LEDs, like all diodes, are current driven devices. The light output is directly proportional to the input current provided. It is not enough to provide a constant voltage to the LED. From one LED to the next, the voltage at a specific current can vary by as much as 0.4V. This can result in a 2:1 difference in the current supplied and will affect the power output from one LED to the next by the same ratio.
There are several methods for supplying current to the LED. An example of an accurate and stable circuit is shown to the left . This circuit is commonly referred to as a constant current source. Note that the supply current is determined by the supply voltage (Vcc) minus Vin divided by R1, (Vcc-Vin)/R1.
In applications where the operating temperature range is narrow (less than 30°C) or the output of the LED is not critical, a simple circuit utilizing a current limiting resistor may be used as shown to the right.
The current value is found by applying the equation I = (Vcc-Vf) / RL. To be absolutely certain of the current flow in the circuit, each LED VF would have to be measured and the appropriate load resistor specified. In practical commercial applications Vcc is designed to be much larger than VF and thus the small changes in VF do not affect the overall current by a large amount. The negative aspect of this circuit is a large power loss through RL.
In some applications current limited power supplies or batteries are used as the power source. If the supply has a maximum current rating less than the LED maximum rating it can be connected directly without any associated electronics. Please note this is not a method we recommend but has been used with varying success by end-users.
For pulsing applications it is important that the LED be driven by a low impedance driver. Opto Diode recommends a low impedance FET output for the final driver stage. If this is not possible, then a load resistor must be placed between the LED and output driver to minimise any inductive spikes that may occur. Instantaneous damage to the LED chip may occur if the circuit is not properly connected. For breadboard prototyping or small production runs we recommend an off the shelf Laser Diode Driver be used to pulse the LED.
