I was kicking around the idea of making a low powered rechargable battery operated device which would recharge via mutual inductance. I have heard of this before but I have never attempted anything similar myself. Any thoughts on how it might work out what type of circuit? This is not so much a request for an answer as it is an open ended discussion. The device would probably be similar in power consumption to a PDA or a cellphone. And the battery would likely be 3.7 Li-ion.

 

You could use a coil to pick up an oscillating magnetic field (generated by electrical field). Then use a rectifier to rectify the current. You will need a coil with many turns. Also, it might take a great deal of time recharging the battery, since the power generated is very small.

This kind of circuit was done to pick up the magnetic field from high tension cables. The objective was to try to power a house. They reached something, but it was not enought (even to power a single appliance).

 

Interesting, I did not know that someone had tried this with such a high voltage. I know there are consumer products available with the criteria I stated before. I think the hurdle to overcome will be the fact that the 2 coils have to be separated. Even if it is only by a couple millimeters it is a problem since they cannot be coupled around a shared iron core. but what if I built the device with a slot in it so a protruding iron rod from the "primary" circuit could fit inside this slot making the engineering challenge less difficult, that way the 2 coils would share a common iron core. What do you think? Am I over thinking the whole thing?

 

http://forum.allaboutcircuits.com/showthread.php?t=7143
Thats what I want to do

And it wont take long to charge if you get the coils and everything right.
search in Google for mit wireless light bulb. they powered a light bulb from across the room using two coils.

look at my last post on page 3. look at that circuit.V.04
i thought the first one could work but in my inexperience(i was my first big circuit design.) i was wrong.
but the guys here are really help full so i should get this to work soon.

 

I think if you can get the transmitter and receiver coils in resonance, you can greatly increase the range. This is the basic idea used by Tesla's Magnifying Transmitter and MIT's Witricity.

 

Oh, thats what you meant by "use a different powersource" Sounds interesting. So MIT has done it huh? those guys annoy me because I am envious of them. Getting the coils in resonance is going to work how? I am not familiar with getting the two coils in resoncance I am only familiar with LRC resonance.

 

Oh, thats what you meant by "use a different powersource" Sounds interesting. So MIT has done it huh? those guys annoy me because I am envious of them. Getting the coils in resonance is going to work how? I am not familiar with getting the two coils in resoncance I am only familiar with LRC resonance.

You don't need to be envious yet, they've only gotten about 40% efficiency so far, which seems pretty wasteful.

I can't claim to understand how it works completely, but it seems to involved evanescent-wave coupling and the near field. The math and physics are a bit beyond me.

LRC resonance is all it is, just try to minimize the R part so we don't waste energy. You can make a coil by itself resonate (with its own parasitic capacitance), but adding a cap will make the resonant frequency much lower, and easier to reach with cheap equipment. Now just take that LC circuit and make an identical copy of it, but replace the power supply with a load. I will try to draw you a diagram when I have time later.

 

You don't need to be envious yet, they've only gotten about 40% efficiency so far, which seems pretty wasteful.

I can't claim to understand how it works completely, but it seems to involved evanescent-wave coupling and the near field. The math and physics are a bit beyond me.

LRC resonance is all it is, just try to minimize the R part so we don't waste energy. You can make a coil by itself resonate (with its own parasitic capacitance), but adding a cap will make the resonant frequency much lower, and easier to reach with cheap equipment. Now just take that LC circuit and make an identical copy of it, but replace the power supply with a load. I will try to draw you a diagram when I have time later.

Well that basically it.

You will have to get a LCR meter to measure the inductance of the coil. then use this formula to find resonant frequency. in radians per second

Or in hertZ

Have you checked out my post at http://forum.allaboutcircuits.com/showthread.php?t=7143&page=5 ?
When I get the ok from the guys here i will start to build it. (i already got all the parts for it)(for free)(ok some of the parts some i had)

 

Well, you can do it without the math. Just hook up a function generator across the coil, and do a frequency sweep. Watch for the frequency where the voltage across the coil reaches the highest peak.

Arthur: I did check your post. I think you will have to tune the coil with those parallel capacitors attached the same way they will be when the circuit is running. You should probably set the 555 to output whatever the resonant frequency of the coil/cap circuit is.

One more bit of advice, you should see what happens when you shrink the duty cycle (by changing the the 1K resistor on the 555). Basically, whenever the voltage is steady DC, the coil isn't going to transfer anything to the secondary, so that energy is wasted.

 

I have the resonant frequncy figured out its about 680khz it works and i but only get .6v on the secondary coil when i used a 9v and the 555 without an fet. The fet will now send more power to the coil so i will have a higher voltage on the secondary. It works like those tooth brush chargers. I have 2 extra bases but the circuits are FULLY covered in epoxy! Like a epoxy cube with wires going into it!

 

What kind of nonconductive epoxy did you use? Doesn't the resonant frequency seem a little high arthur? 680khz? Just wondering. thanks for all of your input guys. Even more help and ideas are welcome.

 

I did not use the epoxy it one of the store bought tooth brush chargers.

680kHz was the resonance frequency of my coils so that what i used.

 

I have seen this done on an "Oral B" electric toothbrush.. there is s coil mounted in the base of the brush, and the base slots (loosly) into a charger.. Charges a AA in about 4 hours. The circuit (only had brief examination) looks like it could be resonant. great toothbrush - this method of charging eliminates problems one would get from water and toothpaste in a socket... the whole thing is completely sealed and watertight. (and the way I use my toothbrush it needs to be....! )

 

I don't think the toothbrush chargers use resonance. They are just acting like normal (although air-core) transformers running at 60 Hz. That's why it has a limited range--it will only charge when the toothbrush is placed right on the base.