Originally Posted by Adiankur
hey, im wondering about LEDs and resistors. I have 2 volt LEDs and am going to run them in groups of 6 for 12 volts, but I am concerned about the upper end of the voltage spectrum for my batteries. 12 volt sealed lead acid battery can have up to what starting voltage? just wondering to know if I will need resistors to keep the life of my leds long, or at all.
LED's are funny creatures. They don't operate as a fixed resistance the way that light bulbs do, they're more like on-off switches. The voltage rating on a LED is the "forward voltage." If the voltage applied is greater than the forward voltage, the LED turns on. If the LED is on in a circuit with no resistance, you have a short circuit, and something will burn up.
What you need to include is a "current-limiting" resistor, which goes in series with the LED to limit current in the circuit. To size the resistor, you need to know three things: the forward voltage of the LED, the maximum supply voltage, and the maximum current the LED can take. As an example, assume you have a 12 volt nominal SLA battery that produces 14 volts at full charge, and six LED's with a forward voltage of 2V and a maximum current of 100 mA. The LED's will always drop their forward voltage, so 12V will drop over the LED's, and the remaining 2V will drop over the resistor. The current through the resistor is the same as through the LED's, so it can be no more than 100 mA. Using Ohm's Law to solve for 2V @ 100mA gives a resistance of 20 ohms. To buy a resistor you need to know its resistance, and also its power, so solving Watt's Law for 2V and 20 ohms gives .20 watts.
With a current limiting resistor, there is always some efficiency loss. The efficiency is given by (forward voltage)/(supply voltage). Note that the value of the resistor does not affect the efficiency.
As the supply voltage drops, the efficiency increases, but the power output decreases. The power output is given by (Power at maximum voltage)*(Supply voltage-Forward voltage)/(Maximum voltage-forward voltage). Note that when the supply voltage reaches the forward voltage -- 12 V in our example -- the power drops to zero and the LED shuts off. Again, the value of the resistor does not affect this equation.
So when you are designing your circuit, what is important is the range of the supply voltage and the forward voltage of your LED's. You have to make a compromise. The lower your forward voltage, the more even the output will be over a range of voltages and the lower your shut-off voltage. However, the higher the forward voltage, the more efficient your light is.
As a reference point, I have some LED truck taillights and they are 4 LED's with a combined forward voltage of 8V. The designer chose stability over efficiency.
In an efficiency-crucial application like bike lighting, it probably makes sense to use a voltage regulator. But that's another topic.