Dynamo powered LED bicycle taillight

BikeLite
Martin uses a dynamo to power his bicycle taillight and wanted to find a way for it to remain lit when the bicycle was not in motion. He sent in this quick hack he used, adding a supercap in the taillight circuit, which powers the light without the need for batteries.

Via the contact form.

This entry was posted in how-to, LEDs, power supply and tagged , , .

Comments

  1. langwadt says:

    I believe most bike dynamos are (kinda) constant current, that is why they are spec for a specific watt bulb and the voltage can be high. a shunt regulator might a better fit and lower loss

  2. Vincent says:

    Regulators consume power. Maybe a serial diode will help backward flow of energy when unpowered?

  3. Martin says:

    The objective of the regulator is to regulate the voltage to avoid the supercap to blow up. If anybody has a good alternative, I’m interested to read it.

  4. philwatcher says:

    Easiest solution:
    Dynamo -> Diode/Bridge rectifier -> 5V zener diode parallel to goldcap -> LED

    From what I see, hub dynamos have around 500mA current, which means that the zener diode needs to dissipate 2.5 watts worst case. An alternative would be a 4.3-4.7 V zener diode and a PNP transistor as voltage follower.

  5. philwatcher says:

    Here is a “link” to a simulation of the circuits I described:
    http://pastebin.com/Lr2JVXqu
    It uses Paul Falstads Circuit Simulator (java applet) and a very long link.
    If you don’t want to open random huge links that attempt to run strange applets, go to falstad.com/circuit, select “File/Import” and paste the code from http://pastebin.com/8HUYvTpd into the window.

  6. Alex says:

    I do not know how the original developer of this idea is good with electronics, but certainly he is not very good designing this analog circuitry. The whole thing is garbage prototype and a bad example of how not to design this type of circuitry.
    Firstly, you want to use highly efficient SM converter, for which you could use a specialized chip or else discretes. 7805 is nonsense here and a big waste!
    Secondly, you want to optimize the use of dynamo for its rated output at certain or average speeds. Polarized electrolytic cap is a bad idea if dynamo gives out AC, you want a bridge rectifier there.
    Thirdly, if you want to go with fully automated activation of the light without a switch, then use a tiny micro to detect activity on the dynamo output, then use a cell phone battery, which has internal protection circuitry and much better capacity to run the LED.
    Fourthly, the LED driver IC is much better option contributing to a better efficiency if SM is employed and gives the setup a very precise current control.
    I think it is enough to criticize this design submission, however it will work for somebody as it is…
    Thanks and good luck.

  7. Sam P. says:

    Hi Martin.Interesting that you’ve used the voltaje regulator – I presume to ensure that you charge the capacitor as close as possible to it’s limit of 5.5V. The problem is that both it AND the charge pump (doubler) circuit are not necessary. Also, I think most of the comments thus far completely misunderstand the demands of your task.

    Your small LED will only draw (drop) around 2.5V and 20ma. So all you need is either a half or a full wave rectifier, your supercap and maybe a few more LEDs depending very much on how you intend the taillight fitting in with whatever other lighting you have planned for the front. If you are not using a front dynamo light then I suggest you use a cheap red power-led to soak up some of that excess current!

    Sam

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