At 04:52 PM 6/24/2004, Harold Hallikainen wrote: >Thanks! I'm not seeing how I can get around the LED Vf requirement. What I >need is a dynamic resistor that sits between the LED and the variable >supply. The supply varies between 0 and 10V. No matter what the voltage, I >want the LED current to be 5mA. (recognizing, of course, that I won't get >that when the power supply voltage is below the LED Vf). This "dynamic >resistor" should adjust between 0 ohms and some higher resistance to limit >the LED current to 5mA. Ideally the circuit draws only 5mA (and all of >that goes through the LED). Classic 2 transistor current source is probably as close as you are going to get: uses 2 transistors of same polarity plus 2 resistors. I like to use 2n4401. Not your ideal zero parts solution but will work down to about 0.1V above the LEDs Vf. Led anode to V+. Led cathode to Q1c. Q1e to V- via Iset resistor (120R). Q2e to V-. Q2c to Q1b and also to 1 side of bias resistor (10k). Other side of bias resistor to V+. Q2b to Q1e. Bias resistor tries to turn Q1 on as hard as it can. Q2 monitors the voltage drop across Iset resistor and throttles Q1 back. I'm guessing that it should work down to about 2V with a red LED. dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 20 years of Engineering Innovation (1984 - 2004) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu