3 ms·

Is the ticking a function of a mechanical relay in a cheaper device, or the coil not being sufficiently glued down and moving as the magnetic field is turned of

• jsheard 2y ago
  The latter I think, I found the volume varied depending on how the coils were aligned. I'll give it another shot once Qi2 brings Magsafe-like perfect alignment to non-Apple devices.
• gorkish 2y ago
  The sound comes from the windings within the copper coils vibrating due to Hall effect in the alternating magnetic field. Yes, the physical construction of the coil matters as to how much it would whine; tighter and more regular windings and appropriate potting/adhesives counteract the physical effects. The easiest mitigation, however, is to just use a switching frequency much higher than humans can hear. AFAIK the Qi standard supports the charger using anything from 80-300kHz and can vary depending on what the charger decides is optimum. Any audible coil whine from Qi would be attributable to harmonic mixing within the physical system of the vibrating coil. Also note that the coil whine could come from either the Tx or Rx coil, so you might easily eliminate it with a different charger that settles on a different frequency or has a better manufactured coil. But if the problem coil is the one in your phone, you might be hosed!