# Wirless Power Tansfer Circuit

Hello so my university is closed because of covid. I am allowed one day into their labs to build and test my WPT device.

My Tx is very large compared to my Rx. I have tested their inductance with a LCR meter. The Tx has inductance 16.7micro H and the the Rx has 1.4micro H. I'm choosing a resonant frequency of 400kHz. That's because the capacitors for the WPT I have available are .01micro F for the Tx and .1micro F for the Rx.

All I have are the capacitors and the coils with wire soldered on. My university has a wave form generator and a NVA machine where some one will help me test WPT.

I was going to place the .01micro F in series with my Tx. The .1micro F in parallel with the Rx. Maybe a bread board for the Tx and directly solder the parallel cap to my Rx. I'm not sure about resistors at all...

My question is, I've never done anything like this. Is this the best way about building this simple circuit. I'm so worried my school is going to allow me in for one day and I'm going to come unprepared and not get any WPT or made a mistake somewhere. Any suggestion to actually building the resonant circuit. Also I know my Rx is very small and very then(I think 18micro meters), however in Maxwell software in ANYSYS, I have power transfer of like 80% at 70mm+(Also the simulation came up with slightly different inductance values but it was close). If I can get anything like that in the lab I will be so happy. Please any help of suggestions for building this circuit.

• For capacitor values I'm using C = 1/(4pi^2 * L * f^2), so for my Rx, C = .113*10^-6. So I hope my .1micro capacitor is close enough. – Krits Jul 24 '20 at 15:17
• So, resimulate it with your actual values and the values of the caps you have. It's not rocket science to re-simulate it. – Andy aka Jul 24 '20 at 15:20
• Yes, well I have the my coils built into Maxwell software. I give it a excitation and it reads an inductance value. So my experimental inductance values are different. If they were spot on I would try right way. I could maybe mess with my simulation capacitors to try to get their value same relative error. My concern is actually building the circuit. I've seen people get WPT with no resonance. So even if I get 10% WPT I'll be happy because then I can find what went wrong. – Krits Jul 24 '20 at 15:26

You say that this is a simple circuit, but it is not. Both coils affect each other (mutual inductance) and the resonant frequency heavily depends on the source and load impedances.

But there exists a lot of know-how in the RFID domain which you can learn from, because RFID seeks the same design goals.

Ok, if your RX coil is much smaller than your TX coil, then you can probably ignore the RX coil first and tune the TX coil for max. magnetic field intensity which is max. coil current.

Assume your TX circuit is a resistive driver (e.g. 50 Ohms) and your TX coil circuit is a parallel resonant LC tank. Then you need to tune the tank not for resonance, but for 50 ohms impedance near resonance, which is the point of max. power transfer from source to TX coil for a given frequency.

In practice, you do not calculate this but you measure and search for max. coil current when slowly varying the capacitor value.

• Any suggestions once I’m in the lab. I thought resonance was only determined by L and C. I know my coils internal resistance and the VNA machine has like a 50 ohm input impedance. Sorry if I’m way off but I thought you had to only worry about matching at like high MHz frequencies – Krits Jul 24 '20 at 17:02
• Also even if I can get a little WPT then I can learn from that – Krits Jul 24 '20 at 17:03
• Sorry one more thing. I thought that mutual inductance wouldn’t matter that much at like 5 cm distance with my coil size – Krits Jul 24 '20 at 17:12
• Yes, resonance frequency is determined by LC, but max coil current by RLC. Yes, at 5cm mutual inductance is small. But you will also have very small efficiency. – Stefan Wyss Jul 25 '20 at 12:41
• (Power) matching has nothing to do with high frequencies. Think of a simple resistor divider. What value must the low side R have to carry max current given a high side R of 50 ohms? Answer is 50 ohms. – Stefan Wyss Jul 25 '20 at 12:52