# LM317 adjustable constant current source - Why is the voltage between Output and Adj pins is not always 1.25V?

I am trying to create an adjustable constant current circuit using LM317 to drive LEDs of different colors.

More specifically, I want the brightness of the LEDs to be the same, therefore, the current flowing through them will be different for different colors. I searched online and found this circuit, which outputs a current between 125-400 mA.

I want the current to be between 1-3 mA, so I played around with the resistance values of R1, R2 and R3 (potentiometer.) When I tried certain resistance values (R1 = 10k, R2 = 2k and R3 = 50k), I found that the voltage between the output and adjustment pins is not always 1.25 V. Why is this so? Are there limits to which resistance values we have to use in order to maintain a potential difference of 1.25 V across the output and adjustment pins?

(P.S. I am supplying 9V into the LM317.)

• I had a similar question. see if it helps: electronics.stackexchange.com/questions/495243/… Commented May 5, 2020 at 7:48
• @TirdadSadriNejad Hi, thank you for sharing but I have found the answers to my problems. :) Commented May 5, 2020 at 8:53