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Low current in wireless electricity

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atocp

Tue Jul 05 2011, 03:47PM

Registered Member #3983 Joined: Sun Jul 03 2011, 03:55PM
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Posts: 11

On another point, re-reading Marko's tutorial and Youtube video, it appears ~20A is being pumped into the transmitter, as the coil heats up. Will have to investigate how much I am passing in.

cheers,
atocp

Forty

Tue Jul 05 2011, 04:42PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

yea i was just about to ask you what sort of input current you were using. frequency generators usually don't output a whole lot. if it turns out that you'll need a lot higher current in the primary, then you may need to up the number of strands in the litz wire to handle it. you may end up having to build your own circuit to oscillate the primary. or use the signal from the function generator as an input to a mosfet driver -> GDT -> half or full bridge of mosfets powered by a seperate dc supply.

atocp

Tue Jul 05 2011, 07:04PM

Registered Member #3983 Joined: Sun Jul 03 2011, 03:55PM
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Posts: 11

Im thinking of putting an n channel mofset connected to a separate dc power supply and firing that through to give me my current. Tried this before, but it didn't really like it, so had to use a darlington and another one, and it still didn't really work.

Can I ask, when you use a signal generator to fire a mofset, should I be using a square wave, or is it ok to keep using a sine wave as I have been using?

Forty

Tue Jul 05 2011, 07:27PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I don't really know. but i imagine that if you use a mosfet driver ic, it'll turn whatever input signal into one that the fet likes.

Ash Small

Tue Jul 05 2011, 09:38PM

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

atocp wrote ...

.Can I ask, when you use a signal generator to fire a mofset, should I be using a square wave, or is it ok to keep using a sine wave as I have been using?

I use a square wave when driving transistors. I assume this results in a faster switching time.

atocp

Thu Jul 07 2011, 06:15PM

Registered Member #3983 Joined: Sun Jul 03 2011, 03:55PM
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Posts: 11

So, an update:

I have created new coils, as per a couple of days ago. Here's the details:
43 Turns
13.5m
0.125mm wire (x8 strands)
Coil diameter 100mm
Turn spacing 0mm
Inductance 361.266uH

Now here's the problem....
I have connected a signal generator to a darlington transistor, and an external 12v supply to the collector. The emitter goes to the gate of a second mofset, and the collector goes to a 12v supply and the emitter goes to the transmitter coil. The transmitter is grounded on the other end, as is the signal gen. Resistors have been put in place, and so have proper capacitors. 2x150pF, plus another to help smooth the DC on the receiver after the diodes. Hope you're still with me, I've gone crosseyed.

I put in 10V at 175mA = 1.75W
I get out 0.6V at 1K ohm =0.36mW

Efficiency transfer = 0.02% ???? @ 45mm

I don't mean to sound moany, but surely I should be getting more than that. Is this down to the coil design now, should I be looking at a redesign? I was going to buy/build an oscillator, but surely I should be looking at getting higher current before I do that.

Once again, any help is much appreciated. I have, in 1 week, managed to go forward 1 step and back 2.

Thank you again for looking and helping me.
atocp

Forty

Sat Jul 09 2011, 04:32PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

whats the exact circuit setup for your RLC circuit on either side? glancing over wiki, it looks like if the resistor is in series with the coil, and the capacitor in parallel with that, of if R and C are in parallel, and then in series with L, then the resonant frequency will depend on R as well. This could give you a mismatching resonant frequency if both sides do not have the same configuration.
0.02% does sound low. you could probably do better with a powered fan turning a generating fan at a distance, so there must be something wrong.
your input power sounds really low as well; i think the magnetic field has to be strong enough to 'reach out' to the other coil. even a 1000 turn, iron cored electromagnet wouldn't be very strong with 1.75W. I might be totally wrong, i haven't done this wireless transfer thing before. but i think if you can crank up the input power, you'll see a nonlinear increase in the efficiency.

atocp

Mon Jul 11 2011, 08:28PM

Registered Member #3983 Joined: Sun Jul 03 2011, 03:55PM
Location:
Posts: 11

Hi Forty,

I have downloaded Eagle in an attempt to try and put the circuit into an easier to see image. I hope it helps, this was my first ever circuit diagram. I've used Logic Works before to create logic circuits, but nothing like this.

It should all be as it is shown, but I will double check when I get into uni tomorrow. I haven't shown it, but there is a load on the receiving coil in parallel with the end/smoothing capacitor. At first some LED's, then I put on a decade box instead (1K Ohm).

I tried to increase the power, but there was no effect on the output level when I put up the voltage after a certain point, and the same with the amps. My previous results were at maximum output, and the reduced level of amps was due to me backing off the amps as there was no power output difference.

I also spoke to some of the Electrical and Mechanical Engineering guys in another department, and they have suggested one possible reason regarding my power problems could be to do with my coil design in relation to frequency, and what both coils are tuned to. The told me to figure out the self-resonant frequency, and aim for that. With a minor modification, I can get it down to 8MHz (MIT use 9.something in their coils, but are huge). So I am going to build a Colpitts oscillator with an 8MHz crystal and give that a go. I am sticking with the Litz wire just now, but after reading on wikipedia that it is meant for 'up to' the MHz range, I may be looking at some copper pipe soon. For now, I can stay with it, as I only need 20cm.

I shall let you know the results of it. I looked online for some oscillators, and saw one from 1-20MHz, variable. But delivery was around 3 weeks. I have 7 weeks remaining, and I still have to design a scooter/moped.

On another point, if anyone here wishes to fill in a quick questionnaire on commuting habbits, and scooters/mopeds, feel free. Totally anonymous, and all the details, ethics etc are on the first page before the start. Cheeky, I know.

Thanks for the help again, and I will post again soon with my results from the 'oscillator of hope'

atocp

1310416079 3983 FT119070 After Mosfet Circuit

atocp

Mon Jul 11 2011, 08:29PM

Registered Member #3983 Joined: Sun Jul 03 2011, 03:55PM
Location:
Posts: 11

mmm... doesn't like the link,

radhoo

Wed Jul 13 2011, 11:25PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

I've just completed a wireless power transmitter, and got some decent results.

Actually I plan to use this to charge the batteries of a robot. The robot must locate the charging station and get close to it. Very easy as compared to making the robot plug in to a connector.

I used flat circular coils (disk shape). See a demo video:

Schematics here:

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