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Registered Member #1822
Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300
I have decided that a good goal would be to be able to charge the cell phone in my pocket in my car with the transmitter in the center console which would put it about 4"-8" away.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
I'm planning to play with this over christmas; I have an ambitious goal of lighting some christmas tree lights wirelessly.
There's a good site with high-Q coils here (it's actually crystal sets, but it's the same principle I think, he gets over a thousand with some of them):
lots of litz wire and widely spaced turns seem to work well, but you may need to adapt it to higher power I guess.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Pretty cool.
I would guess you need more turns on the receivers, that should give better range.
I had a play with this stuff over christmas, wound some coils and capacitively loaded them, and my father built an oscillator. I only managed to get a Q of about 100 though, the insulated wire I was using was fairly lossy. For high Q it looks like you have to avoid wood, paper and polypropylene near the coil, they all are lossy dielectrics, I'm not sure what the insulation was on my wire.
As you did here, getting a range of about 1 coil diameter seems fairly straightforward, witricity currently claim they can get about 7 coil diameters at 10% efficiency, which is really very good.
I also built a theoretical equivalent circuit in LTSpice.
I thought up to that point I understood how it worked, and I wasn't entirely wrong, but playing with the model was interesting; it looks like basically you want to run the transmitter coil at the maximum voltage/current you can; and Q is important but not all-important. Basically free space is high inductance/impedance so you can minimise losses by running high voltages at each end, a bit like a transmission line.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Boston, April 1st, 2010: Researchers at MIT have developed a breakthrough form of power transmission that promises to revolutionize electrical engineering. The aperiodic PVC insulated copper waveguide structure is highly flexible, inexpensive, and can transmit power with 99% efficiency over a distance more than 1000 guide diameters.
J. Random Boffin from MIT's Radiation Lab says: "Thanks to our Waveguide/Insulated Reactive Elements (WIRE) technology, you won't have to worry about cooking your cat with microwave beams whenever you're using the vacuum cleaner. WIRE works just fine with 60 cycles or even DC, so you can take your klystron to the scrapyard and free up a spare room. A hamster-proof version with Kevlar dielectric is in the works, too."
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
GeordieBoy wrote ...
Polypropylene = Good HF dielectric
Yeah, you don't particularly want that. Air is better. PP's got a high dielectric constant which is bad at high enough frequency, because it moves the resonance down, since it gives you a minimum capacitance.
At lower frequencies that wouldn't matter except for the fact that the loss angle is horrible, for high-q coils you don't want it at all, it just steals energy and reduces Q. Things like polyethylene, pvc perform much better. Silk is really, really good unless it gets damp.
wrote ...
> Witricity currently claim they can get about 7 coil diameters at 10% efficiency, which is really very good.
Is 10% good efficiency!?!? I can beat that on efficiency, distance, cost and weight using a simple wired connection!
Can you do this wired connection 7+ft across the room without introducing a trip hazard, and without having to remember to plug it in?
10% is *plenty* good enough for recharging cell phones. They're *very* low average power items. And the system is more efficient at close range- you get maybe 95%; similar to a normal tranformer.
Registered Member #2475
Joined: Sun Nov 22 2009, 10:16AM
Location:
Posts: 3
BigBad wrote ...
Pretty cool.
I would guess you need more turns on the receivers, that should give better range.
I had a play with this stuff over christmas, wound some coils and capacitively loaded them, and my father built an oscillator. I only managed to get a Q of about 100 though, the insulated wire I was using was fairly lossy. For high Q it looks like you have to avoid wood, paper and polypropylene near the coil, they all are lossy dielectrics, I'm not sure what the insulation was on my wire.
As you did here, getting a range of about 1 coil diameter seems fairly straightforward, witricity currently claim they can get about 7 coil diameters at 10% efficiency, which is really very good.
I also built a theoretical equivalent circuit in LTSpice.
I thought up to that point I understood how it worked, and I wasn't entirely wrong, but playing with the model was interesting; it looks like basically you want to run the transmitter coil at the maximum voltage/current you can; and Q is important but not all-important. Basically free space is high inductance/impedance so you can minimise losses by running high voltages at each end, a bit like a transmission line.
Or something like that anyway.:-)
Thanks for your suggestions.
I have tried using more turns on the receiver coils, however, it did not give me as high current. My current receiver coil is 2 turns, able to give me ~ 0.4A which is good enough for charging mobile phones. Also, distance will not matter in this project since the phones will eventually be placed on top of the transmitting coil to charge.
Is wood bad for getting a higher Q? If so, do you suggest I use Plexiglass instead?
My present transmitting coil is a bit thick in size, i was thinking about using much thinner wire (about 14 AWG) or possible use a set of 22AWG and twist them together. Hopefully this will give me better result or efficiency.
I'm glad you created a model on LTSpice. I was also going to create one as well. Would you be willing to show me your model? If so, please email me or PM.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
For short distances like you are talking about, Q doesn't seem so very important; so PP or wood/paper may be acceptable, but details of how you wind the coil can matter as much or more.
For the receiver coil for more range or power, you probably want more copper thickness- winding together insulated wire connected in parallel or proper litz wire is probably the way to go. More turns increases the voltage of course (although not necessarily the power or current).
Registered Member #1721
Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136
Hello:
I love where all this is going, and the benefits are great, as is the convenience factor. The ideas seem to literally span the electromagnetic spectrum. I even read one idea which proposed using light with highly efficient photovoltaic s like GaAs solar cells, by using light beams, light pipes, fiber optics, mostly in the IR & UV spectrum. The UV was nixed due to safety concerns: Q:How did you get that big red spot on your forehead? A: I was sitting in front of my cell phone charger :)
While I am far from being an expert, the microwave part of the spectrum has some nasty "known" side effects like cataracts in the eyes, sterility in men, and EEG measurable brain wave disturbances. This leaves the normal RF, and the VLF range like 250KHz, which is now being used in induction lighting. Shielding is of course a possibility, just like in my microwave oven.
The public at large is of little help in this area, I often still get serious questions like: "If I ever warm up my lunch, with the X-ray machine" I suppose this is a result of the folk expression of "Nuking Your Lunch". The most troubling thing to me is that these kind of questions are becoming more, and more common, as society at large here is becoming more ignorant in general, especially in science related areas. This does not bode well for the future (At lease in the US). I cannot speak for the folks in Europe, or elsewhere. Maybe they have more competent leadership, or better stated educated voters, who can see beyond micro-issues like abortion.
Overall I hope that the technology here, and its implementation, will not overtake the research on the biological effects on human beings, or even wildlife in some cases. With anything new in this arena, the end user would be well advised to do some research of their own from legitimate sources, before setting up "X -Gizmo" in little Johnnies room.
On the plus side this will give us a whole crop of really cool toys to hack for our own pleasure and curiosity. Lots, of fun new circuit boards. Lets not however forget to do our homework on the safety end, just like we do to stay alive with our HV stuff :)
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Wireless Power
