# Magnet wire size for WPT

#### Scattered

Joined Jan 22, 2020
3
Hi,
I'm currently building a wireless power transfer for my project, to charge a phone. I plan to use the resonant inductive method. I want to use series resonance and the diameter of the coil would be 15cm for both Tx and Rx.

In resonance the current will be driven by the coil resistance only, because the (inductive reactance = capacitive reactance).
I need to transfer 5Watts, the coil distance maybe around 5-10cm. I have already built a H-bridge driver to drive the Tx coil. I plan to use around 40KHz switching.

So if 5w is need in output, Tx coil current would be around 450mA. Which size of magnet wire should I choose and the turns and the capacitor?

Currents using Inductance Cal I calculated for 24awg and I would be needed around 25turns, around 0.9ohms and the inductance will be 0.000123H and the capacitor I will be needed is 128nF. Is my calculation right? I won't be able to use litz wire as it will take time to get shipped to here and I only have few weeks to complete the project.

Is 24awg is enough or a over kill?

#### AlbertHall

Joined Jun 4, 2014
10,776
I think you are going to lose an awful lot of power with the coils 5-10cm apart - like only 10-25% efficiency at best. That makes the TX power 20W-50W.

#### Scattered

Joined Jan 22, 2020
3
I think you are going to lose an awful lot of power with the coils 5-10cm apart - like only 10-25% efficiency at best. That makes the TX power 20W-50W.
Well the distance is really not that needed, it could be less than that. But what about my selection of magnet wire?

#### AlbertHall

Joined Jun 4, 2014
10,776
24AWG is rated for 3.5A for chassis wiring i.e. with free air flow for cooling. Your 25 turn isn't going to get free air flow but it's not too bad. It all depends on the current and that depends inversely with the voltage. The higher voltage the transmitter uses, the lower the necessary current is for a specified power.

#### Alec_t

Joined Sep 17, 2013
11,796
Unless the 40kHz is an extremely pure sinewave you will likely be splattering harmonics of that across the radio broadcast bands. Some serious screening may be necessary.