Question: I am trying to make a wireless charger for a small handheld device. I've tried searching on the internet for something read-made, but everything I could find was much larger than I could fit.

The way I understand how wireless power works is that you basically have an air-core transformer with a 1:1 turn ratio - which in theory should pass the power through the air. I therefore made two little coils of 0.1mm magnet wire with ~250 turns each and with a resistance of 15 Ohms.

I used an H-Bridge driver to create a square AC voltage with peak to peak of 5V on the primary coil, but even my best attempt only created ~1V RMS on the secondary coil. The main problem is also, that if I try to power **anything** from the secondary, the voltage immediately drops to 0. My ampmeter registers 2µA flowing in the secondary, while it consumes up to 1A on the primary (which makes the primary very hot).

I tried with AC frequencies from 100 Hz to 60 kHz and I had the best results with around 4-10 kHz.

I thought I could just rectify the output of the sec and use those 5V to charge my battery, but this is apparently more complicated than I thought :)

Do I need some sort of resonating circuit? What am I doing wrong? Any help is much appreciated

Here is an image of my setup:
[![enter image description here](https://i.stack.imgur.com/8rXyU.jpg)](https://i.stack.imgur.com/8rXyU.jpg)

And here is my schematic (really basic circuit):
[![enter image description here](https://i.stack.imgur.com/FvSRU.png)](https://i.stack.imgur.com/FvSRU.png)
OUT1 and OUT2 is where I put the probes of my oscilloscope for measuring output voltage. +5V goes to my bench supply. The Arduino is powered from USB.

Answer: 
What you are trying to achieve is a DIY Qi-Charger! Awesome!

Qi-charger usually work at an operating frequency between 87 to 205 kHz. Usually 100kHz is fine. To assure neat power transmission, it is essential, that both coils are more or less in sync, so you want the receiver coil to have a resonating frequency at about 100kHz (or whatever you choose).

Additionally, a Qi-charger provides a simplex communication from the receiver coil to the transmitter coil to adapt power flow, I think you can omit this, but be aware, that the voltage will rise, when the drawn current drops, and vice versa.
If this happens in a Qi certified device, the receiver coil tells the transmitter coil to adapt power, which is done by slightly changing the frequency out of sync (or more in sync), which changes the efficiency ratio between the two coils.

\begin{equation}
f\_0 = \frac{1}{2 \pi \cdot \sqrt{L \cdot C}} \approx 100kHz
\end{equation}

[![enter image description here](https://i.stack.imgur.com/JToPl.png)](https://i.stack.imgur.com/JToPl.png)

So you can measure the inductor of your receiver coil and then calculate the necessary capacitor you have to put in series with the coil.
**Reason for drop in the supply Voltage:**
From the image, a thin wire with turns is used as a coil. Higher frequencies due to the **skin effect**, the concentration of charge is more near the surface as compared to the core of the conductor. The ohmic resistance of the conductor is increased due to the concentration of current on the surface of the conductor. Hence Voltage drop increases.

The solution will be to use a thick, stranded coil

In order to increase the coupling between primary and secondary coils, a Ferrite plate can be placed in between the coils.

The coil will acts as a **series RLC circuit**, So operating at resonant frequency will increase the efficiency of the system

[![enter image description here](https://i.stack.imgur.com/v5uVw.png)](https://i.stack.imgur.com/v5uVw.png)