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  1. #1
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    Wiring a Busch & Müller rear light with DIY head light

    I bought a dynamo powered B&M Toplight taillight for my bike (TopLight Flat Plus S or here). My current setup uses a DIY headlight with two CREE LEDs and a home-made rectifier (basically, Circuit 6 from http://www.pilom.com/BicycleElectron...moCircuits.htm). I have a Shimano 3N-71 6V/3W hub.

    I can't find any detailed specs on the B&M. It's made to go with the B&M headlights, of course, but I don't have one of those. I know that it takes DC (it has + and - tabs on the case) and that it has over voltage protection. So, a slew of questions:

    - Does anyone have detailed specs on the B&M?
    - Should I wire it in series with my headlights or in place of one of the diodes in my rectifier?
    - My calculations show that my dynamo is pushing around 1000mA +/- through my headlights. What is the ideal amperage for the TopLight?
    - What is the nominal voltage of the rear light? If I were to just use a battery, would I use 1.5V or 3V (or something else)?
    - If I wire it in series with my headlights, will the voltage regulator in the taillight cause problems? As in, will it dump power instead of letting the headlights have it?

    Obviously, some of these are inter-related. Thoughts?
    Last edited by Pedaleur; 12-10-11 at 03:37 PM. Reason: Hub specs

  2. #2
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    You should wire in parallel. The nominal voltage is 6V AC, but 6V DC works as well. Below 4.5 DC or so it will cease to work. It takes of the order of 50mA in steady operation and likely 40mA more when charging the standlight. The voltage protection is likely meager, i.e. if you were to run it at say 8V DC for a longer time, you would likely blow it. What I am saying is either from a direct experience with the TopLight Flat or from similar lights in the B&M lineup.

  3. #3
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    By adding more of a load you may get more power out of the dynamo, in fact its quite likely. But your pedaling load will increase (very slightly)

    Many bottle dynamos have some form of clipping circuit installed to lower the peak voltage. You can check by using a multimeter on the AC setting - or if it doesn't have one, a bridge rectifier and a capacitor in parallel with the meter.. to see what it charges to. But as long as both lights are connected, the likelihood of either one's current going so high it burns out anything is small.


    My headlight can run on 6-12v and it also has a regulated output for a taillight. I don't remember ever checking the voltage, if I did, I dont remember.. it works well driving a single high power (1 watt) LED with no resistor. That setup works really well, but its unusual in commercial lights. But you could build the same thing - or use a commercial LED driver.. (Buck/boost converter)

    If you insert a low value resistor in series with the taillight, it will limit any current extremes, but also waste some energy heating that resistor.

  4. #4
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    Quote Originally Posted by 2_i View Post
    You should wire in parallel. The nominal voltage is 6V AC, but 6V DC works as well. Below 4.5 DC or so it will cease to work. It takes of the order of 50mA in steady operation and likely 40mA more when charging the standlight. The voltage protection is likely meager, i.e. if you were to run it at say 8V DC for a longer time, you would likely blow it. What I am saying is either from a direct experience with the TopLight Flat or from similar lights in the B&M lineup.
    Thanks. This makes sense.

    Of course the trouble is that I have two LEDs each running somewhere between 3.7-4.0V, which puts me in a bit of a bind: wiring in parallel with one the voltage is too low; with both too high. I guess I could put a small resistor in to lower the voltage, but I hate wasting energy, especially on lighting.

    [Actually, one follow up: the TopLight has + and - labels on it. You say it is built to take AC, so these are just referring to "hot" and ground, as labelled on the hub, and not necessarily +/- DC. Is that correct?]

  5. #5
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    Quote Originally Posted by Pedaleur View Post
    Of course the trouble is that I have two LEDs each running somewhere between 3.7-4.0V, which puts me in a bit of a bind: wiring in parallel with one the voltage is too low; with both too high. I guess I could put a small resistor in to lower the voltage, but I hate wasting energy, especially on lighting.

    [Actually, one follow up: the TopLight has + and - labels on it. You say it is built to take AC, so these are just referring to "hot" and ground, as labelled on the hub, and not necessarily +/- DC. Is that correct?]
    Are your two front LEDs back to back? 4 LEDs up front could save the situation. For voltage lowering you might also stack up diodes. I suggest you just start experimenting. As to the Toplight polarity, I am confused since I have a Toplight Flat in front of me and no polarity is indicated. In older Toplights there was a difference between the ground and live connections, but not anymore. In any case the worst that can happen, I think, regarding polarity is that you will be using only half of AC. Just connect 6 V to it one way and another and see what happens.

  6. #6
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    Quote Originally Posted by 2_i View Post
    Are your two front LEDs back to back? 4 LEDs up front could save the situation. For voltage lowering you might also stack up diodes. I suggest you just start experimenting. As to the Toplight polarity, I am confused since I have a Toplight Flat in front of me and no polarity is indicated. In older Toplights there was a difference between the ground and live connections, but not anymore. In any case the worst that can happen, I think, regarding polarity is that you will be using only half of AC. Just connect 6 V to it one way and another and see what happens.
    Yeah, the LEDs are in series.

    And you're right about the experimenting. It's just that I don't have access to all the equipment (particularly, a decent power supply) I had when I built the light in the first place, so I figured I'd ask around first. Your comments have certainly narrowed it down, so thanks!

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    Why not just get a 3W red LED and wire it in series with your headlight? It will make for a much brighter tail light anyway. Perhaps at the cost of some of your pedaling output. With 3 LEDs you might need a voltage doubler if you bike at low speed often though.

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    If I were you I would stick a multimeter in there and measure both the voltage and current peaks just to get a feel for the situation. Also, somebody told me here recently that some dynamos end up charging any caps placed in parallel with the output of a bridge rectifier very high, if there's no load. Sometimes he said as high as 100 volts, (with a 12 generator, so I guess figure -perhaps 50 volts..) All that goes away when there is a constant load, but the minute the load is removed it starts charging higher and with no load the instantaneous voltage when the LEDs are re-connected could be high enough to fry them. I think if you buy from China you can pay under five bucks for each LED driver - plus shipping, so even though you'd have to wait, it might be a good investment!

  9. #9
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    A few weeks ago I stumbled across a German (I think) site where a person examined five (I think) European combination taillight/reflectors in detail, both in terms of their output and electrically. Ever since I first saw this post, although I could swear I bookmarked it, I lost it and Ive been unable to find it again. If I find it, I'll post the URL here.


    The info on this site is similar.

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