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Capacitive power transfer: how would you approach this?

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Pinky's Brain

Thu Jun 28 2012, 07:30PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Steve didn't propose an extra capacitor ... there is effectively a capacitor there whether you include it or not, that's the whole point we've been trying to make. It's that capacitance without a capacitor which makes the circuit work, the reason you should put it in the diagram is so you can reason about and simulate it.

Ash Small

Thu Jun 28 2012, 08:36PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Pinky's Brain wrote ...

Steve didn't propose an extra capacitor ... there is effectively a capacitor there whether you include it or not, that's the whole point we've been trying to make. It's that capacitance without a capacitor which makes the circuit work, the reason you should put it in the diagram is so you can reason about and simulate it.

Aah, ok, that clarifies things a bit, but when I put the capacitor in there I was able to measure 2.2V accross the other (top) capacitor.

I was under the impression that you had suggested it would need a second capacitor to work.

I'll have another look at this tomorrow if I get a chance.

(I realise that there is some capacitance there (stray capacitance, if that's a good term to use))

2Spoons

Thu Jun 28 2012, 09:58PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Just remember that when you start poking your scope around it too will provide a ground return path, both DC and AC.

Ash Small

Thu Jun 28 2012, 10:59PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
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2Spoons wrote ...

Just remember that when you start poking your scope around it too will provide a ground return path, both DC and AC.

I realise that, 2 Spoons, that's one of the reasons why I'm not going to jump to any conclusions until I've tried it at a much higher voltage, and ideally measured any percieved 'energy transfer' with my DMM, or some other method as well.

(My 'scope is around 40 years old, and, I suspect, has 'soggy electrolytics', as well. I've doubted what it's told me in the past, and run into problems before because I wasn't sure if I could trust it,)

The definitive proof of any potentially viable solution to this issue of capacitative energy transfer will be if we can light an LED, for starters.

Steve Conner

Fri Jun 29 2012, 09:42AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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"Connect your scope across it"? In that case the return path is through the ground clip of the scope, of course.

In Ash's test circuit, there isn't enough stray capacitance to make it work without explicitly adding the second capacitor. But you could easily dream up some physically large circuit driven by a Tesla coil where it would work through stray capacitance alone.

Ash Small

Fri Jun 29 2012, 10:05AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4245

Steve Conner wrote ...

"Connect your scope across it"? In that case the return path is through the ground clip of the scope, of course.

In Ash's test circuit, there isn't enough stray capacitance to make it work without explicitly adding the second capacitor. But you could easily dream up some physically large circuit driven by a Tesla coil where it would work through stray capacitance alone.

Yep. I've had another look at the Murata document I linked to above, and I now see what you were getting at Steve.

The circuit could be made to work using two capacitor plates in the transmitter, and two in the reciever, or, using the 'ground' plates indicated in the Murata schematic.

Voltage would need to be higher, you'd need two resonant circuits, one in the transmitter and one in the reciever, both resonating at the same frequency, a step up transformer in the transmitter, possibly air core, but at frequencies up to 1MHz it could be ferrite cored, and a similar step down transformer in the reciever. frequency would be limited by the speed of the diodes used for rectification in the reciever.

Would it be better to use 'loose coupling' of air cored xformers, or 'close coupling' of ferrite cores?

Steve Conner

Fri Jun 29 2012, 10:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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The closer the coupling, the easier it gets to transfer power efficiently. That goes for both capacitive and inductive coupling.

If I were designing a wireless power system, I'd always go for the tightest coupling possible within the constraints of the wider problem.

Ash Small

Fri Jun 29 2012, 10:38AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
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And one other question I've been puzzling over. What form should the ground plates take, considering the size/space constraints of a viable system?

Steve Conner

Fri Jun 29 2012, 10:44AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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Posts: 6706

The size and space constraints depend on the application, so the form of the electrodes/coils should also depend on the application.

Ash Small

Fri Jun 29 2012, 11:15AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

The size and space constraints depend on the application, so the form of the electrodes/coils should also depend on the application.

Of course. I should have been more specific.

Suppose, for example, you wished to charge a laptop. Ideally you'd want the transmitting plate and recieving plate to be as large as possible. This would be ~equal to the 'footprint' of the laptop, say 30cm X 40cm (~12" x 15").

What would be the most compact form for the ground plates to take, bearing in mind that they still need to function effectively?

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