Easy Dynamo build- Mtbr.com
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  1. #1
    mtbr member
    Join Date
    May 2011

    Easy Dynamo build

    Mostly my contribution on this forum has been asking dumb questions & applauding. This time however I have built something a bit different. While I greatly admire the machining skill shown in this forum & the ingenuity shown by those without mills or lathes, the only thing I can fabricate is a story, so this build was done with not much more than a soldering iron.

    I should note from the get go that these lights are not going to win a lumen war, out the front anyway – a dynamo is limited to 0.5A, so I would have to go to 4 XM-Ls to get anything like the lumen output of 1 XM-L driven at 3.0A. That 0.5A limit however is exactly why dynamo lights are really, really easy to build – you don’t need to limit the current & the LEDs limit the voltage. My first light, to tide me over until I figured out what I wanted was Matin’s circuit 1, consisting of 1 XM-L $6.39, 15° optic $0.86, 1 easy2led housing & mount $16, and 4 diodes $2.40. Total cost $25.65. Cheap, as long as you don’t factor in labour costs.

    Dynamo hub This was the easy bit. Having heard good reviews about the dyno hubs from Shutter Precision I chose a PD-8 – it’s small, discrete, 6 bolt disc brake & only weighs 400g (less than my 4 cell battery pack). When I first got the hub I was a bit concerned as I could barely turn it over by hand. Turns out to be an illusion though as I cannot feel the drag, unlike say the bottle dynamo I had as a kid which was more useful as a brake than it was for powering lights. It was an opportunity to practice another new skill, wheelbuilding, but you will find plenty about that on other threads.

    I use these lights for commuting, & had initially settled on a 3 led set up with 2 x xml up front and a red XP-E for the taillight. Running at 0.5A the cooling requirements are not high, but I happened to have 3 easy2led housings, so I used those. The only difference to the common dynamo set-up is that I decided to run the taillight in series, basically because I didn’t want to reduce the power available to the front leds, I wanted the red led to operate at full power & the red led would comfortably handle the 0.5A of a dynamo.

    Standlights however had me stumped & I had all but given up. There are two problem with using a capacitor in parallel with the LED as a standlight (1) it will only charge up to the voltage forward of the LED & (2) it discharges at the full current the capacitor can deliver. The result is a very short standlight time. Then I came across a simple standlight circuit that involves some electrickery. It uses the potential difference across 2 LEDs to charge the capacitor but a diode means it discharges through just 1 LED. A simple resistor then reduces the current on discharge, on my circuit below at ~20 mA.

    The front lights

    2 x easy2led housings, 2 x XM-L. I started with a 30° optic and a 15° optic, but have since switched to using 2 x 10° optics. I used XM-Ls because I had them on hand, but they don’t add anything by way of light at 0.5A over say an XP-G or XP-E2. The main feature of XM-Ls is being able to run at 3A while the rest are out of the game at 1.5A. The XM-L2 however would be a good improvement – an increase in lumens of 25% at 0.5A for the same power, however it is not readily available in a 3C tint.

    In the back of 1 housing, hidden by the shrink wrap is the super capacitor that powers the standlight (C5), in this case 1.5 farad, a diode (D6) & resistor (R3).

    The rear lights

    1 x easy2led housing, 2 x XP-E Red, 1 deodorant ball diffuser. In this case the XP-E is the best of the Cree red leds – while an XP-E2 red ultimately produces more lumens, it does so by running up to 1.0A.
    As with the front lights, in the back of the housing is a 1.0 farad super cap (C4), a diode (D5) & resistor (R2). To run the standlight circuit, I needed 2 LEDs. To fit 2 red LEDs in 1 easy2led housing, I took some brave pills & then picked up my aviation snips & simply cut off the “excess” aluminium star, making sure I left bits with the traces to the LED, faintly visible in the pic below. The circuit below runs 1 XP-E as a standlight at ~20mA which runs brightly for 4 or 5 minutes before gradually fading away.

    0.5A through two red lights has been described as “crazy bright” & I can happily confirm that is 100% correct. It is highly visible, even in daylight & with my deodorant ball diffuser I can tell that the rear is working because of the red glow over the entire rear of the bike. I will see if I can come up with a safe and interesting way to take a picture of the rear light.

    The rectifier & other bits

    The dynamo output is AC, so you need a rectifier circuit (D1-D4). No printed circuit boards, no surface mount devices for me just simple through hole mounts on a small circuit board including the other bits that don’t fit in the light housings, Capacitors C1-C3, & R1. This time I remembered to take a photo before covering it with shrink wrap Once wrapped it is hidden away in the steerer tube.

    The circuit currently looks like this

    The good

    I am super happy with the rear light, currently running 2 x red XPEs at 0.5A. Crazy bright when moving & a decent standlight, bright for at least 5 minutes. The front lights put out an acceptable amount of light at the speeds I ride at & the standlight, while not bright enough to ride with, should be enough for visibility when stopped at traffic lights.

    To be improved

    My set-up includes a li-ion headlight & puts out enough light for my commute, but for proper off-road riding I would want more. Putting more LEDs in series will cover that, but at the cost of decreased low speed performance. Ditching the rear light would help.

    Beyond that though would need switching, either manually or automatically – eg the Exposure Revo has 4 leds but only 2 light up at low speed. Yet to figure out how to do that simply. Also if you could drive more current through the standlight circuit, this would also improve low speed performance, at least while the supercapacitor lasted.

    Parts List
    The prices are what I paid, as best I can remember, not including postage.
    1 SP Dynamo PD-8 AU$120
    32 DT Swiss competition spokes AU$34
    1 rim I had lying around
    3 easy2led housing think I paid $25 each but they are now US$12 ea
    2 handlebar clamps US$4
    3 Cable glands M8 bag of 5 AU$13.50
    1 rear mount – scrounged from the dead lights box
    2 XM-L LEDs US$6.39 ea
    2 10° optics pack of 5 AU$4.35
    2 XP-E red LEDs US$4.20 ea
    1 deodorant ball
    1 capacitor 4700uF 16V AU$2.50
    2 capacitor 1000uF 63V pack of 5 AU$12.15
    1 capacitor 1F 5.5V AU$6.20
    1 capacitor 1F 5.5V AU$6.95
    5 Schottky diodes 1N5819 AU$1.85
    1 resistor 47 Ohm pack of 10 AU$0.50
    2 resistor 47K Ohm pack of 10 AU$0.50

  2. #2
    mtbr member
    Join Date
    May 2005
    Nice system! Any way to add a flashing circuit to the rear light? That seems to be more eye-catching when I am driving and come up behind a bike.

  3. #3
    mtbr member
    Join Date
    May 2011

    Easy Dynamo build

    Thanks. With the lights in series I am not sure how to get just the rear to flash. A smoothing cap may work but I am thinking it would need to be big

    One idea I have though is to make the standlight flash, perhaps even on & flashing using mattthemuppet's trick with a little blinky led

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