Inductive Charging Circuit Design Help

[ryanrocket]

In Short: I need help designing a circuit between a receiving induction coil and a battery load.

Longer Explanation:
I am entering a science fair project this year which requires wireless charging of a drone. So far, a have a transmitting coil that takes 12V at 2 amperes and the receiver coil is able to pick up 5V at 2 amperes when in an optimal range. However, I discovered that whenever I add a load to my receiver coil (the drone battery), it causes the receiver coil's PCB to make a strange noise and does not charge the battery.
Keep in mind: After testing, we found that this battery can charge with anywhere from 3v to 12v at 2-3 amperes.
So, I concluded that the battery was simply drawing too much from the receiver coil, which caused some malfunctions in the coil's PCB circuit.

I need a circuit (between the coil and battery) that can:

• Prevent the load from drawing too much from the coil
• Maybe step down 5v to a lower voltage (3.7?) but for more amperage.

Sorry if I come off as stupid, I know very little about electronics and I have no idea what components I would need to make this circuit operate without malfunctioning.

A quick recap on readings:

• Transmitting coil input: 12V at 2 amperes
• Receiver coil output: 5V at ~2 amperes
• Battery requirements: 3-12V at 2-3 amperes

For reference: the drone is a mavic mini and the coils can be found here: US $8.54 3% OFF|DC 9V 12V Large Current Wireless Power Supply Charger Module 5V 2A Transmitter Receiver Charging Coil Module DIY XKT 412|charger module|power modulewireless charging module - AliExpress - https://www.aliexpress.com/item/32310638853.html?spm=a2g0o.cart.0.0.c4f63c00gJ8oNd&mp=1

Any help/guidance is greatly appreciated.

 

[SparkyChick]

Welcome to the forums

Did you connect the battery directly to that PCB?

 

[ryanrocket]

Welcome to the forums

Did you connect the battery directly to that PCB?

Yes, as stupid as it sounds that was what I was doing.

 

[SparkyChick]

Then the noise may have been the coils of the inductor vibrating as a result of the unexpected operating conditions

Assuming the receiver still works, at the very minimum you'll need something to prevent current flowing from the battery into the receiver module. Given the circumstances I might be inclined to use a suitably rated Schottky diode as these tend (unless I'm much mistaken) to have a much lower forward voltage drop than regular silicon rectifier diodes.

After that, depending on the battery chemistry, you'll need a suitable charge controller unless you can 100% guarantee such a device is built into the battery and you may need something like a buck converter to raise the voltage to the range required by the battery charge controller. If the drone has a USB port that can be used for charging the battery (somehow I doubt it for reasons of weight/space), then you could make a simple voltage regulator and provide the power to charge via the USB port, putting the responsibility for charge control back into the hands of the drone, but the power you feed to a USB chargeport must be a stable 5v... if in doubt, lookup the USB standards documents and check what the voltage is and the required tolerance.

 

[ryanrocket]

@SparkyChick

Because the drone takes power from a micro USB cable directly from a basic wall power-to-usb, such as this one, I would assume that it does not need a charge controller?

I took what you suggested to quickly put the above diagram together. A 5-volt regulator that feeds to a diode and resistor and eventually out. Ground simply passes through the regulator and to the output. The regulator is a normal 7805. Would this suffice? Also, assuming the 5v regulator has a minimum voltage, what would happen if it gets less than the minimum?

 

[SparkyChick]

The low input/output voltage difference can present a problem to some regulators. It's been a while since I did any serious amount of electronics design, but in your situation I think I would be looking at LDO (low drop out) regulators... if memory serves such devices are better at handling the situation where the desired output is close to the input voltage.

Whenever you select one of these devices, you should refer to the datasheet as these often have reference designs... the ubiquitious 7805 regulator for example, the LM7805 from Fairchild for example should have some decoupling capacitors on the input/output... and given the variable nature of the supply source, a couple of smoothing capacitors on the input/output might be a good addition.

The resistor... what purpose does that serve? If you're looking to limit the current to prevent damage a fuse is better alternative as it won't cause a voltage drop of anything like a resistor.

 

[ryanrocket]

The low input/output voltage difference can present a problem to some regulators. It's been a while since I did any serious amount of electronics design, but in your situation I think I would be looking at LDO (low drop out) regulators... if memory serves such devices are better at handling the situation where the desired output is close to the input voltage.

Whenever you select one of these devices, you should refer to the datasheet as these often have reference designs... the ubiquitious 7805 regulator for example, the LM7805 from Fairchild for example should have some decoupling capacitors on the input/output... and given the variable nature of the supply source, a couple of smoothing capacitors on the input/output might be a good addition.

The resistor... what purpose does that serve? If you're looking to limit the current to prevent damage a fuse is better alternative as it won't cause a voltage drop of anything like a resistor.

Ok, I swapped the resistor with a resettable fuse. Is there a specific decoupling and smoothing capacitor you could recommend? I know next to nothing about them.

 

[SparkyChick]

You should refer to the datasheet for the regulator. Such devices will nearly always include example base designs in the datasheet.

To be honest, I'm kind of hoping that someone who is more knowledgeable in these areas than I will jump in and provide some more technical advice

 

[DPG]

As above, check the data sheet. Although to be honest I think with your level of knowledge you may be better buying a pre-made regulator circuit. But watch out for the issue SC mentioned - you need to ensure the difference between input voltage and required output voltage is not too small.

 

[SparkyChick]

No need to agree... LOL

As best as I can tell, you're moving in the right direction. The diode I would put on the input side of the regulator not the output. If this is ultimately going to be connected to the USB port, you won't get any back feeding from that and the diode then become just a protective device to protect against reversed polarity on the input.

 

[ryanrocket]

@SparkyChick Thank you for the advice I redrew the circuit.

@DPG But watch out for the issue SC mentioned - you need to ensure the difference between input voltage and required output voltage is not too small.

What do you mean by "not too small"? Also I'm decently comfortable with making the circuit and printing the PCB, as I've already done it for the transmitting side of the circuit. I just can't figure this one out for whatever reason. I'm stumped.

 

[DPG]

Have you downloaded the data sheet yet?

 

[ryanrocket]

Have you downloaded the data sheet yet?

Yeah, i downloaded the datasheet for the LM7805 and the Schottky diode I found.

 

[DPG]

Does it tell you the minimum voltage difference allowable between input voltage and output voltage?

Sorry, what actually is the input voltage you have?

 

[ryanrocket]

Does it tell you the minimum voltage difference allowable between input voltage and output voltage?

Sorry, what actually is the input voltage you have?

It does not define the min. input voltage for the 7805 specifically, it just gave a range for all 78-- models: 5v - 18v. My input voltage (when in optimal range of transmitter) is 5V at around 2 amps