Plates vs Coils – An alternative approach to wireless power transmission

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.

9 comments

  1. kwhitefoot

    >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.
    Nonsense.
    The current in the secondary coil is caused the changing the magnetic field. This cannot keep changing indefinitely in the same direction so the current in the primary that generates the magnetic field must be alternating and then so will the current in the secondary.

    • bigjosh2

      The magnetic field generated by the transmitter does alternate. In one direction, it generates a forward voltage across the diode and current flows though it. When the magnetic field switches to the other direction, it generates a voltage that reverse biases the diode, so no current flows- the electrons just pile up on the negative side of the diode. Then, when the field reverses again back to the same direction we started with, the electrons that got stuck pushing against the diode are free to flow back though the coil and around to the other side of the diode to create a circuit. Slightly simplified explanation (at the frequencies we are dealing with the electrons don’t have time to go back around the loop, but overall there is a flow), but hopefully explains the point. Make sense?

  2. JC

    This looks great. Just curious but what is the required surface area to generate a usable voltage? ie do the aluminum plates require that much surface area as shown in your video or can it be reduced in size and the voltages and cycle altered to still be effective enough to light the LEDs? It looks very big to charge anything short of an IPAD or laptop.

    • bigjosh2

      This is a very crude proof-of-concept. Much smaller plates and much larger power transfer are possible. Stay tuned for optimizations! BTW: Note that output voltage is actually not dependent on plate size (at least in the ideal case), it is dependent on field strength – which is primarily dependent on driving voltage and plate separation. Bigger plates do give you more capacitance, which lets you drive more current at a given voltage level.

  3. Antonio Carmelito Lizada

    What will be the effect of the area of the capacitive plates to the amount pf current flowing through the LEDs?

    • bigjosh2

      Larger plates mean more capacitance which means that for a given voltage swing you can have more current flow though the load (the LEDs in this case).

  4. The Green Gentleman

    Hey! You got featured on Hackaday! I saw their write-up, and I had a question: if the first plates are charged with 30V, what portion of that makes it to the second plates? It’s clearly enough to light up an LED, but I imagine there’s considerable fall-off. Thank you for sharing this!

    • bigjosh2

      There is about maximum 1.7 volts between the two second plates. This is because the LEDs are diodes and so as soon as the voltage tries to rise above the LED’s threshold (~1.7 volts for yellow), then the LED will start conducting which prevents the voltage across them from rising any further.

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