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How to convert 6V~3w input to 5V 500mA output (USB)

Discussion in 'General Electronics Discussion' started by rambler, Aug 5, 2011.

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  1. rambler


    Aug 5, 2011
    So I have a slightly strange project, and I'm a total beginner with this stuff, so bear with me if I'm asking stupid questions! I have a power source that generates 6V~3W, but that power varies because it doesn't generate constantly (it's a hub on a bicycle, so the faster you go, the more it generates, and it doesn't, so far as I know have regulator, as that part is usually in the lights), and I want to bring it down to 5V 500mA to power a gps that gets its power from a USB. So there's the project. I think I need some sort of diode to bring the voltage down, but I also need something to smooth it out, which would be some kind of cap, wouldn't it? Like I said, my grasp on all of this is way too loose, but I do hope to understand it better soon. Thanks much, and I'll try to provide any extra information that might be useful, if it's needed, but right now, that's about all I can think of.
  2. Laplace


    Apr 4, 2010
    I also have a 6V-3W hub generator which powers an array of 16 18,000 mcd white LEDs as a bicycle headlight. If the bike goes very fast the generator has enough voltage to also switch in series a 1 watt LED with a focused beam. I use a Schottky diode bridge rectifier to maximize the DC voltage and am working to convert the design to an active bridge rectifier.

    But realize that the generator provides an AC voltage (and it is not a clean sine wave). The no-load voltage can exceed 70 volts but comes down quickly as the current load increases. Also, the AC frequency varies over a wide range. I would think in terms of charging a battery followed by a 7805 voltage regulator.
  3. BobK


    Jan 5, 2010
    Agree with Lapace. The output is too variable to power the gps directly. What happens when you are stopped at a light? You need to power from a battery and use the generator only to recharge the battery.

  4. rambler


    Aug 5, 2011
    Ok, that's fine. I kind of thought that there might be some way to use some capacitor set up that might allow for some kind of very short-term storage, and therefore regulation of the output. Of course that thought probably stems from my super-clueless status! Oh well, isn't ignorance bliss! Well, so I had thought about trying to charge a battery that I could later use to power the gps, but I guess I'm not sure what kind of a battery I might be looking for, and what would be involved in charging it.
    On a side note, one of the things that made me think of doing this is a product that came out last year that allows one to charge their phone or gps from the generator hub. Here's a link:
    I would, in an ideal and possibly non-existent world, like to make something like that that was unadjustable, and much smaller, which is why I started looking into this in the first place. With that unit, they do say that with some devices you need to use a battery in between the charging unit and the device being powered to smooth out the voltage.
    The last thing that I would like to make clear, though it may be of no consequence in any case, is that I'm hoping to charge the gps, not power it. That is an eventually kind of thing though since the gps I have at the moment can't be charged and used simultaneously, which is kind of silly, but I guess they didn't forsee people like me....
    Thanks so much for the replies! I really appreciate the help.
  5. TBennettcc


    Dec 4, 2010
    Yikes. $300 for that E-WERK system and the battery, and you still have to buy the dynamo! (And the light if you want one...) You must be on your bike an awful lot...
  6. rambler


    Aug 5, 2011
    Haha, well yeah, it's also pretty expensive, which is another reason I'd like to do it myself....
    But I am on my bike a lot, but it's rare that I would have a need for something like the E-WERK, so the price is completely absurd in my case.
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Does the GPS have its own battery? Obviously the dynamo is going to give you somewhat less than a continuous supply of power (especially if you stop :)).

    If the GPS misbehaves when the input voltage drops below 5V, you might need to consider disconnecting the output once it falls significantly below 5V. The rapid reduction in load is likely to vause the voltage to rise rapidly, so you will want to do something to also ensure the GPS doesn't get presented with the equivalent of a 5V square wave.

    You might consider a circuit where the dynamo charges a LiPo cell, with the power from the LiPo cell (and any additional available from the generator) are used to generate a 5V rail. The availability of the 5V rail should be dependant on the LiPo battery voltage. You could turn it ON at 4V and turn it OFF at 3V.

    This might take the form of a buck regulator (to 4.5V(ish)) followed by a boost to 5V with the LiPo battery supplying an alternate low voltage rail in the middle.

    Quite complex, but would probably make for a very good source of 5V that is also relatively independent of the alternator's instantaneous output.
  8. ikansw


    Aug 8, 2011
    Another rough and simple idea is to use a full wave rectifier, wire the DC output to a large cap (1000 uF) and then to a three terminal regulator (7805). The output 5V goes to the GPS device (through an optional SPDT switch, explained later). This will keep your GPS running as long as you are pedalling and probably a minute after you have stopped (as I suspect that the current it requires would be much less than the [email protected] specified maximum). Having the GPS device wired to the 7805 through a SPDT switch would allow you to manually switch over to a backup battery and continue beyond the stored charge. It wouldn't be automatic, and you will have to act in a timely manner ! But trust me, it will make a great conversation starter when people see you flip the switch :)
    Last edited: Aug 8, 2011
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