# LM317 Get Stable Output voltage at 1,25 V

I am currently trying to use the LM317 with a stable output voltage of 1,25 V. The general formula is $$Vo = Vref\cdot\left(1+\frac{R2}{R1}\right) + Iadj\cdot R2$$

I used this circuit to create the 1,25 V output voltage:

simulate this circuit – Schematic created using CircuitLab

This circuit is directly according to the formula. R1 is infinitely high, R2 is zero. In general I measure the 1,25 V output voltage. But when the load is drawing current, everything gets unstable. Then I measure voltages higher and lower than 1,25 V...

I read some things regarding the minimum current in Adj Terminal. Therefore I thought about calculating R1 to create Iadj at ~50µA (according to ST LM317 datasheet). But whats with R2, should it be shortened to ground in this case?

I also thought about inserting a series resistor at the output, just to create a small load of ~1mA. Maybe the LM317 needs a minimum load... Are there any suggestions? Thanks for your help.

• What load are you applying? Does it vary, is it constant? How unstable is it, do you have a waveform? What kind of output cap do you use (i.e. what ESR does it have) – PlasmaHH Jul 21 '17 at 8:41
• At the load are various resistors creating a load current between 0,5µA to 50 mA. Relays are used to switch between the load resistors. – Merki Jul 21 '17 at 8:46
• Output caps are normla electrolyt 10µ capacitors, ESR is neglectable.. – Merki Jul 21 '17 at 8:47