Building my own standlight...
I've got a Shimano Dynohub powering a B&M Lumotec headlight and a B&M DToplight. Last year I had the same setup powered with a homemade 6V NiMH battery pack (6 "D" cells). Like most others I love the Shimano hub, but having all of the lights go out at a stoplight stinks. I'd like to use a smaller battery pack from all-battery.com as a fallover standlight, so when I'm slowing to a stop the relay kicks off and the battery powers the headlight. When I take off again and get over 5 or so mph, the relay closes and I'm back on the hub power.
To that end, I've gathered the parts I think I'll need: full wave rectifier (Radio Shack 276-1152 Silicon Bridge Rectifier) to convert the 6+ volts AC off the Shimano to 6+ volts DC, NTE962 voltage regulator to clamp the excess voltage to 6 volts, SPDT relay (NTE R48-5D5-6L), a good switch, and 3 mono speaker jacks (gen in, batt in and out).
I Am Not An Electrical Engineer ;-), but my understanding of how relays work (and tested with some bare wire from the battery pack) is that when 6 volts is passed across the coil pins of the relay, the switch closes. In my case I'll want to wire the Shimano (rectified and voltage regulated) 6V+ to one of the switch pins, and the 6V+ from the battery pack to the other. The common pole in the middle of my relay is connected to the positive terminal of my out jack. I think I can put all of the negatives together on the same circuit.
My question then is is how I can wire the Shimano across the coil? Do I just tack a small wire from the 6Vout pin of the regulator to one side, and the GND from the regulator to the other? That would mean two wires off the Vout pin--one to the coil and one to the NC side of the switch.
The relay I have is low resistance off the coil (340 ohms) but I don't want to lose all of my power just running the relay.
If this works like I hope, I'll do a writeup with schematic and photos.
Reeks of aged cotton duck
According to the NTE documentation, that relay has a coil power consumption of 100mW... I don't know what that hub puts out, so I don't know how much impact that 100mW will have.
I'd be curious to see a schematic of your setup...
They are rated at 6 Volt, 3W, so I don't think that will hurt too much.
You don’t need the relay or the voltage regulator, been there and tried that. Ni-MH batteries will regulate the voltage and give any excess current a place to go in the form of recharging the batteries. I use 4 AA solder tab Ni-MH batteries for 4.8 volts with the dynamo off and 5.2-volts with the dynamo engaged and making power at speed. The batteries place a high impedance load on the dynamo effectively regulating the system voltage to 5.2-volts with the dynamo engaged. 6-volts will work just as well. The regulated voltage with the dynamo engaged should be about 6.3-volts with 5 1.2-volt Ni-MH batteries. The reason I use 5-volts is the ability to run and recharge a GPS unit and a Cellphone from a USB port I have wired up to the batteries. USB ports have +5-volts and ground to power USB devices and it's common to find GPS units and phones that use that power for recharging as well. There are 4 pins in a USB connector. Pin-1 is +5-volts and is the red wire in the cable. Pin-4 is ground and is the black colored wire in the cable. I just dropped by a dollar store and purchased a USB extension cable and cut it stripping back the casing exposing the 4 wires. The other 2 wires are for data and may be cut even with the casing as these wires are not required or necessary and you don’t want them shorting to the power wires. I built my own LED headlight and taillight. The Headlight is a Luxeon 1-watt LED and Fraen 10X20 elliptical lens and holder. I came up with 4.9-ohms resistance to regulate current to 350mA by paralleling resistors and watching a current meter between the fully charged batteries and LED as I rode around with the dynamo engaged so I could not possibly overdrive it with too much current. My schematic is a little outdated. I have moved the 470mF capacitor from the rectifier board to the headlight LED leads. This was done so the LED could be driven directly from the Dynamo in the event of damage or loss of the batteries. This will decrease the life of the LED, as the full available current from the dynamo will flow through it. It should still last in excess of 5,000 hours. This would give plenty of time to replace the batteries when possible instead of trying to ride at night with no lights. Here is my schematic.
When S1 and S2 are closed the LED's are using 380mA with 120mA used to recharge the batteries. This is the recommended slow recharging rate for Ni-MH batteries and is about right when riding in city stop and go traffic. If riding long periods without stops I disengage my hub dynamo by opening S2 for 15 minutes of every hour to prevent overcharging the batteries. For bottle and BB type dynamos you would of course mechanically disengage it but S2 should be retained and opened when the bicycle is parked to prevent slow discharge across the bridge rectifier.
When powering USB devices I have S1 open and never use USB devices when also powering the LED lights. It is also easy to overcharge the batteries in the USB device so be extremely careful to watch the power meter in the device when the dynamo is engaged.
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!
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 thatís 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.
Originally Posted by JeffRoss
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 donít 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.