Hi there -

I have a handful of LM350s, looking to create a variable voltage source - 9v to 0v (or wherever the Reference Voltage can safely be set - 1.25v?). The basic diagram, from the datasheet, is included here.

I'm a little uncertain about picking resistor values, however. At the moment, I only have 100k pots for R2. The datasheet suggests 240 ohms for R1. I'm pretty sure that would be a bad pairing. Is it possible to use a different value for R1 to match my 100k pot? Or do I need to do something to restrict the pot?

 

Without a negative voltage, the lowest output voltage is 1.2-1.3V.

The datasheet states a minimum load current of 10mA, so R1 should be more like 120 ohms. You can use a 100k pot (1k would be more appropriate with R1=120 ohms), but a smaller pot would give more adjustment accuracy; maximum output voltage will be determined by input voltage and the value of the program resistor (R1).

EDIT: Added "adjustment" above for clarification.

 

For 9V output the pot should be no more than 1kΩ (which would give a maximum output of about 11V) for a good adjustment range and resolution with a 120Ω resistor for R1.