Originally Posted by
astronomerroyal
I've been waiting for a post like this for a while. First, please accept my hearty congratulations. Let me say that I've built something similar for my bicycle trailer, but I've been wanting to ask the opinion of someone knowledgeable in this area.
Background: My power source is 12v dynamo + rectifier/smoothed + voltage regulator-> ~10.4v DC. I have a little box of electronics that has a voltage meter (for fun) and a voltage regulator that outputs 5v for USB devices and my 1W LED headlight (ebay thing, it has a built in current regulator on the heatsink). The original 10.4v is also fed back to some rear LED lights I made. But as per your allusion, I can't run both rear and front lights in parallel. I assume it's some sort of gross impedance mismatch.
Q1). I don't know if there's a standard solution, but do you think if I made a 555 timer 'switching circuit' that switched between rear and front lights at an imperceptibly high rate, this would be be a 'good electronic engineering solution'? If I added an adjustable switching rate I could make it act as a flasher too.
Q2). I've never been sure exactly how to rate NiMH voltages, but under load they're usually about 1.2v (1.4v open circuit, fully charged), so do you find that 4.8v reliably drives USB devices? The iPod, for example, seems to be notoriously picky about the exact pin voltages.
Q3). Ignoring the voltage drop, is there any merit in adding a diode in series with the USB? I imagine most devices have their own protection built in.
I'm sure i have photos of it somewhere.
All the best,
AR.
Your trying to make it complicated. 4 Ni-MH batteries will regulate the rectified voltage from the dynamo to 5.2 volts. You don't need the electronic voltage regulator at all. The reason this works is due to the design of the dynamo and the increasing impedance of the batteries as voltage exceeds 4.8 volts. At 5.2 volts the batteries present an impedance so high it's impossible for the dynamo to produce more than 5.2 volts. As for a diode between the batteries and the USB device, you can add one if you like but it's not really necessary. All modern USB powered devices use protection and regulation circuits protecting them from the power available at the USB port. The diode would also drop the voltage from the batteries by .86 volts when using the same diodes used in the bridge rectifier. Standard rectifier diodes will drop the voltage 1.2 volts.
As for running a wired taillight, You can remove one of the diodes from the bridge rectifier and replace it with 15 SMD LED's. Another option is to use 4 5mm LED's hooked up as a bridge rectifier circuit and a resistor to control current found experimentally hooked to the AC output of the dynamo. This is all covered at this link.
http://pilom.com/BicycleElectronics/DynamoCircuits.htm
I separated my lighting circuit from the USB circuit so I could switch on the headlight and allow any USB device connected to the USB connector to remain connected as you can not power both lights and USB devices. There is simply not enough power to do both.