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Capacitive power transfer: how would you approach this?

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TBJ

Wed Jun 27 2012, 07:46PM

Registered Member #5374 Joined: Mon Jun 18 2012, 06:54PM
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Hi Guys,

I have a little project at the moment - to build a capacitive wireless power transfer system.

I would like to transfer about half a watt of power over a distance of maybe 5-10cm using capacitive coupling, and I have no idea how to approach the problem. I have built a few inductive wireless power systems before (using a modified ZVS driver) and have achieved reasonable levels of power transfer, but that's where my knowledge runs out.

What sort of voltage level would i need to use? Should it be a reasonant system using the capacitance as the variable element in a series LC circuit? Anyone done this before?

There is a company called Murata who is bringing a system that transfers power using this method to market, but there isn't much about it, and it seems that not many people have attempted this before, although inductive coupling is relatively easy to do.

Cheers.

Ash Small

Wed Jun 27 2012, 08:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
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Pinky's Brain

Wed Jun 27 2012, 10:08PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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So basically they are using two capacitors, one from the sender to the receiver and one from the receiver to ground (you'd probably use a metal casing or something, so you'd get some decent capacitance to ground) with a resonant tank at the sender ... that's one option.

I see a second option though ... you could use alternating electrodes in a checkerboard pattern at both the sender and receiver (with the receiver having smaller ones). The fringing electric fields fall off quite rapidly of course, but still ... might be possible.

It would be a little more complex though, because depending on alignment the receiving electrodes can be at pretty much any polarity ... you have to actively pick out and correctly connect the ones which are in phase with each other.

Ash Small

Thu Jun 28 2012, 12:22AM

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

how would you approach this?

This is hugely oversimplified, but with a few inductors thrown in for good measure (I'm sure SC, or others, could advise) would this not be a starting point?

1340842928 3414 FT140804 Capacitive Coupling

2Spoons

Thu Jun 28 2012, 01:26AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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A resonant system is definitely called for. Over that distance the capacitance will be very small, (unless you've got a sq metre of area), so to get 1/2W you will need to go to high frequencies, 10s to 100s of MHz would be my guess.

Ash: have you actually looked at what you've drawn and tried to work out the current flow?

Pinky's Brain

Thu Jun 28 2012, 05:49AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Ash Small wrote ...

This is hugely oversimplified, but with a few inductors thrown in for good measure (I'm sure SC, or others, could advise) would this not be a starting point?

There needs to be a return path to ground for current to flow ... now in practice there will always be parasitic capacitance back to ground of course, but best to draw it in and have some idea of it's magnitude so you can do some simulations.

Ash Small

Thu Jun 28 2012, 09:16AM

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

Pinky's Brain wrote ...

Ash Small wrote ...

This is hugely oversimplified, but with a few inductors thrown in for good measure (I'm sure SC, or others, could advise) would this not be a starting point?

What I've drawn IS hugely oversimplified, but surely this is, in part, how a plasma ball works, for example?

If I'd added a capacitor between the AC source and ground, possibly, and if the AC source was, for example, a Tesla coil secondary, surely the potential of the capacitor plate above the AC source would fluctuate?

I was considering mechanisms for 'electrostatic power transfer' when I drew it, as 'electrostatic' seems to be the keyword here.

Would you agree that the part of the circuit comprising the capacitor plate connected to the diodes would pump some charge into the top capacitor?

Where would you start if you were designing a system for electrostatic power transfer?

EDIT: Are you saying it would require a second set of capacitor plates to work ( return path), and that a system with only one set of capacitor plates for energy transfer won't work under any circumstances?

Steve Conner

Thu Jun 28 2012, 09:37AM

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

The Murata modules appear to be modified CCFL inverters, so I think these would be a good starting point for a DIY system.

Tesla coils are the original capacitive power transfer system. Tesla didn't believe in Maxwell's displacement current: he thought that the power flowed from the base of the transmitting coil to the base of the receiving one, through the ground.

If you think about that, it sums up the issues you have to face in capacitive power transfer. Unless you're transmitting proper radio waves in the far field, you need a "circuit" with two paths. In Tesla's case, one path was the ground and the other was displacement current flowing between the transmitter and receiver toploads.

Ash Small

Thu Jun 28 2012, 10:14AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
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Then may I ask one further question?

If you have a small, battery powered plasma globe, and you, for example, touch the globe, the streamers will tend to be attracted to your finger, due to the increased capacitance.

you become charged by this process, yet where is the return path?

If you were to replace your finger with the capacitor plate in the circuit below, would not 'some' charge flow in the circuit and charge the capacitor at the top?

Steve Conner

Thu Jun 28 2012, 10:56AM

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

The return path is through the capacitance between your body and the surrounding environment, which is more or less "ground". This is about 100pF, the human body being quite a large lump with dielectric-soled shoes.

Your circuit is incomplete, it doesn't show any return path. Try to redraw it including the stray capacitances that it relies on to work.

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