If you want to transfer more power... I suggest that you use two 'horseshoe'
shaped magnetic cores with the open ends facing each other. The best case would be if you could make them touch... but even with a gap it should concentrate the flux much more than your current design.
you might look for ferrite "C" cores, there are other shapes you could use as well.
I don't see a schematic posted... but I'd caution against paralleling transistors unless you have some series resistance (usually in the emitters) or you match them for Vbe (voltage base-emitter) drop so they share current well. If not, one will probably get more current than the rest and fail first (either shorting and killing the circuit quickly, or opening and leaving the rest to run even hotter and fail slowly...)
Not knowing what knowledge this young man has, as Engineers in the field we need to be careful with what instructions we give him. While outside thinking is indeed necessary for innovation, a firm comprehension of the fundamentals is necessary. That can only be obtained through understanding and asking those with greater knowledge than the questor currently has.
With that said, I think it would help young Mr. Duffy out if others in our field that may have some thoughts on how to improve his charging circuit would share them with him as he may not know where to begin to change and experiment to attempt better charging times. It is another step to outside thinking as well. Learning from others and bouncing ideas off of more knowledgable people. Even if you run into someone who states, "Oh that can be done that way...", it can be turned into a "Because you said it couldn't I need to prove or disprove that to myself first!" philosophy.
Great Job on your first Gadget Freak Mr. Duffy! If you keep at the circuit I am sure you will find the answer. Ask everyone you can for clues, thoughts, and ideas on what might make the battery charge faster! And remember, like TJ says, don't take anyone's answers to heart unless you understand them and know that they apply to your question. While it is up to *you* to learn, others can help as well.
John, in order to more efficiently transfer power from the primary coil to the secondary coil, you need to have a high permiability core to concentrate the magnetic fields of the two coils. That is why transformers have iron or magnetic cores. I have a Hoemedics toothbrush that uses a dimple in the housing that allows a magnetic core from the charger to closely couple the two transformer coils. The other technique is to use resonance of the two coils by adding parallel capacitors to tune the two coils to the same frequency to allow better power transfer. Good luck with your experiments.
Yes, I had originally intended to use a core, but since the PVC cap is rounded, I cannot fit a core inside the sender coil, and the reciever coil is crowded by the circuit. Thanks for your help anyways.
The efficency of the lantern itself is actually super high. The circuit with a fresh alkaline AA will last about a full week before running out. As for ICs I dont have acess to anything beyond my local radioshack and science surplus store. This gadget can be made from most low-end electronics stores with component parts.
As for the "horseshoe" cores, I dont know where to get any, and I'm not sure why the schematics werent posted. I sent them in with the build instructions. If you do want to make one, you can find the original article that I wrote on MAKE:projects. (It was written before this one was posted). The transistors actually work okay, as my current unit still runs cold after about 3 months of use. It may not be the "best" way, but it doesn't require any fancy or unusual components.
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