Inductive Charging: a Simple Approach (TfCD)

Inductive charging, sometimes referred to as wireless charging has become an emerging technology with promising prospects for charging all sorts of devices we use in daily life. The principle behind this, called resonant inductive coupling, is not necessarily new. This Instructable has the goal to make you understand how charging coils work and how you can create them yourself, in an easy way. A step-by-step approach is given on how to create the most simple circuit for wireless energy transfer, how to build your own coils and how to see at what frequency resonance occurs.

Some theory

In this Instructable you will be taken through some theory about electromagnetism. Wireless energy transfer makes use of air core coils. Where transformers (used in adapters) make use of a solid iron core to channel flux, the air coil creates a field around the coil. The transfer of energy is made possible by transferring a current through the primary coil, which induces a change in the flux of the coil. The change of flux causes an induction of a certain voltage. If this is done with an alternating current, the frequency of flux change is equal to the frequency of the alternating current, resulting in an oscillating voltage induced by the coil. The change in magnetic flux can be picked up by a nearby coil (e.g. when it's placed on top of the emitting coil). This results in an alterating current with the voltage induced by the coil inside the circuit of the secondary coil.

This Instructable is aimed at people that are not necessarily experienced with electronics. The aim is to show the aforementioned principle as simple as possible, making it accessible for people with a curious mind. The last part of this Instructable is somewhat more complex, going into some of the theory about resonant inductive coupling.

Let's make a list of all the different components we need to build the circuit:

• Function generator (also called an oscillator): creates alternating current
• Oscilloscope: the oscilloscope shows the waveform
• Two coils (more about how to create them in the next step)
• Some connecting wires
• 1 LED (the colour does not matter)

While this may seem like a terrifying step, it is actually not that hard to make a coil. You will need some enlamelled copper wire (preferably with a diameter thickness of over 0.8 mm, which translates to AWG 21 or over). As can be seen, the choice for our coil has been to give it an (rather arbitrary) inner diameter of 50 mm. Any inner diameter can be used, as long as you have something cylindrical to roll it around. Make sure to leave enough space to connect the coil on both ends. (Duct) tape can be used to make the turns of the coil stay in one place. Since the copper is enlamelled, make sure to strip off the plasticized layer (with a knife or by heating the ends) at the place where you are going to connect it to your circuit, else the coil will not conduct any electricity.

Theoretically, the precision of the turns as well as the distance between them will influence the total inductance of your coil. The inductance of the coil is something that can be calculated, and will be explained in step 5. Although it is not a necessity, make the first two coils identical in regard to turns and inner diameter. This will ensure a 1:1 (or at least close to that) ratio of the voltage that is transferred. If they are not identical in the number of turns, this ratio may differ, since the energy will be transferred to a different voltage and current. This secondary voltage and current are, when multiplied, equal to the primary voltage and current.

Technically speaking, two circuits are created: the emitter coil circuit and the receiver circuit. The emitter coil circuit is very simple and consists of one coil, connected to the function generator. Connect the coil to two wires and connect these wires to the function generator. Make sure the settings on the function generator are right (AC, sinusoidal wave). The receiver coil circuit consists of the other coil, connected to a LED and the oscilloscope to see the frequency picked up by the receiver coil. This receiver circuit is closed without a direct power source, it is often referred to as the slave coil.

As can be seen on the pictures, there is no direct connection between the two circuits. Switch on the function generator and set the amplitude peak-peak to a value of 20 V (could be any other voltage). Subsequently set the frequency to be adjusted in the region between 10 to 100 kHz. Next, place the coils on top of each other. By adjusting the frequency to a value of around 55-60 kHz a vague signal on the LED of the receiver circuit will be visible. On the oscilloscope, the same frequency as set at the function generator will show. When turning the frequency up a tiny bit more, a change in the oscilloscope frequency will show, which translates to roughly twice the frequency as set on the function generator. This can be seen in the video.

From when the doubled frequency is apparent and the LED starts to shine, a different characteristic shows up on the screen of the oscilloscope. This characteristic, either peaking towards the positive or negative side is caused by the behavior of the LED, which is a diode.

Increasing the frequency will make the transfer of energy more efficient, as can be seen in the voltage change. The frequency that delivers the highest efficiency is dependent on the coils that are being used. Increasing the frequency will also make it possible to increase the distance between the coils, from being on top of each other to a height of 15 mm apart. It is however important to keep the axes of the coils lined up.

As shown in the previous steps and in the video, energy transfer can happen at a different frequency, which varies among coils. But what happens if you are not able to tune the amount of frequency to fit the coil? This problem is tackled by adding a capacitor to the circuit creating an LC circuit (L stand for inductance, C for capacitance). The optimal resonant frequency is calculated using the equation as shown in image 1. L stands for the amount of inductance produced by a coil and is a set number depending on its dimensions measured in Henry. C is the amount of capacitance depending on the capacitor chosen measured in Farad.

L is calculated using a formula (as shown in image 2 and 3) which takes into account the dimensions of the coil.For purpose of ease an internet based calculator can be used such as this one: http://www.66pacific.com/calculators/coil_calc.as...

Our coils had an inductance of 50 microhenry (uH). You can easily calculate the inductance of your own coil. A big challenge is to accurately determine the amount of inductance from a homemade coil. Since it is hard to get the wires to align perfectly, the amount of inductance may be somewhat off from your calculation, because in reality your coil may look a bit more messy.

After knowing the inductance of your coils, you can create an LC-circuit by inserting a capacitor of your choice, like in image 4. The choice of capacitor will have an influence on the resonant frequency, as shown in image 1.
In case of the previous calculation the actual optimal resonance that was measured was about 67 kHz, putting the 58 kHz (as we calculated) well within range of the resonant frequency, which ranged from about 35 kHz to 99 kHz in this case.

Why is this important?

Within our setup we are making use of a function generator and like demonstrated we have used it to find the resonating frequency by trial and error. In all regular and most prototyping applications you will not have a function generator or the like which is able to regulate frequency available. Therefore this allows you, by selecting the right capacitor with your coils, to match the resonance frequency with the input frequency of your system.

Replacing the function generator

In case you do not have a function generator available you can make a simple circuit with which you are able to replace it. The circuit makes use of a transistor to turn your direct current into an alternating current. The components determine the frequency of your input and are therefore it is important to pick well. Subsequent to the frequency produced by your DC to AC circuit the appropriate capacitor should be selected.