Originally Posted by
JeffRoss
Hi n4zou,
I've seen your schematic quite a few times now, but I've always thought that with my riding I'd end up frying the NiMH battery pack in short order. Most of my daily commute is rural, only a mile or so is in the city, and even at that I have very little stop and go.
Why do you need the capacitator? Does it smooth out the below cut in speed "pulse"? Or maybe that's because you are driving an led while I'm driving a halogen headlight and an led tail light. The B&M tail light doesn't care what its input is, though, because of its design.
Couldn't you use a zener diode to keep the battery pack from discharging across the rectifier? Although the way I see my standlight box, the battery pack doesn't have a path to the recitfier at all.
Thanks for the suggestions!
Jeff
The capacitor scavenges a bit more power from the dynamo. I ultimately decided I didn't need it with batteries. It also occurred to me I might need to power the LED lights without the batteries in the event they were stolen, lost, or damaged so I connected it across the headlight LED to smooth out the DC from the bridge rectifier in the event I needed to ride in the dark without the batteries. Without batteries or a capacitor the DC voltage from the rectifier is not steady and will cause the LED light to flash at slow speeds and that’s hard on the LED as well. The capacitor will smooth out the ripples preventing that. Here is a link to quite a few circuits for powering LED's from a dynamo, all without batteries.
http://www.pilom.com/BicycleElectron...moCircuits.htm
Note how an effort is made to properly load the dynamo to prevent excess current by using 15 or more 1206 style SMD LED's in the taillight. When you add up the current requirements of a 1-watt LED and at least 15 SMD LED's you find it's just a little more than 500mA so the power system is balanced just as a 2.4-watt headlight and .6-watt taillight would do for the dynamo.
I do a considerable amount of nighttime riding as I patrol an MUP with a Trail Watch volunteer group. I needed a lighting system that was unlimited in its duration and did not go out when I was stopped. I've fixed many bikes in the dark using my headlight as a work light. Considering your not going to be stopping very much all you need to do is simply disengage your dynamo from the batteries allowing them to discharge a while so you have room for more excess current produced by the dynamo as recharging current so you don’t overcharge the batteries. After you build your system just place a milliamp meter between the bridge rectifier and the batteries. Engage the dynamo, turn on the lights, and ride watching the meter. The excess current can be read from the meter. You can then calculate how much time is required to recharge the batteries by how much current is recharging them. You'll find the slow recharge rate for Ni-MH batteries is 120mA and rapid recharging is at 500mA is suggested by most battery manufactures. You'll just need to measure your recharging current and calculate how much time you run with the dynamo engaged on your commute. As your running a higher voltage system and your system is designed for 6-volts you might find you have very little excess current and you may never need to worry about overcharging your batteries.
The bridge rectifier blocks the batteries from the dynamo so the drain is very slow through them. In a perfect world there would be no drain across the diodes and S2 would not be required, but nothing is perfect! It might take a week to even notice the drain on the batteries but it is there. It's also a nice way to disconnect the batteries from the electrical system on the bike when the batteries need to be removed.