Introduction: Basic Wireless Power Transfer
About a hundred years ago, a crazy scientist well ahead of his time established a laboratory in Colorado Springs. It was filled with the most eccentric technology, ranging from massive transformers to radio towers to sparking coils which generated bolts of electricity dozens of feet long. The laboratory took months to set up, represented a significant investment, and was financed by a man who was not exactly known for being particularly wealthy. But what was the purpose of the thing? Quite simply, the crazy scientist aimed to develop a method of transmitting electricity directly through the air. The pioneering man was envisioning a world in which we would have no need for tens of thousands of miles of power lines, no need for millions of tons of copper wire, and no need for expensive transformers and power meters.
Renowned inventor Nikola Tesla was a man whose brilliance pushed the science of electricity and magnetism forward by many years. Inventions such as the AC motor, radio-controlled machines, and the modern power infrastructure can all be traced back to him. Yet despite his profound influence, Tesla never succeeded in developing a means of transmitting power without wires at his lab in Colorado. Or if he did, it was either impractical or he simply lacked the means to develop it into maturity. Never the less, his inventive legacy lives on, and while we might not be free of the burden of massive electrical grids today, we do have the technology to send power short distances without wires. In fact, such technology is readily available at an electronics store near you.
In this Instructable, we will be designing and building our own miniature wireless power transfer devices.
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Step 1: Materials
Relatively few materials are required to build this simple device. They are listed below.
1. A battery-powered fluorescent light. These can be bought at the local Wal-Mart, Dollar General, or hardware store for just a few dollars. Any one of them will do, but try your best to choose one in which you can easily reach in and detach the fluorescent tube from its socket.
2. Enamel-coated magnet wire. You'll need several dozen feet of wire for this project. The more you have, the better. Additionally, it's best to use thinner wire, as more wire packed into a smaller space will equate to greater range and efficiency. My choice of wire here is not ideal - I'd much rather it be thinner - but it was all I had on hand when I designed this project.
3. Spare copper wire. This is not necessary, but it helps out a lot. If you happen to have alligator clips (preferably four of them), you're in even better shape.
4. An LED. Any LED will do the trick, but for this application, brighter is generally better. The color does not matter, for the voltage supplied by the device will be more than sufficient to light any color of LED. Resistors are not required.
5. (Not pictured) - Sandpaper, a C or D cell battery, and a lighter. These things aren't necessary to the success of the project, but they will come in handy as you build the various pieces of the wireless power device.
Step 2: The Primary Coil
To begin, start by taking a section of magnet wire (anywhere from twenty to fifty feet, depending on the thickness of the wire) and winding it into a coil. This is where a C or D battery comes in handy, since you can simply wrap the wire around it repeatedly. Try to make your coil as neat as possible. In addition, ensure that you completely and thoroughly remove the enamel insulation at each end of your coil. This might require a lighter to burn the insulation off (as is shown in the picture), as well as sandpaper to remove it completely.
When you're done with the coil, slide it off the battery (or leave it on whatever you wrapped it around; in my case I used a leftover spool from a previous project) and tie it up using tape or zip ties. The last thing you want in this case is a rapidly-unraveling coil of wire. If it unravels, it will get tangled up, knotted, and may even become unusable. To prevent this from happening, hold both protruding ends of the wire against the coil as you secure it.
Step 3: The Secondary Coil
The secondary coil, like the primary one, can be any length of wire (preferably longer than 20 feet, once again), and need not be the same type or thickness. However, much the same as the primary coil, it must be made out of enamel-coated magnet wire, it must have the insulation removed from each end, and it should be roughly the same size and shape as your first coil.
When you've completed the secondary coil, tie it up and then attach your LED to it. This is where spare wire and/or alligator clips start to come in handy. I was fortunate enough to have a coil which was thin enough that I could just wrap the wire around the LED's leads, but if my coil had been made of thicker wire (as the primary one was), it would have been best to attach the LED to it using thinner copper wire or clips.
At the end of the day, it does not matter which side of the LED gets attached to which lead of the coil, as long as the two ends of the coil are firmly and securely connected to the bulb's terminals.
Step 4: Wiring It All Up
If you haven't done so already, remove the fluorescent bulb from your battery-operated light and locate the terminals which were previously connected to the bulb. Make sure at this point to turn the device off. The current is not strong enough to be deadly, but it can give you quite the painful shock if you happen to touch bare wires to both terminals at the same time.
Once you've located the terminals, wire your primary coil into to them, connecting one lead to one terminal and the other lead to the other terminal. Ensure that you have a secure connection. Alligator clips can work wonders here, but if you don't happen to have any (like me) you can jam large bolts into the terminals, or you can even attach balled-up aluminum foil to the ends of your coil and then stick them into the connections. However you go about doing this, just ensure that your connection is stable and steady.
Turning to the secondary coil, you need not do much except to make sure that it's securely connected to the LED.
Step 5: The Circuit in Action
All we have left to do is to fire it up! Making sure once more that all your connections are good, lay the secondary coil on top of the primary coil and flip the switch to turn the 'light' on. You should see your LED come to life. If it does not light up, check your connections again. This is a fairly forgiving project, and so it likely won't take long for you to troubleshoot the source of your problem.
As you experiment with the circuit, you should notice that you can lift your secondary coil off of the primary coil and the LED will still remain lit. This proves that you are 'wirelessly' transferring power. Try sliding some papers, a book, or any other non-conductive object in between your two coils. In most cases (unless you've got a really thick book) the LED should stay on. In my own personal experience with other builds of this project, I have been able to place the secondary coil as far as six to eight inches from the primary and still see a faint glow coming from the LED.
Step 6: How It Works
In essence, this device is what we would call an air-core transformer. Normal transformers (like the ones on power poles, the ones found in phone chargers, etc.) consist of two or more coils of wire wrapped around a piece of iron. When alternating current (AC) power is passed through one coil, it creates a rapidly-switching magnetic field in the iron, which then induces a current in the second coil of wire. This is the same principle that electrical generators work off of - that a moving magnetic field will cause electrons to move in a wire.
Our device works in a very similar (albeit slightly different) manner. As it so turns out, every battery-operated fluorescent light has a small circuit in it that takes the low-voltage DC (direct current) from the batteries and steps it up to a much higher voltage, somewhere on the order of a few hundred volts. Without this high voltage, the fluorescent tubes would be unable to operate. In order to generate this higher voltage, however, our fluorescent light-driving circuit needs to convert the steady DC power from a battery into another form of electricity known as pulsed DC. Pulsed DC acts the same as AC electricity in a transformer - the 'pulsed' nature of the current essentially creates a magnetic field in the wire which collapses and reforms thousands of times each second. This pulsating DC enables a tiny transformer embedded in the circuit to step the power up from six or twelve volts to several hundred. But due to the way in which the power supply works, the electricity at the terminals is 'pulsating' at a rate of several thousand times per second. We can essentially say that the high-voltage electricity coming out of the device is 'buzzing.'
When this pulsating DC power is fed into our primary coil, it turns the coil into an electromagnet which is projecting a rapidly changing magnetic field. As we bring our secondary coil near to the primary one, a current is generated in it on account of the pulsating magnetic field. This current then passes through the LED, causing it to light up. The further away from the primary coil the secondary gets, the less of an effect the magnetic field has on it, and the less current gets generated. Likewise, this effect can be 'countered' by adding more wire. More wire means more magnetism in the primary coil, and more wire in the secondary coil means that more of that magnetic field can be captured.
Because of this, we can call our project an 'air-core transformer' because we are constructing a device which has two coils - a primary and a secondary - and works off of pulsating magnetic fields. However, unlike traditional transformers which utilize iron to 'transmit' the magnetic field from one coil to another, ours has nothing to carry the magnetic field. Thus, we say that it has an 'air core.' To put things in a nutshell, this small, simple device is just a different take on a technology as commonplace as the clouds in the sky.
Enjoy your wireless power transfer device, and thank you for reading!