltage drop. The P/N is BAS40TW-TP, however these are VERY small parts so you will probably have to order individual schottky diodes for this one. Just use ones with a low voltage drop AND a low reverse leakage current.
OK! Enough of this long-winded theory and background info, let's get to the actual good stuff!
The long story, well, it's not much longer. Take a coil, make it resonate at a particular frequency using a capacitor, then place it near a similarly tuned coil and use the oscillating magnetic field of the first to cause the second to resonate. Use a clever AC to DC converter and voila, you have a method of wireless energy transfer.
After some sleuthing on the internet, I went about devising the first part, an oscillator. Various homebrew methods have been used (see: Wireless Power Instructable) but weren't very good or just temporary solutions. I used the suggestion on wikipedia of using a Colpitts oscillator. This is a decent solution because it's dead simple to build and, most importantly, it's a current oscillator and not a voltage oscillator. As current through an inductor is what generates the magnetic field, this is what will drive both coils.
The second part is fairly easy to understand, that being the two coils. Although they don't have to be the same physical size, they do need to resonate at the same frequency. The combination of number of turns and diameter determine the inductance, and some capacitors were added to obtain the correct oscillating frequency. It gets tricky when you get into the details however (and they get very, very detailed, so I won't put the majority down here) as you need to select the diameter of wire to go with the amount of current going through your coil, which will determine the amount of resistance in the coil, which will impact the viability of your oscillator. To make it somewhat easy, go with 24AWG enamled magnet wire.
You now get to pick a some-what arbitrary frequency for your circuit. This I decided to go with 80KHz, it happened to be a nice middle ground between easiness and efficiency. Then you pick a capacitor value that's commonly available, I picked 150nF. This took a while to select because you need to get an inductance that is within the realm of being hand made. Using the equation:
frequency = 1/( 2 * pi * sqrt(inductance * capacitance / 2) ) (from Colpitts oscillator)
we use the capacitor value to try to get the inductance in and around 20uH to 70uH. Air-core inductors around those values are easy to make. I used a value of 53uH.
From here you need to use this handy inductor calculator to try to figure out what diameter and number of turns are needed. I used values of ~22 turns at 6cm diameter, with an arbitrary length around 4-5x the wire thickness for the secondary, and ~13 turns at ~15cm diameter for the primary. These values will be your STARTING POINT ONLY. You have to experiment to get it right (covered in the next couple steps).
Note that you are using the same inductance and capacitance for both the resonating coils, this is so it's easy to tune. Don't go crazy with different inductances and capacitances or else you won't get it to work.
OK, the last part of this picture is the AC to DC converter. This is what will shape the received AC into something we can use to charge a capacitor or a battery at a usable voltage. I used a CW generator here to great effect; it allowed me to tune the slave coil to produce exactly the right voltage without going over the charging voltage. I determined (through experimentation) that a two stage generator would be enough, and that will generally be fine when trying to generate ~5V. For the capacitors I arbitrarily chose 2.2uF caps, and for the diodes I chose a nice Schottky diode array with a very low 0.38V forward voltage drop. The P/N is BAS40TW-TP, however these are VERY small parts so you will probably have to order individual schottky diodes for this one. Just use ones with a low voltage drop AND a low reverse leakage current.
OK! Enough of this long-winded theory and background info, let's get to the actual good stuff!
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We are having problems choosing the best oscillator. You state that the Colpitts's is ideal, but commentators try to put that aside. What's your current choice on this one?
Secondly, could you please draw how you would add multiple BJTs? I don't see how you could enter more transistors in parallel?
What I'd also like you to know is that we might use a DS1077 programmable Oscillator! I'll post the outcome.
Thanks in advance!
I only say the Colpitts is ideal because it's easy to build and it's a current-oscillator instead of a voltage oscillator. In the long run it doesn't matter too much, all you want is a bunch of current moving back and forth through the primary coil.
To add multiple BJTs just add them so the pins match up. Pin 1 on BJT1 will connect to Pin 1 on BJT2 and so on.
The picture you attached is a bit fuzzy, but it looks right. Remember that the output is ~5V, it's very hard to predict exactly what voltage it will be!
I was successful building both the receiver and the transmitter, thanks for your instructable!
We are going to do some research on the q factor of this setup, and therefore we need to know the resistance. What's your input regarding the resistance measurement of this system? I have no idea what the R is in this RLC circuit! ;O
Regards,
Bart Verhaegh
First thing you need to do is make sure your oscillator is working correctly. You can use a voltmeter for this, just try using the 'ACV' setting and seeing what the voltage across the primary coil is. It should be around 5V. If it's not then you have a problem in the circuit somewhere, try double checking the values of all the components you are using. If the voltage is good in the voltmeter you might want to use an oscilloscope to double check that you have a clean sine wave across the primary coil. Double check too that the frequency is correct: if it's not it means that the coil or the caps aren't correct.
After you verify the primary is working, remove the primary coil and replace it with the secondary coil (so you have the coil you are using for the secondary directly hooked up to the oscillator). Then using an oscilloscope check the voltage and the frequency; it should be the same as when using the primary coil. If the frequency is off then you have a problem.
Also double-check that the capacitors you are using are the correct ones. I bought a package of caps and it had a few that weren't the correct value so make sure you double check.
Hope that helps,
-Devin
1.the master coil how many turns should i make?
2.the slave coil how many turns should i make?
3.the master coil is the transmitter and the slave coil is the receiver right ??????????
4.the most imp question....
will the master coil give off the 5 volts and the slave coil get that foil wirelessly????.....and in the diagram of the slave coil u showed connections for 5 volts and ground are those inputs or outputs...??????
could ya xplain the whole thing??
3. Yep
4. Nope, the master coil doesn't 'give off 5 volts', I don't know what the nominal voltage on the master is. The slave should however get pretty close to 5 volts when within a short distance.
I don't think you are understanding what's going on here. You should probably read the entire thing before starting, because you seem to be missing the point of this :). The slave coil and caps 'receive' power transferred from the master coil. The '5V' in that diagram is roughly 5 volts output, when compared to that ground point.
If I understand the description you are suggesting two pathways for the secondary
- Just use the two capacitors and the 4-diodes to get V(sec in mV) X (1.41X1.41X1.41X1.41)
in step three you provide an applification with a transitor ( +-5V supply) and you get the voltage from the ground-node between capacitors
The primary resonates at 50 KHz
How do I find the resonance of the secondary?
Send info also, with drawings to hitemag@ims.demokritos.gr
The secondary and primary coils will resonate at the same frequency.
That being said, remember that square waves contain a lot of extra energy in the high frequencies because of the rough edges. Sine waves are much nicer for this application, so as long as the output is sine-ish, you should be good. Using square waves would just mean lots of wasted power in the primary side.
1. Switching transistors wastes less energy, since they have to act like a resistor at half-on instead of a full-on or full-off switch.
2. The input impedance of the resonant coil looks like a short circuit at resonance, and an open circuit at all other frequencies, so if the fundamental frequency of the square wave is at resonance, it will act just like the sine wave case, but the high frequency components of the square wave will see no load. It will bandpass filter the energy by blocking high frequencies, not by shunting them.
I'm not sure if I agree with the second point. My circuit is causing a current oscillation primarily, the voltage oscillations are caused by the current. Or to put it more correctly, the current is being forced in the circuit, voltage does whatever it wants. When you switch to a square voltage oscillation I'm not sure it works as easily as described, since the load on the secondary is causing all sorts of complicated 3rd order effects :).
Meh, doesn't matter, both the method you describe and the method I've worked out achieve the same end, and both will be surprisingly inefficient when implemented, haha.
A transistor can't "force" current through a high resistance. It's not a current source.
"The resonant network filters the higher harmonic currents. Thus, essentially only sinusoidal current is allowed to flow through the resonant network even though a square wave voltage (Vd) is applied to the resonant network."
Yes, both will be inefficient, I'm just trying to learn the most efficient way to do it.
My secondary does actually have more turns but it has the same inductance (since it's a smaller diameter). Even if the voltage is stepped up from primary to secondary, the voltage increase isn't that much. With the CW gen, you can get a much more substantial voltage increase with a couple more parts.
If you double the number of turns in the secondary, it doesn't double the output voltage? It's different for resonant transformers?
Also you are right, this circuit has really poor regulation. That's ok as long as you don't suck down too much current, or charge a super cap like I did. It works excellently for charging large caps because it will deliver the max current possible in all situations.
plz help me
may u give me full detail (circuit dia.,calculation, formula etc) of this project.
What is the value of capaciton in primary coil and secondary coil , what is no of turns and what is wire gauge of primary coil and secondary.
what is the operating voltage and current and power
and what is the operating frequency.
plz mail me this details
abhishek79shrivastava@gmail.com
may u give me full detail (circuit dia.,calculation, formula etc) of this project.
What is the value of capaciton in primary coil and secondary coil , what is no of turns and what is wire gauge of primary coil and secondary.
what is the operating voltage and current and power
and what is the operating frequency.
plz mail me this details
abhishek79shrivastava@gmail.com
So to start, the coil as shown is exactly where it should be. Remember that the current in that transistor goes from top to bottom, with a tiny bit coming in through the side there (in other words, current from collector to emitter, with a bit coming through the base, see wikipedia). The current has to go through the inductor at the top.
Now for fixing the finicky nature of you oscillator. If you see the wikipedia link of the original oscillator you'll notice that the values for all the primary parts are different. There are two things you could try that right off the bat might help. You could change the 100R resistor to different values (50R? 1k?) and you can try dropping in another transistor in parallel (I briefly mentioned this in the article, I ended up with 3 on my circuit). Both of these should change the saturation behaviour of the circuit, and might make yours start working. Also double check that the 5V supply is rated for at least 500mA (I never bothered checking how much current is being drawn, I doubt it's more than a ~200mA though). If you have it you can try using a 6V supply as well.
Good luck!
If i were to change to value of inductor and capacitor. would i need to adjust the resistor values around the BJT to compensate it?
Thanks :)
cause I'm using a 18AWG enamelled copper wire.. However, i could not obtain any form of sin wave off the oscilloscope.
If so, can you build me one?
PM me for the trade value...
This is pretty kewl BTW...