dynamo help needed

Hi all,

I've got a dynamo on my bike - a litespin with a regulated (AC) output of 6V and 3W. I figure this to be 0.5 amps (please tell me if I'm wrong). I've already got a circuit that I think might be able to handle the AC to DC.

Can anyone give me a few ideas about what i'd be able to run on this sort of output in terms of LEDs? I'd like to be able to run a luxeon, but i'd be happy with a cluster of 5mm LEDs. I can't get my head around the various watts, amps and volts, and how they power LEDs. Could I run another regulator circuit to make it more suitable? Or just hook up a heap of 5mms in parallel and don't bother with anything apart from a resitor?

Any help greatly appreciated!

Thanks,

wumblethump

Howdy,

Are you really sure it's regulated? That is if you double or triple your speed, it doesn't get brighter?

No big deal, really. Yes, you can run a Luxeon (or two even) on 6 Volts AC, 3 Watts (yes, it's half an Amp).

As a first try, use a "bridge rectifier" to convert to DC. Ground one AC input, hook the other to the hot side of the dynamo. Hook the Luxeon from plus to minus (no resistors or other parts for now). Then take it for a ride.

Yes, consistant light output will need a regulator (unless it really is regulated at the dynamo, in which case a single resistor could do). A much better idea might be to store the 'extra' in a battery to keep the Luxeon on at low speeds, however. That is to say if you have enough 'juice' to light two Luxeons at nameplate ratings (and you do), you could possibly light a single one full time.....

Anyway, hang a bridge and a Luxeon (or a handfull of 5mm parts in parallel) on the output and wind it up and see what you get (pun intended).

Cheers, and welcome to the fun!

Doug Owen

Hi Doug,

Yes, the dynamo is regulated so it has pretty much the same output at 10km/h and 50km/h.

I'll give the bridge rectifier and a 5mm cluster setup a go for now - the dynamo power -> battery option is a great idea - I'll upgrade to that when I feel comfortable with the electronics.

Just on the battery idea, given that the output is 6V AC, would I need a bridge to charge the batteries? Also, would I need to have 6V worth of battery, or could I charge a typical D NiMH straight from the dynamo?

Thanks for your help - much appreciated!

Wumblethump

I don't know if you're aware of it or not but there's a mail list called Bikecurrent that offers an extrodinary depth of knowledge in all areas of bicycle electrics. There are several dynamo experts there (I'm not one of them). It's nearly as friendly and helpful a place as CPF. You can find more about it here.

Cheers
Duane

Most interesting. The physics makes a dynamo produce (or try to) voltage directly relative to speed. That is if it produces 2 Volts at 10 rpm, it will produce 4 at 20, 6 at 30 and so on. I suspect it's clamped? Does it get warm if it spins and the light is off?

Anyway, you should probably add a small resistance, a few ohms to control the current once it get's going fast enough to light the LEDs. The value of the resistor will set the current, lower it to raise current levels.

Yes, you'll still need to rectify the AC to DC to charge the battery. While you could set up to charge a single cell, I'd suggest 3 or 4 since you'll need at least 3.5 Volts or so to drive the LED without complex voltage converters.

Keep us posted?

Doug Owen

I'll second the recommendation to ask the folks at BikeCurrent, especially as I'm one of those folks. /ubbthreads/images/graemlins/wink.gif

I'm currently using a 5 watt Luxeon with my Schmidt hub dynamo, and it's a simple arrangement. Just add a full wave rectifier, and connect the Luxeon. The 0.5A limit of the Schmidt provides all the regulation that is required.

The circuit can be made a bit fancier by using a rectifier bridge made of mosfets for a bit more output at low speeds, but that's not necessary.

Regarding the Lightspin, it's a different sort of beast than the typical bike dynamo. I don't recall if there is some pwm'ing that it does, but the output is not a simple regulated 6vac rms. It's designed to drive an incandescent bulb, and I can't say what it would do if you connected a led to it. The other BikeCurrent guys might know more details. /ubbthreads/images/graemlins/thinking.gif /ubbthreads/images/graemlins/thinking.gif

If you do use an led/leds, what ones were you thinking about? Proper heatsinking is definitely a challenge, as it getting suitable optics/reflector to produce the beam pattern desired.

Steve K.