I am making a device that will be wireleslly charged and I decided to go for a standardized Qi transmitter/receiver. The device is a kind of bulk and I want it to have an option to get charged no matter what side it is lying on. I thought of putting a Qi coil on each side and getting the charging voltage on battery port when the device is lying on any side.

Let's say there will be 3 coils. How do I connect them to the same battery charging circuit? If I connect them in parallel, 2 coils will present huge load to the one coil that is currently active. If I connect them in series, It will present additional impedance to the source and may limit the output current.

I thought of putting them in parallel and adding a diode in series to each branches. This way, the diode is going to be reversly polarized when one coil is active and there will be no overload. But at the same time, it will work as rectifier and kill all negative half-periods when it's conductive.


simulate this circuit – Schematic created using CircuitLab

Is there any reasonable way to do this?

  • 2
    \$\begingroup\$ The diodes won't help things. I don't know how (or if) you can use multiple Qi coils, but the diode business is right out. \$\endgroup\$ – JRE Apr 25 '16 at 12:42
  • \$\begingroup\$ Using two separate coils (each tuned with their individual capacitors) can reasonably co-exist. Inductance will half and capacitance will double resulting in the same tuned frequency. Circuit Q-factor will halve but I wouldn't call that a huge increase in load. It may not work because of the Q halving but it may not work if the two coils are too close because of induction. \$\endgroup\$ – Andy aka Apr 25 '16 at 12:54

Qi wireless charging relies on coupled LC resonators (Inductor in parallel with a capacitor).

Your diodes solution will not resonate so it will not work.

Besides that some communications happens when you place the receiver coil on the transmit coil by means of shorting the receiver inductor. This also cannot happen in your diodes solution.

You will need a separate Qi power receiver and coil on each side of your device.

  • \$\begingroup\$ Will it be enough to put all receivers' outputs in the parallel? It won't damage the circuit? \$\endgroup\$ – Roker Pivic Apr 25 '16 at 13:41
  • \$\begingroup\$ That depends on the receivers. If you use the 5 V output types these might work OK since these might be designed to connect to the USB 5 V. When you would charge via USB there would be 5 V at the output of the receiver but that would be OK. But you must check the datasheet of the receiver to be sure. If unsure, use Schottky diodes (the 1n4148 gives too much voltage drop) to combine the 3 outputs. \$\endgroup\$ – Bimpelrekkie Apr 25 '16 at 13:52

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