Power Source Considerations
The voltage versus current characteristics of an LED are much like any diode. Current is approximately an exponential function of voltage (see Shockley diode equation), so a small voltage change results in a large change in current. If the voltage is below the threshold or on-voltage no current will flow and the result is an unlit LED. If the voltage is too high the current will go above the maximum rating, heating and potentially destroying the LED.
It is therefore important that the power source gives the right current. LEDs should only be connected to constant-current sources. Series resistors are a simple way to passively stabilize the LED current. An active constant current regulator is commonly used for high power LEDs, stabilizing light output over a wide range of input voltages which could increase the useful life of batteries. Low drop-out (LDO) constant current regulators also allow the total LED voltage to be a higher fraction of the power supply voltage. Switched-mode power supplies are used in some LED flashlights.
Read more about this topic: LED Circuit
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