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enter image description here

The image above shows a 20W phone charger wireless transmitter.

The transmitter details stated

Charging power: 5W/7.5W/10W/15W/20W automatic identification on its receiver coil)

How does the transmitter detect the power needed by the receiver?

Is it through the number of coils in the receiver? For example, a phone which can be charged with high power has its own special receiver with more number of coils and conversely, a phone which can be charged with low power has its own special receiver with low number of coils.

The receiver is also made up of coils as shown. I am quite confused.

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  • \$\begingroup\$ Data can be sent back through the magnetic field to be received by the transmitter in some designs. Do you have a schematic for the one you show? The transmitter will have no idea how many receive coils are used. \$\endgroup\$
    – Andy aka
    Commented Jan 12, 2023 at 15:04
  • \$\begingroup\$ They talk. I would not be surprised if they use NFC, but they could use pretty much anything. \$\endgroup\$
    – Neil_UK
    Commented Jan 12, 2023 at 15:50
  • \$\begingroup\$ Qi allows data transfer. Check WPC for details. \$\endgroup\$ Commented Jan 12, 2023 at 15:53

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Here's a picture from OVERVIEW OF WIRELESS POWER AND DATA COMMUNICATION explaining how data transfer from the Qi receiver to the Qi transmitter is used to regulate the power magnetically coupled to the receiver: -

enter image description here

This is just one commercial example. There are other variants of this and, I've also developed my own prior to the emergence of Qi. It also back-modulated the magnetic field so that the transmitter could regulate power transfer.

How does a wireless charger transmitter detect the power required by the receiver?

The receiver tells the transmitter either by modulating the magnetic field (so that the transmitter can detect this as data) or by sending a separate signal. The "separate signal" can be achieved optically or via RF wireless but, is not limited to either; if you wanted, you could "talk" to the transmitter ultrasonically if you wanted.

Is it through the number of coils in the receiver?

No, it's impossible for the transmitter to tell how many coils the receiver has and, even if it could detect this, power has to be regulated on demand from the receiver hence, a basic data transfer mechanism has to be employed.

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