Dynamo Light - Front and Rear- Mtbr.com
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  1. #1
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    Dynamo Light - Front and Rear

    I have been reading about dynamo lights for the past week and have been intrigued to how much testing has gone into fine tuning.

    I am hoping to start building a dynamo light circuit that powers 6 XT-E (3S2P) up front and then have 3 XP-E Red on the rear with a flasher.

    The idea is to use Martins circuit 7Dynamo LED Light Systems for Bicycles (electronic circuits) with the string of 6 leds but in 3 series 2 parallel. I would also like to have a a flashing rear light by using a 555 timer.

    Is there any reason that this circuit would not work?

    When current is flowing through the rear flasher will it cause the the front lights to dim?

  2. #2
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    You don't get any real benefit from putting the XTEs in parallel. Three at 500mA will be about as bright as as six at 250mA.
    For the rear, you'll get 200mA out of the 555 which will be annoyingly bright on a flasher. I would ditch the 555 and just run a single xpe in parallel with the XTEs, limit the current to around 50mA.

  3. #3
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    Some brief comments (because I'm at work), I can discuss more in depth later

    1. Having tried the same thing recently, yes, the 555 timer will cause the front lights to flash too.
    2. Even a single XP-E in the P3 bin from Cutter (Red-Orange, actually) is extremely bright at 500ma.
    3. What kind of riding are you doing with this bike that needs so many LEDs? If you're going to be sticking to trails, you don't need anywhere near as many rear lights. If you're going to be using it primarily in urban traffic the power draw from 9 LEDs is going to require an extremely large and expensive standlight capacitor bank, otherwise all that light will instantly vanish when you stop (or even slow down) at an intersection.
    4. Having built two dynamo lightheads using an XT-E and XB-D, I can say they don't work very well with Carclo TIR optics; they don't focus very well and have a prominent yellow spot in the middle, make sure your optics work well (vs an XP-G, XP-E, or XM-L) before investing too much money in XT-E's, housings, and reflectors/optics. I'm planning on running some tests this weekend using the optics with other emitters.

  4. #4
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    You don't get any real benefit from putting the XTEs in parallel. Three at 500mA will be about as bright as as six at 250mA.
    For the rear, you'll get 200mA out of the 555 which will be annoyingly bright on a flasher. I would ditch the 555 and just run a single xpe in parallel with the XTEs, limit the current to around 50mA.
    The reason for the 6 LED's up front is so that the voltage supply needed isn't so large when the hub is starting up. The next reason is this light will double as a MTB helmet light with a battery and a bar light for road riding with the dynamo.

    To limit the current going to the rear XP-E i could just increase the 47 Ohm resistor.

    Having tried the same thing recently, yes, the 555 timer will cause the front lights to flash too.
    Were you able to successfully find an alternative for the rear to flash? ie put two flashers out of phase 180 Deg and position them facing the other way so when 1 is on the other is off vice versa.

    What kind of riding are you doing with this bike that needs so many LEDs? If you're going to be sticking to trails, you don't need anywhere near as many rear lights. If you're going to be using it primarily in urban traffic the power draw from 9 LEDs is going to require an extremely large and expensive standlight capacitor bank, otherwise all that light will instantly vanish when you stop (or even slow down) at an intersection.
    The riding will be mixed, both MTB and Road, but i wanted a universal light for both applications. it is not crucial that i use 6 XT-E, could reduce to 3 if the current draw is so low from the dynamo.

    Having built two dynamo lightheads using an XT-E and XB-D, I can say they don't work very well with Carclo TIR optics
    Damn, did you find any other optics/reflectors that worked well?

  5. #5
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    I tried several setups with regards to the flasher, but to be fair I only tried series configurations. The closest I came is putting larger (>1F) capacitors in parallel with the white LEDs, but there was a huge penalty when it came to brightness. I finally gave up and just put a dedicated flasher on my GF's bike. XP-E + Carclo 20mm wide optic + 5-10Hz strobe = brighter than any red light under $100, easily.

    There may be a parallel configuration that makes it work, but the thing to be aware of is that putting LEDs in parallel limits max current. 2 LEDs in parallel will only ever get to 250ma each (assuming no tuning caps), and splitting the current 3 ways allows only about 170ma... not that much, no matter how many LEDs you have in series after that split.

    As for tuning caps, they're the best thing you can do to your dynamo system. Not sure when Martin did his website, but bi-polar caps are fairly cheap. I got a variety of sizes from Digi-Key for about $.70 apiece. Get several sizes because each dynamo performs differently. When I started with a 220 I would get a "peak" of power at about 8 mph... 430ma. At speeds above that it would fade to 30ma, no matter how fast I went. With a 100uF cap I can get about 850ma at 17-18mph, fading down to about 600ma around 27-30mph. I'm planning on adding a ~73uF capacitor bank I can switch to for very high speeds (the maximum supported drive current of aa red XP-E is 700ma. Be sure to heatsink well if you plan on overdriving it).
    Also, at higher power levels (850mA at 9V = 7.6 W) , the tuning cap gets HOT. I ended up wrapping mine in aluminum tape and taping it to the side of my housing (an old candy tin on my rear rack) to provide a thermal path.. If you have extra aluminum scraps and a drill press you can probably drill a hole slightly larger than the cap and attach it to the aluminum block to use it as a heat sink.

    No, I haven't yet figured out what the best optic is, but I'm going to do a new batch of lightheads this weekend and will pay more attention to mounting the optics. Sometime next week I hope to put up some tests of a custom lighthead I had fabricated with various different emitters/Carclo optics.

  6. #6
    Frt Range, CO
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    Quote Originally Posted by mitchellluke View Post
    Is there any reason that this circuit would not work?

    When current is flowing through the rear flasher will it cause the the front lights to dim?
    LM555 is open collector, you need to connect the LED string's anode to the positive rail and the cathode to the output pin of the LM555.

    For LM555, max current is usually 200mA, depending on the LEDs forward i/v characteristics, you may need a limiting resistor.

  7. #7
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    The concept you have there will work, but I'd advise being able to switch the rear light off. I have used the K-Lite dynamo with my own rear light and the Exposure dynamo with their redeye rear light, as well as making my own versions and, in all cases, my friends very quickly told me to turn off my rear light as soon as we went off road as it was distracting, dazzling even, for the rider behind.

    Your rear light is in parallel with the main light so the power you split off from the main light to drive the rear light will be a direct reduction in the available (and finite) current for the main light, this could be really annoying if your main light pulses, even if it is a very weak pulsing effect!

    Finally, as Kerry says, at the drive currents of a dynamo, I'd go for a simpler three series LED arrangement, and lose the second string.


    Look forward to seeing the finished light, and hearing how it performs :-)

    Rob

  8. #8
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    Quote Originally Posted by znomit View Post
    would ditch the 555 and just run a single xpe in parallel with the XTEs, limit the current to around 50mA.
    you would just limit it with a voltage divider?

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