Does Voltage Divider Impedance Matter with LM317?

[janneman]

Originally Posted by Soundminded

Yes, a voltage divider consists of to or more resistors in series, the voltage dividing in proportion to each resistor compared to the total resistance. When you connect a load, you are connecting it across one of the resistors. If the load resistance is not an order of magnitude larger than the resistor it is being connected across, it will reduce the parallel combination of itself and that resistor and will therefore reduce the proportion of voltage across them. This is basic DC electricity.

The load is connected from output to gnd, in effect in parallel to the series string, and not across one of them.

BTW, aside from the minimum load for the regulator as noted, you also have a small Iadj current coming out of the adj terminal.
To make sure that that current doesn't impact the design value for the resistive divider, make sure that the current through the divider string is at least an order of magnitude larger than the iadj current. That will also take care of the minimum load, killing two flies with, ehhh, two resistors.

jan didden

[Soundminded]

Originally Posted by janneman

The load is connected from output to gnd, in effect in parallel to the series string, and not across one of them.

Across means in parallel. The string should also end at ground. It should be in parallel with one or more resistors but not all of them. Any other arrangement and it's not a voltage divider.