Quote:
Originally Posted by Terry_VK5TM
R1 at 510 Ohms is more than double the recommended value of 240 Ohms and the pot only needs to be 5k to give a 2 - 30 v range.
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True. The 240 ohms is a bit of a misnomer too though. Someone copied it from the datasheet decades ago, which was honestly pretty poorly written in context to how people tend to use the things, and it became religion.
A neat trick to making calculations easier is to use a 120 ohm resistor there. This does two things:
Firstly it makes the other resistor value easy to calculate in your head as the reference adj is 1.25 volts. So for 12 volts you just take 1.2 volts off and multiply by 1000 then round down to the nearest E12 so 1K. This pops out around 11.7 volts, slightly under intentionally, so you can then slide a 100 ohm trimmer on the end to give 11.7-12.7 volts for precise trimming or use a precision E24 in there instead.
Secondly this divider also sinks 10mA which is the minimum required DC load current for regulation. That means it'll be quite happy with whatever is or isn't connected to the output. With a 240 there it's only about 5mA which is borderline unstable with about a 10% chance of doing something weird.
There are some real cock ups with the LM317 out there. QRP labs sell a few ...