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Very Low Cost Mathless LED Driver
Intended for people with interests in LED lighting, but are allergic to the math needed for calculating current limiting resistors, and have very little money. A good example would be an art student or custom car person, although it its ease of use could be appreciated by anyone. If you’re not so allergic to math, the equation is:
R_led=(V_input-Vf_led)/If_led
The device should regulate current through an LED, with a wide range of input voltage. The design uses a pair of PNP transistors as a current source with a target output of 10-15mA with 3-20V the target. Simulation in LTSpice predicts the circuit is sane. Dissipation in the LED drive transistor is under 200mW, and should be within specification of the MMBT3906 used to 155F ambient temperature.
BOM:
- 2x MMBT3904 (SOT23-3) $.025
- 1x 1K Resistor (0805) $.01
- 1x 68R Resistor (0805) $.01
PARTS TOTAL: $.07
The board design is single sided SMT, to be milled on the taig using Eagle’s pcb-gcode ULP.
Posted by rmrubin 
September 2007