Inductive Charging Circuit Design Help

[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

 

[DPG]

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

Ah sorry, I think I've misunderstood. I thought you were regulating down to 5V from a higher voltage. I don't think I've read the thread properly.

 

[SparkyChick]

The LM7805 from Fairchild has a dropout voltage of 2v. What this means for you is that the minimum input voltage to get a stable 5v output is 7v. This rises as the load increases. This is why I specifically mentioned low dropout regulators... these have a much small dropout voltage and may be suitable, but it's entirely possible you would need to look at alternative solutions.

One possible alternative is using a zener diode to limit the voltage. Google something like "zener diode voltage regulator" or "zener diode voltage limiter".

 

[Lucien Nunes]

A lot of randomness here.

1. We need to understand more about the output of the inductive charger receiver module. You originally mentioned connecting the battery to the coil, but presumably meant to the output of the receiver module. That has its own voltage converter by the look of things, but the info is not very clear about what it outputs. 5V 2A to 12V 700mA, or something. What actually comes out... a regulated voltage? A voltage that is always 12V off-load but falls with increasing load? Before trying to connect any drones or batteries I would begin by trying various resistive loads and seeing what it delivers. Personally I would plot a curve.

2. Having discovered the nature of the output, then we can decide what additional converter, if any, is needed between its output and the drone charging input. I would definitely avoid using 7805 or similar linear regulator, as that could end up wasting over 50% of the limited available power as heat and decreasing the maximum charging rate as a result. A switching buck 3-terminal regulator such as a Traco TSR2-2450 might be a better choice, which is functionally equivalent to a 3-terminal 5V 2A linear but with very little power loss and heat dissipation. But as yet it is too early to say whether this type of regulator is suitable.

3. Alternatively we can look at the option of bypassing the drone's charging circuit and charging the battery directly.

But first things first, characterise the output of the inductive coupler output module...

 

[SparkyChick]

@ryanrocket ... see what I mean about someone with more capabilities than me

 

[Lucien Nunes]

I am not sure what you mean by:

Battery requirements: 3-12V at 2-3 amperes

The battery appears to be a 2-cell Li-Ion with a nominal 8.4V charging requirement. Less than 7.4V it won't charge at all, 10V or more it will probably overheat and be damaged. If you are referring to the Mavic charger input, that might be specced for USB-C flavoured voltage options and probably isn't designed to go as low as 3V.

Going back to my point about a linear regulator being wasteful, just take a look at the conversion stages that the power will be going through before it gets to spin the drone motors. For argument's sake I'll take switching stages as 90% efficient, battery 85%:
PSU: 230V AC to 12VDC, 90% out.
Transmitter: 12V DC to HF AC, 81% out.
Receiver: HF AC to 12V? DC, 73% out.
Linear regulator: 12V to 5V, 30% out.
Battery charger: 5V to ~8.4V, 27% out.
Battery charge/discharge efficiency: 23% out.
Motor driver: 21% out.

21% gets to the motors, 79% wasted as heat, of which the linear (e.g. 7805) regulator is responsible for 43%. In real power applications design we try at all costs to avoid this daisy-chain of conversion stages. Ideally you want the inductive receiver to charge the battery directly, but that requires custom electronics.