Thanks for all your effort! It looks like we'd need about 700mv across Iset, then another 50mv across Q1 CE (assuming saturation), so that'd be a minimum of 750mv. Pretty good! I'll try it out. Thanks! Harold > 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 > -- FCC Rules Online at http://www.hallikainen.com -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu