seeking bike headlight using generator

I've got an old Sanyo 6v 3watt generator that I'd like to set up with most effective headlight possible for a reasonable effort and cost. I understand that 3 watts won't provide the same lumens of a 10watt halogen bulb but these generators have very little drag and it would be nice to not have to deal with battery maintenance for city riding where the street lighting is adequate. Any recommendations for type of LED and reflector unit?

LeeG, How much electronics knowledge do you have? You could use a step-down DC-DC converter and drive a 1W Luxeon LED or three, using either NX05 type optics or the /O type Luxeon LED. Each LED needs a constant 3.30 volts or so for brightness. In any case it's much easier to pedal with a battery pack than a generator, even if it is only 3 Watts.

LeeG,

This subject has been discussed in numerous threads over a the "Bikecurrent" forum here:

http://www.topica.com/lists/bikecurrent/?cid=7620

Good Luck,

Dan

Watch out. You do not want to use a standard step down DC-DC converter with a bike dynamo. A bike dynamo is designed to produce roughly constant current provided that it is moving fast enough to produce sufficient voltage. This means that if the current drawn by the load is not sufficient, the voltage can go through the roof when the bike is moving quickly.

Most step down DC-DC converters draw _less_ current as the input voltage goes up. This means that the combination of the DC-DC converter and the dynamo would be unstable; if the DC-DC converter doesn't draw sufficient current, the dynamo output voltage will go up, and the current drawn by the DC-DC converter would actually go _down_. If you want to use a DC to DC converter, you will need to find one that is specifically designed to work with a dynamo supply.

However there is a much simpler approach. Because the output of the dynamo itself is roughly constant current, you can pretty much use the dynamo itself as the device which properly limits the current to the LEDs. All you need to do is have a sufficient number of LEDs to handle the rated current from the dynamo. You will also need to rectify the output of the dynamo, because it is AC and the LED requires DC.

A standard 3W dynamo is generally designed to operate with a 500mA output at a nominal voltage of 6V. If you have LEDs rated for a total of 500mA or greater, then you will not need any additional current limiting. A single 5W Luxeon with an appropriate rectifier would make a dandy front light...if you can get one. A pair of 1W Luxeons would also work quite well. For a 'be seen by light', a small set of Amber LEDs in the front, and a small set of Red LEDs for the rear, will be quite bright and visible even at low speed.

-Jon

I think I proposed a fully independant pushbike lighting system a few months ago, I'll see if I can dig it out!
I've not built the system yet as all the transport spend is going on my motorbike.
My idea included a bridge rectifier, a regulation circuit and some kind of storage battery which could live inside a torch body tube strapped to the underside of the crossbar. Luxeons are definitely the way forward!
Much of the engineering that goes into a generator powered pushbike lighting system has already been developed for HPT's, so maybe you could copy the circuit from an HPT. (See earlier posts about spring powered torches.)

thanks everyone, the search is on. Lightbright, I agree that a couple pounds of batteries will deliver more watts than I'd want to sacrifice through a generator but the Sanyo mounts on the bottom bracket with the axis of the generator parallel to the wheel axle so the drag is MUCH less than the side wheel mounts. For a bike that might be used for touring or when the batteries aren't charged fully it's a good backup.

Originally posted by Jonathan:
If you want to use a DC to DC converter, you will need to find one that is specifically designed to work with a dynamo supply.

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<font size="2" face="Verdana, Arial">Do you have an example of such a converter?

However there is a much simpler approach...you can pretty much use the dynamo itself as the device which properly limits the current to the LEDs.

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<font size="2" face="Verdana, Arial">How does this limiting work? These dynamos are notorious for blowing out lightbulbs at higher speeds. Why would it be different for LED's?

A standard 3W dynamo is generally designed to operate with a 500mA output at a nominal voltage of 6V. If you have LEDs rated for a total of 500mA or greater, then you will not need any additional current limiting.

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<font size="2" face="Verdana, Arial">I've found that it is easy to load down the output if the current is exceeded, resulting in dim LED's,low voltage, and high drag. I wanted to try keeping the current draw lower and limiting the voltage with a regulator; I figured lower current it the coils would decrease electromagnetic drag.

A pair of 1W Luxeons would also work quite well.

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<font size="2" face="Verdana, Arial">Do you mean in series? Will not the Luxeons be overdriven at higher speeds without some regulation? I have measured up to 9V from one of these dynamos under load by an incandecent bulb, drawing more than the Luxeons in series would. I would fear for the LEDs.