Inductive power transfer is all the rage. There are now easy chips that do all the hard work for you. It even came built-in on my new phone!
But what about capacitive power transmission?
How come I’ve never seen a product that uses plates rather than coils? Is capacitive power transfer possible? Practical? Let’s break out a roll of tinfoil and find out!
Here is an demo of the simplest inductive and capacitive wireless power receivers I could come up with…
Both types are able to send enough power though the insulating glass to light some LEDs.
What advantages might a capacitive system have over an inductive one?
Inductive systems use coils to generate and receive a changing magnetic field. Capacitive systems use plates to generate and receive a changing electric field.
Coils can be expensive to manufacture and assemble into a product. They have bulk and come in a limited range of practical shapes. The magnetic field they generate can potentially interact with parts that are behind the coil (especially stuff like motors), so you usually want to put some heavy magnetic shielding behind the coils.
Plates can be very cheap (or even be free) to integrate into a design. Any conductive material in any shape can be a capacitor plate- its location is what makes it a plate. A plate can be a piece of foil, a spray-on conductive paint, or an un-etched area on a printed circuit board. The electric field generated between the plates is mostly canceled out behind them, so the plates act as their own shields.
So now we know that capacitive power transfer is possible. Tune in next time to see how we can make it practical.
FAQ:
Q: Why did you use two LEDs for the capacitive receiver, but only one for the inductive receiver?
A: To transfer power capacitively , you use an electric field to push electrons from plate A to plate B though a load (an LED in this case), and then you reverse the field to push the electrons from plate B back to where they started on plate A and the cycle repeats. An LED is a Light Emitting Diode, and diodes only let electrons go one way, so with only one LED, the electrons would flow though the LED the first time you pushed them from A to B, but when the electric field reversed they would be blocked by the diode and get stuck on plate B. Adding a second diode gives the electrons a way to get back to where they started. If you had super high speed vision, you’d see that only one of the two LEDs is one at a time. This would not be a problem with other kinds of loads, like (say) an incandescent light bulb that lets electrons flow both ways.
Note that we only need one LED for the inductive coil since there is a loop though the coils, so the electrons can keep going around and around in the same direction and never get trapped.
Q: What is driving the transmitting coil and transmitting plates?
A: Both are being driven by a sine wave generator. The coil is getting 15 volts peak-to-peak at 1MHz while the plates are getting 30 volts peak-to-peak at 5MHz.
Q: What is the relative efficiency of the two systems?
A: I don’t really care for this proof of concept. I just wanted to show that capacitive power transfer is at least possible. For the kind of applications where it might be practical, we likely will only need small amounts of power so efficiency is probably not as important as cost, simplicity, and design flexibility.