Hello,
I'm working on a project to make an inductive charger. After working on preparatory calculations, I am stuck on making a circuit even work to test different configurations of coils. I have a prototype that works from adafruit. It has 12VDC input and through a sending minicircuit, somehow creates 45 VAC across the two ends of the inductor coil in an oscillator (15 loops, circle is ~3.5cm in diameter). This gives about 3-5 VDC output on the leads of the receiving coil minicircuit.

I have put together a test circuit on my breadboard with a LM741 op amp and low pass filter, using the same inductor with a 47uF capacitor with calculated resonant frequency 20 kHz in the oscillator. I have about 9 VAC with respect to GND, but there is no voltage across the two ends of the inductor.

Is a step-up transformer needed here? I'm pretty sure it is, but even if I get that, it won't solve the problem that I have zero voltage across the two ends of the inductor.

I really need the help soon! I have been working on the project for a while and time is running out.

Thanks for any help.

2 ways come to mine for this, 1 is the 2 coils as you describe, Would try to get wingings closer together, Router if necessary.
Never installed the Sat through Glass, Figured it HAD to be 1 of 2 ways, 1 with the 2 coils, but Rf through a Transformer is like going through a choke(little gets through)? may pass voltage and handle signal trough some active way, since power in now on the other side of the glass.
2nd way, and haven't got to test yet but transfer power with light.
Op Amps are Great, I built into Fender Stratocasters since the 70's, but for Power, used 2 9 volt Batteries, with the positive of one and the negative of the other go to ground, then you would have a positive and a negative 9 volts, 18 volts between the 2. I didn't want AC near it, even though Op Amps Cancel anything common to both Inputs(Common Mode Rejection), Plus you can get a million to one gain.

Have Both coils as close as possible and as aligned as possible.

How many Volts and Amps do you need? the Voltage you are reading may be no load?

Don't worrying about converting to dc, until you can get good ac across, secondary winding are typically smaller.

10 times as many windings will provide 120 volts from 12.

Ratio of windings is the same as voltage(double the windings/double the voltage)

With this inductive circuit I bought from adafruit (I want to build my own with similar capabilities), I want to focus on the sending mini-circuit because there are no active devices in the receiving circuit. The receiving circuit can have no contact, only filters, and a rectifier and whatever else that needs no power input.

All of the high voltage must come from the sending circuit. My prototype from adafruit (schematics unavailable) has inductance around 0.14 uH, oscilloscope says f = 66kHz. It is DC input 12 V, so there is an oscillator at the end, Cap = .04 pF based on tank circuit formula f=1/(2*3.14*sqrt(L*C)). Okay, so it converts DC to AC to jump the air gap.

Somehow, the inductor has 44V difference from one end to the other (not wrt ground). I'm wondering how 12VDC input can turn to 44VAC across the two ends of the inductors. I don't think the op amp is even used because any voltage it would amplify with 12V would be saturated at 12V. On the sending mini-circuit, there are two 8 pin chips whose ID has been scratched off (so secretive!). A few capacitors and maybe resistors, that's it.

I thought that there is a voltage drop across an inductor with AC because the current is changing direction (L*di/dt). This would mean, the higher the frequency, the higher the voltage drop?

With just an input from a function generator at ~25V, I get around ~0.2-0.3 V on the other side with very small separation at 0.5-1.0 cm, just using the coils.
This is asking a lot, but I have been trying to develop an inductive charger for weeks and this is as far as I've gotten. I feel like if I answer that one question about the voltage drop across the inductor, or what those two chips on the sending circuit might be, I could make it through.

If you are able to help, it would so appreciated!

Sorry... the cap = 40pF, that makes more sense.

Earlier when I said Router, I meant to Router under the desk or whatever Material separates the 2 coils.
No Active components on the receiving end? Then why does it need power?
I wouldn't worry about The circuit so much as being able to pass AC volts between the 2 coils, May have to use Higher voltages to make up for losses. When you get good AC to the receiving coil, then, rectify, filter or whatever, Under a load the voltage will drop. What is the Load?
Good Luck.

The project was proposed to me for a wireless data transmitter. I meant that I can't have an active device on the receiving end, like an op amp, because that would need external power, and the idea is to power the second coil without any wired connections to anything (no DC supply to power hypothetical op amp).

I use the function generator to get 25 VAC in the first coil and get only about 300 mV on the second coil when separated by a few mm. I will try another configuration for the coil, but this is not the main fix. I've already made two inductors with 50 loops that are stronger compared to adafruits 30 loops (tested both shapes) just can't get it to get an output of 3-4 V with 0.7-1.0 cm separation. It's a crazy mystery right now.

You have 25 volts on the coil, what current do you have with primary coil attached?

I can't get a direct measurement with a multi-meter, it says 0 A DC and AC, but I with 50 loops around my little plastic cylinder, I calculated 0.3 Ohms, giving 83A, AC. This number seems ridiculous. I had a complete circuit to ground (opposite end of function generator) with only the primary coil and it didn't get hot. Also when I connect the primary coil to this ground, the oscilloscope loses all readings, wherever I put the probe, and the function generator's readings (I'm using a T-plug so I always see the function generator on the scope). I get a voltage transferred when I have the + end connected to the primary coil and nothing else connected.
I tried making an oscillator with DC and feeding it into a capacitor with the coil, but that didn't work either.

You can buy this already built, disect if necessary to answer some questions you may still have about your project:

Securitron's Powerjump

Securitron Magnalock Corporation : Access Control Accessories : PowerJump