First post here.

I have a built a wireless power transmitter that consists of 2 flat spiral coils that transmit about 20 watts across a short distant to power a heating element. The coils inductance is roughly 7uH.

At the moment I have the oscillator circuit so that it is powered off until the receiving coil is placed over the transmitting coil, in order to sense that the coil is real and not a foreign object. To do this, I have placed a Hall effect sensor on the base with the transmitter and a small magnet in the middle of the receiver. Obviously under RF power, the induced magnetic field in the transmitting coil heats the sensing magnet up pretty hot! It works, but it not elegant.

I would like to do something different but not sure how. I'm pretty sure the transmitter coil inductance will slightly change when the receiver coil is near. I would like to detect that change in inductance and trigger the oscillator. I tried to build a RLC circuit which is pinged at a known frequency then measure voltage. This works at low power I found, but now at high power as I damaged my circuit. Can anyone provide any guidance on this?

During transmission, the entire circuit draws 6.5A @ 12V with a load designed for max power transfer.

Also, if there is a better/simpler/elegant method to accomplish this, please tell!


  • \$\begingroup\$ Are you using tuning caps and, what is your operating frequency. \$\endgroup\$ – Andy aka Dec 4 '15 at 0:42
  • \$\begingroup\$ Yes, each coil has a capacitor. The frequency is about 65kHz. \$\endgroup\$ – Omar K Dec 4 '15 at 2:30
  • \$\begingroup\$ Did you check the WPC standard (Aka Qi)? They have a similar system, at ~125KHz. \$\endgroup\$ – Lior Bilia Dec 4 '15 at 21:53
  • \$\begingroup\$ Just checked it out. Looks like section "Annex B Object Detection" is what i am looking for! Thanks! \$\endgroup\$ – Omar K Dec 4 '15 at 22:21

If you make your induction coil system self oscillating then when properly set up the switching losses will be low .This means that its running at its resonant frequency .Now when your object comes near the effective circuit inductance changes .If you move your object the change in frequency will be visible on a frequency counter.I got a freq change of 8% which was adequate for running some sort of decision circuit.I know that your job is different but if you get a reasonable freq shift then you will be able to reliably detect absence or presence of the object .You could do this detection at low power and go to full power when the object is present .On my job i used buck convertor to control input to induction heater .I suppose you could strobe your system to save power .Frequency detection is easy for people to do in code these days .

  • \$\begingroup\$ Awesome, Thanks for the response. I need to look at my circuit and see how I can integrate this into my design. This is definitely the optimal method. \$\endgroup\$ – Omar K Dec 4 '15 at 22:23

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