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I am a high school student doing a lab report on changing the number of turns of a coil and seeing the effect it has on the output voltage from a wireless charger.

I am trying to take a Qi Wireless Charger and have a coiled copper wire above it with its voltage being read from a multimeter. I have placed the coil on top of the charger and connected both terminals of the coil to a multimeter reading AC voltage. However, when I do this the voltage oscillates from 0 to a random number and back. I am unable to get a stable voltage reading and was wondering if there was a solution to this which does not require a Qi wireless charging transceiver. I have attached a picture of the setup and the inside of the charger.

setup

qi charger

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Look up those chargers. They run at around 140KHz. Most multimeters are designed for roughly 60Hz. (Get your multimeter manual, see its specification for frequency range of AC measurement.) The QI charger frequency is 2300X too high, so instead use an oscilloscope.

I just checked a charger. If no phone is being charged, the QI puts out a 2Hz repeating pulse of 170KHz, 250mVp per turn on a coupling loop, when probing for a phone (the phone then communicates back, to only start charging when needed.) With a scope you can still perform your experiment, just measure waveform peak on your pickup coil.

Also, if you add more turns to your coil, and if those new turns are many mm away from the QI coil, then that's cheating, and your results will be all screwy. You have to add more turns that are right against the coil in the QI. In other words, your own coil has to be hoop- or pancake-shaped, not cylinder shaped.

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I am unable to get a stable voltage reading and was wondering if there was a solution to this which does not require a Qi wireless charging transceiver.

There is not.

Qi chargers do not transmit power continuously. They use backscatter modulation to communicate with the device being charged; full power is only delivered once the device has confirmed that it is present and ready to be charged. Without a receiver in place, all you're going to see is a small amount of power that's transmitted to probe for a receiver.

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