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How can I keep an RLC resonant circuit tuned

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Kaets

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Hi all,

I'm in the middle of a project to design an inductive charging system. For the primary circuit, I've got an RLC circuit that transfers power to the secondary coil when at the resonant frequency.

The problem is that, for this project, the distance between the two coils isn't always the same, so the mutual inductance can change, effectively changing the resonant frequency. So if I have the primary circuit by itself perfectly tuned to the supply, when I bring the secondary coil close to the primary, it's not tuned anymore.

Is there any way I can make the primary circuit automatically adjust itself so that it stays tuned? I guess this could either be done by automatically adjusting the capacitor, or adjusting the supply frequency?

This is going to be quite important once I start trying to design switching circuits to drive the primary coil (up to now, I've just been powering it from a function generator), because if the circuit isn't tuned, then the current and voltage will be out of phase, and the switching device will end up wasting a lot of power and overheating.

If anyone can help, it would be much appreciated!

Thanks
 
WHY/how does your coils move?

if its a RLC maybe adjusting R would be easier with a "slider" pot, that mechanically follows the coil?
 
Hi, thanks for the reply.

The charging system is for a small automated underwater vehicle. The primary coil will be mounted on a docking station so that will stay put, but the secondary will be in the vehicle, so it might be in a slightly different position each time it comes to charge the battery. It'll only be slightly different positions but could be enough to change the resonant frequency slightly.
 
why not use a electro-magnetic dock to align it in then? (with 2 magnets on the boat)
 
Actually, that could be a better idea - I'm over complicating things! Thanks for the help, I'll try the electromagnet idea.
 
I am not sure how big your underwater electric vehicle is going to be, but do you realise that wet mateable electrical connectors are a mature well established product (but very expensive).

There are several companies making them, look here:



In days gone by, before these connectors were developed, it was usual to use inductive couplers for subsea power connectors.
These were effectively transformers which easily split into two parts, the primary and the secondary, the trick was to make them watertight.
These usually operated at about 400hz and were not resonant in any way.

JimB
 
Hi,

Thanks for the reply, I'll definitely look into those connectors, but I think they may be a bit expensive.
Perhaps like you say, both sides of the transformer don't need to be resonant circuits.

Thanks again!
 
if its a RLC maybe adjusting R would be easier with a "slider" pot, that mechanically follows the coil?
The resonant frequency of an RLC circuit is little affected by the R, it mainly affects the Q of the resonance.
 
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