Resistor and potentiometer values to obtain 1.25-12V variable with an LM317

I want to make a variable power supply up to 12V. For that I want to select resistor and adjust potentiometer values. I have calculated below values from this website.https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator

R2 is a 10K poteantiometer and R1 is 1000. When I put these values in above farmula, I am get 12V Vout. Are these values okay? I have seen many tutorials where potentiometer R2 is 2k and R1 is 220.

• Don't forget to calculate the power dissipated in the 317. It will be given by I x the voltage drop across the LM317 and at higher currents will generate a lot of heat. Jul 14, 2021 at 8:41
• Are these okay values? Read what the LM317's datasheet tells you about the current flowing out of the ADJ pin. You want the current though R1 and R2 to be much larger (I would make that current at least 10x larger). Jul 14, 2021 at 8:46

The problem with using those values is that you are not inherently meeting the minimum current draw from the LM317 so it will tend to go out of regulation (higher than expected output voltage) with a light (or no) load.

At maximum input-output differential (40V) the datasheet typical value is 3.5mA and the maximum is given as 10mA. So to be completely safe R1 should be less than 125 ohms.

In practice you can usually get away with 240 or 220 ohms with modern LM317s, especially with lower input voltages, and that is what many designs use, but you should recognize that it's rolling the dice slightly.

One way to go would be to use a 2K pot rather than a 10K, and then your R1 would be closer to 240 ohms.

You could also add a current sink to the output to draw (say) 5mA, but that's a bit of extra complexity. For example:

simulate this circuit – Schematic created using CircuitLab

The diode is used as a crude reference (you could also use a voltage divider if the 12V rail is regulated). It will draw a constant current of about 5mA for any output voltage above about 650mV, easily handling the 1.25V minimum from the LM317.

Using too high resistances will also affect the stability since the adj terminal bias current is not perfectly constant, as Bimpelrekkie points out.

• Thank you so much for this detailed answer, I will use R1 = 220 but if i still use 10k POT would it still be okay? If i use farmula Vout = 58 voltages. Jul 14, 2021 at 9:19
• If you use 220 ohms then it will reach 12V with the pot at 1892 ohms. So 4/5 of the pot electrical rotation will either give you more than 12V or saturate at input voltage- dropout voltage (the latter is a couple of volts, see the datasheet). You could shunt the pot with ~2.5K but it would be very nonlinear. Jul 14, 2021 at 9:22
• thank you for the answer. Jul 14, 2021 at 9:36