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AC or DC current

  1. Feb 25, 2014 #1
    Hey, I hope I don't bother any of the serious electrical engineers, but I was wondering if anybody could help me with a project I'm working on.
    I want to build a generator that I can attach to my stationary bike that will charge my phone. It has a very simple design, two permanent magnets that create a magnetic field around a coil which gets rotated by the bicycle. The part that I'm stuck is what to do now with the two ends of copper wire from the coil. The potential difference produced will be sinusoidal, and I know that wall sockets use 60 hertz AC current, which is what I normally charge my phone with, so is it even necessary to use a commuter and another coil to make DC current? Or can I let the current alternate and connect the circuit up to a USB converter?
     
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  3. Feb 25, 2014 #2

    PhysicoRaj

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    What is the output voltage of your generator? What is the rated charging voltage of your phone?
     
  4. Feb 25, 2014 #3

    phinds

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    Do you actually plug your phone directly into the AC socket?
     
  5. Feb 25, 2014 #4
    Oh yeah, thank you. And I haven't tested the output voltage yet, but I don't think it'll be greater than 240V, which is what my phone is specified to take.
     
  6. Feb 25, 2014 #5

    PhysicoRaj

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    No! your phone won't take 240V surely! Are you serious? What phone is it?
     
  7. Feb 25, 2014 #6

    phinds

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    You didn't answer my question.
     
  8. Feb 25, 2014 #7

    russ_watters

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    240V would be the input of an AC charger...
     
  9. Feb 25, 2014 #8

    PhysicoRaj

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    And the output? OP has to specify that.
     
  10. Feb 25, 2014 #9

    PhysicoRaj

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    If OP's generator has an output voltage equal to o/p of the charger, he can just rectify the generator o/p, filter it and connect directly.
     
  11. Feb 25, 2014 #10

    sophiecentaur

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    A rectifier, reservoir capacitor and a Voltage regulator are what you need. All very easy stuff, once you know what the output of your dynamo is and the required volts for your phone. The voltage regulator may not be essential but it will protect the input of your phone - which is not possible to repair.
     
  12. Feb 25, 2014 #11

    phinds

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    You think maybe we were trying to get him to figure that out for himself?
     
  13. Feb 25, 2014 #12

    russ_watters

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    It looked to me like the person I responded to misunderstood the answer he was given.

    I was thinking there is a language barrier here, but I guess others are thinking the OP doesn't know the phone charger has a different input and output.
     
  14. Feb 25, 2014 #13

    jim hardy

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    Which gets rotated - the coil or the magnetic field?
     
  15. Feb 25, 2014 #14
    The coil
     
  16. Feb 25, 2014 #15
    They're right actually, I wasn't thinking and I just googled "iPhone 4 charger voltage" and I just found somewhere that the CHARGER could take 240v, but I can't find the voltage that the phone can take anywhere.
     
  17. Feb 25, 2014 #16
    You don't connect the phone directly to the wall socket, you plug the USB into a box which I assume has a DC output and an AC input. Is this correct?
     
  18. Feb 25, 2014 #17
    Thank you!
     
  19. Feb 25, 2014 #18

    sophiecentaur

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    If your phone is an iPhone (or most other phones on the market) it can be charged from a USB output, which provides a nominal 5V and can supply various amounts of current, depending on the Computer involved. The iPhone has quite a fast internal charging circuit (compared with some other smart phones) but you should still think in terms of an hour of two of cycling. Alternatively, you might consider using an intermediate battery (say a 6V lead acid) of high capacity, which you could charge at a much higher rate with some brisk pedalling for a shorter time and which could charge the iPhone over the necessary time. This may not suit your circumstances, of course, but it's a possibility.
     
  20. Feb 25, 2014 #19

    jim hardy

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    (gets rotated)

    So there's got to be commutation; or slip rings and rectification.....
    there exist bicycle lighting generators that'll do that, about 6 volts maybe half amp..
    wheel+generator.jpg
     
  21. Feb 25, 2014 #20

    sophiecentaur

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    That dynamo has seen a few miles in the dark and the rain, Jim!

    Actually, it's normally the magnet that's rotated and the stator is the coil. No slip rings required, that way. A good rectifier circuit is much more effective than a commutator and rotating coil for getting DC power out.
     
  22. Feb 25, 2014 #21

    meBigGuy

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    CAREFUL!!!!!!!!!!!!!!
    I'm seeing some serious misinformation regarding USB charging systems, especially how the iphone works. I've commented on it before, but the bad information still seems to show up. When the iphone (or any phone) connects to a USB port it enumerates and communicates with the computer. If you just hook an iphone up to 5 volts it will not charge. There are lots of different posts around on how to mess with the iphone DP/DM pins to get the phone to charge. Those pins also affect the rate at which it will charge. (I don't have the spec handy --- sorry)

    Also, most USB ports are limited to 500ma (called an SDP or standard data port), so you don't get the fast charge features. There are more and more PC's or hubs with CDP (charging data ports) that can supply more current. But they require a special sub-threshold handshake protocol to identify the high current feature.

    If you are going to connect a homebrew charger to an iphone you should read about how it was accomplished by others. If you decide to proceed, I can look into it for you.
     
  23. Feb 25, 2014 #22

    meBigGuy

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    Actually, this should make it clear that USB chargers are not as simple as some seem to think.
    http://datasheets.maximintegrated.com/en/ds/MAX14578AE-MAX14578E.pdf

    Figure 3 contains the DM/DP resistor values required to emulate different chargers. There are two values for apple to enable 500ma or 1A charging.

    The final answer is pretty simple --- a 5V supply with sufficient current capability and the right resistors on the DP/DM pins depending on the phone you want to connect to. I think there are other standards also.

    I'm not sure how well the iphone will respond to voltage droop when you stop pedaling. It isn't supposed to happen, and I don't know how or if the phone protects against it. You might damage your phone. (but I don't know for sure). Commercial hub dynamo connected chargers may have safety circuits for this. Of course the ultimate (as posted previously) is a dynamo/battery/phone-charger system.

    Here is an interesting new product via kickstarter: http://sivacycle.com/
     
    Last edited: Feb 25, 2014
  24. Feb 26, 2014 #23

    sophiecentaur

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    All this is absolutely true and, if the OP is serious about this project, it goes without saying that he will need to make use of some of that information. The fact is that he will need to connect his phone to a 'box'. Anyone knows how much more convenient it is to wire up a 'receptacle' than to terminate a cable so he will end up with a USB receptacle on this box and make his connection to the phone with a proprietary 'USB cable'. The phone 'talks USB' and so must this charger. The cheap and cheerful USB mains chargers perform adequately with a minimal "communication" with the phone (can it be more than which pins are earthed and which are connected to Vcc?)

    If you read the posts from the OP, I think you will divine that what is required is something that works - at all, and not anything that involves the sophistication of high current USB options. You clearly have a lot of experience about the USB interface but it is possible to operate successfully with a reduced subset of the full knowledge. If you give him the impression that he will need to know the lot, then he will be put off the idea, completely. So far, in this thread, we have got as far as deciding that 240V AC is not what's required (:wink:) and my contribution that USB uses 5V was surely not harming the progress of the discussion.

    I fully agree with you that he should see what others have done, before launching out on buying components. Personally, I would recommend starting one's electronic constructor life with a project that didn't involve a £n00 iPhone!
     
  25. Feb 26, 2014 #24
    This project is really more of just an attempt at a design, not actually building the system, at least not now. I'm still just in high school physics, and I was just trying to apply what I've learned about magnetic fields and current to something practical in the real world. I'm sorry that I seem extremely ignorant about this, and I thank you all for helping me out. But I certainly was not planning on just hooking the bike device straight up to my phone.
     
  26. Feb 26, 2014 #25

    sophiecentaur

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    That's a relief for us all !! :approve:

    No need to apologise for ignorance. You have the rest of your life to sort that out. Stick with PF and try to ignore the occasional Rantings you will encounter (mea culpa, as much as anyone else)
     
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