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Overview The LED brightness is kept constant by this constant current source circuit no matter what power supply is used making it ideal for high power LEDs for any number and configuration. Explanation The input voltage of the circuit ranges from 2V to 18V with up to 0.5V less than the input voltage. The power source used and Q2 are the only real limit to the current source. The voltage from the power supply is stepped down by Q2 that functions as a variable resistor to match the need of LEDs. A heatsink will be needed by Q2 if there is a high LED current or if the power source voltage is a lot higher than the LED string voltage. Also, a lot of power can be handled with large heatsink since the Q2 transistor specified will work up to 18V power supply. Q2 can only dissipate about 1/2W before getting really hot without heatsinks at all. The start of Q2 as a variable resistor commences by turning ON R1. An over-current sensing switch is performed by Q1 while R3 is the sense resistor or set resistor that triggers Q1 when too much current is flowing. Rest of the project
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