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Wireless power- my own setup and questions

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LAN_403

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Fri May 01 2009, 03:03AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The values for the capacitors aren't critical, it usually works to just throw a good mix of values on there to catch different possible frequencies.

To measure the output power, rectify and feed into a capacitor (to convert it to DC), and use a resistor to load the output. Ideally you would get a large variable resistor and make a graph of voltage vs resistance (which you can then square and multiply by resistance to get a graph of power vs load), but it would probably work fine to use a fixed value resistor for comparative measurements.

As to the mosfet, the IRFP460 is a pretty common choice. With a powerful gate driver you should be able to get it up to 10MHz without too much trouble, and it is good for a fair amount of juice.

As to getting solderable breadboard ('perfboard') just google it

twist2b

Fri May 01 2009, 08:02AM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

Do you really need a machine? Can't you pull this off via a TL494? It can create both sharktooth and square waves, and with a capacitor you can create a sine wave (constantly positive).

I dunno, just saying, that would be a cool method.

Oh, and just interested, how did YOU test resonance?

Proud Mary

Fri May 01 2009, 08:19AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Don't function generators usually have to work into a matched load, e.g. 50R?

Marko

Fri May 01 2009, 09:52AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

For those who haven't seen it yet, a teaser:

For power levels as low as for a single led, even audio frequencies without any resonance at all are just ok:

I concentrated on transmitting more significant amounts of power though, instead of just lighting led's.

I used a self-resonant royer oscillator, which gives an advantage of always keeping the transmitting tanks circuit always in tune, only putting real power into it. I've had several ideas but settled on that for a small tabletop demonstrator that doesn't involve high voltages.

A function generator is more challenging to use for this demonstration due to tuning requirement on both sides and limited power level. I would do it this way:

- The waveform must be sine wave. Power factor of square wave or any other waveform will be bad for feeding a parallel resonant circuit.

- I would first tune the transmitter loop alone for the maximum real power draw, at it's stand-alone resonant frequency - which can be verified by a led connected to a wire coil of several turns without any capacitance. This does not tell anything about the resonant frequencies of the system when the receiver is brought in, but is a start.

- Once the receiver is coupled to the transmitter, their old resonant frequencies disappear and fourth order system with two resonant modes is born. It could be understood that, as the receiver is brought close to the transmitter, it detunes it.

Hence, you'll need to tune both the signal generator and the receiver tank for whenever you change the coupling.

Receiver can be tuned either by bringing a ferrite core into it to decrease resonant frequency, or by bringing a copper plate behind to increase it.

- Final problem is impedance matching. LED requires very little current but relatively high voltage, making it's working impedance as high as 1000ohms, and if you are using only a few turns for your coils you need to construct a step-up transformer to light the LED at greatest possible distance. A tappable winding of 100 turns or so air-coupled to the receiver coil would probably be about right.

- If you want a rectified DC output which you can measure, just use a bridge of 1N4148 diodes and a 100nF capacitor. This will work well for powering small motors too.

Cheers,

Marko

rp181

Fri May 01 2009, 12:26PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

@ Marko
I do want to transmit considerable power later, I am just getting the feel of it now. I do plan on going to a self resonant circuit. Thanks for the information
@Harry:
The BNC has a 50 ohm termination.
@...
For the solder-able breadboard, I meant something like this:

The white breadboard.

twist2b

Sat May 02 2009, 02:53AM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

Marko - Can you make a video showing what happens when objects are in the magnetic path?

Also, where did you get the sine generator? It = awesome

Oh, and can you post the schematic? That would be too cool :)

rp181

Sat May 02 2009, 03:35AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Some news.
I contacted support about the power supply issue, the problem is not EM fields and transients. This supply operates in constant voltage or current, so it constantly adjusts the output. With the supply being switched that fast, the control electronics can't keep up. He suggested a LCR circuit to issolate the circuit.

Have you tried any non-round coils? I want to try a low profile one, so I made on with 3 oval rings of 12 guage, 5 turns each. This lets me curve it a little, giving a larger range. It resoantes at 1.5mhz exactly with a 5.6nf capacitor.

twist2b

Sat May 02 2009, 10:47PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

If you ever get to try a higher Mhz, let us know.... it SHOULD give you longer range.... I am just interested to see how well it matches with MIT's math.

rp181

Mon May 04 2009, 12:08AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

This is weird...
I setup the wireless system again, with the same coils and fres. The only difference was power supply was a rewound MOT, fullwave rectified, and a 1200uf filter cap, for 24v out. While I was running it, the ceramic capacitor on the receiving coil got burning hot, so i tested the output, with a half-wave rectifier and a 3900uf capacitor. The output was giving 130v! It charged the capacitor at about 3 joules a second.

How is the output voltage so high? Could it be acting like a boost converter? I am uploading a youtube video right now.

EDIT here it is:

Scope is on the input, anode and cathode of IGBT.

UPDATE: The voltage boost only happens below the resonant point of the source coil. resonant is at about 1mhz, voltage boost starts at 940khz and below.

twist2b

Mon May 04 2009, 04:48AM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

HAHA, I bet you got so excited :P

You should do the same thing but get some distance... seriously everyone keeps just doing short distance wireless transfer, its just a "special" transformer then -_-

Distance is the impressive thing.

Though rp181 - Your equipment is AWESOME, how did you get your hands on so much awesome crap??? :P

due to college happily cutting holes in my pockets for me, I have not really had the chance to grow much more in components or equipment -_-

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