March 20th, 2012


The simple and easy single transistor NPN constant current sink for driving LEDs.

I constantly see people on Reddit /r/ece, /r/electronics, etc asking for advice on how to drive LEDs. It happens at least once a week. The classic approach is to take the input voltage, subtract the LED's forward voltage, and then use V = IR to compute the appropriate resistor. And that does work.

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Here it is:

Click on the image to open circuit simulation in a new window/tab. (NOTE: Requires Java capable browser.)

Simple, no? The only thing you need to know is how to calculate the resistor. Here's the formula:

Rbase = 100 * ((Vconstant - 0.65) / Iconstant)

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