So here is the deal, for the 2nd time in less than a year, my halogen bike light on the front has popped due to cold. This is rather bothersome and I can't find any nice inexpensive LED alternatives in my country.

So I set out to do it myself, I have possibly gathered a list of components, but I am somewhat new at building electronics that doesn't just hook up to a pre-made micro controller prototyper.

So I would like to ask you kind people if my current assumptions and hardware list is correct or if I am totally in the woods and if I am, how I should go about doing it instead?

The list: 1*3W 650~700mA Constant Current Regulated LED Driver (8~40V Input), 1*2W 160-180LM LED (Rated voltage: DC 3.0V~3.2V - Rated current: 600~650mA), 1*6.3V 3300uf Aluminum Capacitor

The dynamo is a 10 year old AXA HR Traction, possibly without power control (rated at 6 Volt/3 Watt).

I was thinking I open the housing of my bike light, take out the old stuff, connect the wires to the current regulator, connect it's output to the capacitor (for power at intersections) and then hook up the LED and run it back to the regulator?

Will this actually work? The numbers seem right to me, but again I have no actual education in electronics.

  • 2
    \$\begingroup\$ Can you provide a link to the datasheet for the LED Driver? The dynamo generates AC, you'll need to rectify it. \$\endgroup\$
    – Jason
    Feb 21 '13 at 12:08
  • \$\begingroup\$ In addition to Jason's comment, a circuit would be helpful too. \$\endgroup\$
    – user17592
    Feb 21 '13 at 12:19
  • \$\begingroup\$ Connect the capacitor to the input of the regulator - and don't expect it to do more than smooth out the wobble from the dynamo. You'll need a bridge rectifier from the dynamo too. \$\endgroup\$
    – pjc50
    Feb 21 '13 at 12:41
  • 1
    \$\begingroup\$ A 3300 uF capacitor is only going to keep a 500-mA LED lit for about 20 ms. If you really want it for light at intersections (20 seconds or so?), you'll need something like a 5-Farad ultracapacitor, and you'll probably want to arrange a charging circuit for it that keeps it from consuming all of the available current when starting out. \$\endgroup\$
    – Dave Tweed
    Feb 21 '13 at 12:43
  • \$\begingroup\$ Unfortunately that is all I have to go on with the LED driver. How would I go about rectifying it? I tried making a circuit here: imageshack.us/photo/my-images/526/circuit.png \$\endgroup\$
    – user19269
    Feb 21 '13 at 12:43

If it is to be used for smoothing the capacitor should not be in series with the circuit to the regulator. A capacitor in that location can assist with voltage provided at lower speeds, but that's getting fancier than is relevant at this stage.

An excellent page is Dynamo-Powered LED Light Circuits for Bicycles.
This has a large number of ideas and circuits.

Note that the writer has a few wrong ideas so take due care about why rather than what - but you will probably find a circuit to suit your there.

MANY relevant ideas

Note that dynamos TEND towards constant current and in many cases your can get more power just by raising voltage.

Headlight conversion http://karlmccracken.sweat365.com/2012/02/22/pashley-dynamo-headlamp-upgrade/

Dynamo to cellphone charger http://www.arenddeboer.com/bicycle-usb-charger-using-a-hub-dynamo-update

Dynamo to Minty Boost. http://forums.adafruit.com/viewtopic.php?f=15&t=19515

BSc project - related to 1st ref http://www.pilom.com/BicycleElectronics/BicycleElectronics.htm

OK discussion - http://in.answers.yahoo.com/question/index?qid=20110709001827AA3kp21

  • \$\begingroup\$ ...Note that dynamos TEND towards constant current and in many cases your can get more power just by raising voltage... What does this mean? \$\endgroup\$ Feb 21 '13 at 14:05
  • \$\begingroup\$ @abdullah It means that most modern dynamos have power control, which makes sure it has a steady powerflow even at low speeds. But I unfortunately think that mine does not. Thanks Russell I will take a look at it. \$\endgroup\$
    – user19269
    Feb 21 '13 at 15:01
  • \$\begingroup\$ @abdullahkahraman - read the first reference. He goes into it in detail. Dynamo is current limited due to iron saturation. Double V and V x Imax doubles. Triple V and V x Imax triples ... \$\endgroup\$
    – Russell McMahon
    Feb 21 '13 at 15:03
  • \$\begingroup\$ I really don't get it although I have read the first reference. How come increasing the voltage will give the same output current, hence more wattage? Is it a free energy generator? \$\endgroup\$ Feb 21 '13 at 17:20
  • \$\begingroup\$ @abdullah kahraman, I believe that would involve more physical input energy... but free energy would be way better. \$\endgroup\$ Feb 21 '13 at 20:37

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