# Thread: 8 Watt Hub generator LED system

1. ## 8 Watt Hub generator LED system

I've finally got a working prototype for my LED system. Basically, a single front and rear light come on at low speed, and the rest switch on at around 8km/hr. About 620lm of front facing Cree LED light when riding over 15km/hr. See http://www.malleegum.com/bike/project4.html for details.

2. That's a great looking setup, should earn you plenty of respect from other road users! What are your housings made of and how did you make them? Have you posted your construction setup in the bike lights database yet?
Regards,
Dylan

3. Wow, some clever stuff going on there!
Love the home made housings.

4. I only don't understand why you use resistor. A dynamo can be seen as a current a source, a resistor woudn't change anything to that.

5. Thanks guys.

I dont understand the question about resistors. The starter lights are in parallel, so need resistors to determine the proportion of current the LED's see. If I didn't use starter lights and the LED's could handle max current, sure I'd just put them in series and forget any fancy switching circuit. The problem is at slow speed an array of LED's in series will not be as bright as a single LED, they will infact strobe poorly because only the peak of the rectified voltage will be quiescent. Also, this system lends itself to NiCd support for the starter lights, which is my next project. I hope that was clear.

6. That is a pretty cool setup. How much does that bike weight with those racks, and what do you carry on them? I guess you either commute or tour with it?

7. I'll try to explain it about the resistors.
In most light set-ups a batterypack is used. A battery is a voltage source (so constant voltage and the current changes to keep te voltage constant), but you'd need a constant current for a led. That is the reason and that you'd need a leddriver when you use a battery. A resistor is a bad leddriver, the current isn't really constant (it depends on the input voltage) and it just dissipates all the voltages that there is to much.
For example: Led forward voltage = 3,5V, battery voltage=6V, led current=0,5A. Then the resistor will dissipate: (6-3,5)*0,5=1,25 Watt.

But you don't use a battery as energy supply for your leds, you use a hub generator. A generator can electrically be seen as a constant current source that delivers 500 mA. When you connect one led, it delivers 500 mA to it. But when you connect 4 leds in series it still will deliver 500 mA (when you'd ride fast enough), so the voltage is now 4 times as high. Your generator won't deliver more then 500 mA, so your leds are completely safe when you connect them without resistors.

When you'd want more light, you should look into tuning capacitors. You can double your output with one (but is a bit big) capacitor.

8. Holy crap- talk about DIY! Congratulations, Scuppy. Especially for the housings.

9. Originally Posted by super-fast
I'll try to explain it about the resistors.
In most light set-ups a batterypack is used. A battery is a voltage source (so constant voltage and the current changes to keep te voltage constant), but you'd need a constant current for a led. That is the reason and that you'd need a leddriver when you use a battery. A resistor is a bad leddriver, the current isn't really constant (it depends on the input voltage) and it just dissipates all the voltages that there is to much.
For example: Led forward voltage = 3,5V, battery voltage=6V, led current=0,5A. Then the resistor will dissipate: (6-3,5)*0,5=1,25 Watt.

But you don't use a battery as energy supply for your leds, you use a hub generator. A generator can electrically be seen as a constant current source that delivers 500 mA. When you connect one led, it delivers 500 mA to it. But when you connect 4 leds in series it still will deliver 500 mA (when you'd ride fast enough), so the voltage is now 4 times as high. Your generator won't deliver more then 500 mA, so your leds are completely safe when you connect them without resistors.

When you'd want more light, you should look into tuning capacitors. You can double your output with one (but is a bit big) capacitor.
What about 2 LED's in parrallel when they have significantly different Vd (red and white)? Don't forget I need a drop high enough to want to turn on the other lights, switching as soon as I hit the white LED Vd is pointless. I think it's a good compromise, no other switching system I've tested (and thats quite a few) will deliver as much current to the high beam lights, which is of primary concern, not increasing total power consumption by a few watts at speeds over 25km/hr. My trigger uses 3.6mA, thats not even quiescent for a regulated system. Feel free to correct my circuit and post it here if you like (but if you just remove the resistors, you'll have to change the zener trigger too). I can then graph power comsumption for both systems and functionality (ie how many lights are on at a speed). I wont be back for a week though (Morton National Park is calling me).

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