Dave wrote on 10/16/15 at 12:17:10:
I also believe I have to run enough LEDs in series to reduce the voltage on each LED down to it's operating voltage (or use a resistor to step the voltage down). |
Running LEDs in series distributes the voltage load over more components evenly. This reduces waste power to heat.
For instance, if the bike is generating 12VDC (it won't be, it will be higher than that in real life that's why you need regulation instead of resistors) you have 12VDC to spread out in the circuit. If you used one LED and one resistor, those two components would be forced to use all that voltage (Kirchhoff's circuit laws).
White LEDs only need somewhere between 2.8 and 3.4 volts (varies with manufacture) to operate at FULL POWER. So because our power source is much higher than 3 volts (we got 12 and higher) we can run more LEDS in series to match that closer and use the available voltage more efficiently. But you need to leave a little overhead for the device you are using to regulate the current.
Assuming a 3V LED, what is often common in 12V applications is to run 3 of them in series for a total forward voltage (Vf) of 9 volts. This leaves 3 volts left over for use with your current regulation device.
Resistors don't regulate voltage, they regulate current. The amount of current they pass is proportional to the amount of voltage running through them. Because available voltage can swing in automotive applications, its best to use a current regulator of sorts. The easiest get-up-and-go would be a linear regulator.
You will not be happy if you don't use a current regulator. Especially if you are trying to make your lights change intensity by opening up another circuit through them.
Also, LEDs will change their forward voltage based on the temperature they are currently at. As they get hotter (like when they are turned on), their forward voltage drops. This drop in forward voltage will cause them to want to pull more current (diode magic). Now you have an already hot LED trying to get itself hotter even faster. You are now experiencing thermal runaway and will drastically hurt your LEDs lifespan. Again, you need to use a form of current regulator.