Wireless power transmission is a system where energy is coupled from a transmitter to a receiver without a physical connection. It was the dream of Nicola Tesla to transmit energy wirelessly from a remote power station to homes and factories without wires. Unfortunately, this has never been realized with any level of efficiency. However, there are several programs still exploring the possibilities.
“Power can be, and at no distant date will be, transmitted without wires, for all commercial uses, such as the lighting of homes and the driving of aeroplanes. I have discovered the essential principles, and it only remains to develop them commercially. When this is done, you will be able to go anywhere in the world — to the mountain top overlooking your farm, to the arctic, or to the desert — and set up a little equipment that will give you heat to cook with, and light to read by.” Nikola Tesla, The American Magazine, April 1921
Some contenders have been lasers, microwaves, infrared, and our project for this tutorial, resonant electromagnet coupling.
In this tutorial, we will couple a very moderate amount of power from a transmitter running at about 260kHz to a receiver that will charge a battery and feed a USB charger.
The oscillator runs at about 260kHz. This is governed by the resonant frequency of the tuned tank circuit made up of L1 and the parasitic C of the transistor, which is several thousand pF. The frequency is expressed as:
L1 is 20t on an 11-cm or 4.5-inch former, center-tapped at 10t giving an inductance of about 80uH, which will resonate with C at about 260kHz.
The transistor I used is an RF type, but it needn’t be that fancy. However, we require a small heatsink as it runs quite hot. I ran it at 9V, and it worked best there. I found it by varying the drive to the transistor with R2. The effective capacitance was varied, resulting in the ability to shift the frequency slightly. This is great as we can adjust it to match the resonant frequency of the receiving coil and thus, improve the efficiency.
The transmitter draws 1A at 12V or 12W.
Below is an example of the simplest receiver.
The coil is a similar 18-turns, center-tapped on an 11-cm former. As the LED D1 is a rectifier in itself, it works at AC here. When the coils are close and tuned for resonance, the LED lights proving power has been transferred. However, as the power in the LED is the voltage across it (about 2.3V) and the current flowing through it (about 20mA judging from the brightness), the power transferred is V*A = 2.3 * 20E-3, which is only 46mW. Considering there is 12W going in, the efficiency is 0.046/12 = 0.38%.
So now, we can see why Nicola Tesla’s ideas still have a long way to go!
A Battery Charger and USB 5V Output
Shown below is a more complex circuit. Instead of the LED, we use a TP4956 Li-ion battery charger module which can work from 1V to 7V input and charge the battery.
The battery, in turn, feeds an 80538 module which converts the 3.7V of the battery to 5V for the USB output. D2 is a 6V Zener as the TP4956 has a max input of 7V.
Working Lash-up of the above circuit
Uses of Wireless Power
Although the dream of kilowatts of power arriving in your home wirelessly has not yet materialized, there are many other uses for wireless transmissions.
We already have cell phone charging stations where the phone has a built-in receiver, and you just have to place it on top of a charging pad. Devices used by divers like torches need to be very waterproof indeed, needing to sustain pressures of up to 100 bar. But having sockets for battery chargers makes the devices much harder to protect, so wireless charging is perfect for this case.
Doctors implant devices under the skin in some medical applications and require surgical removal to replace the battery. Once again, here comes wireless charging to the rescue.