Voltage divider circuit

[ilan1]

Sparkfun electronics sells this breadboard power supply:

https://www.electro-tech-online.com/custompdfs/2006/02/Breadboard-PowerSupply-v2.pdf

that is switchable between 5V and 3.3V. Could somebody clue me
in on exactly how the equations in the voltage divider in the above
schematic gives 3.3V and 5V?

I apologize for my abject ignorance!

Ilan

 

[spindrah]

Sparkfun electronics sells this breadboard power supply:

https://www.electro-tech-online.com/custompdfs/2006/02/Breadboard-PowerSupply-v2-1.pdf

that is switchable between 5V and 3.3V. Could somebody clue me
in on exactly how the equations in the voltage divider in the above
schematic gives 3.3V and 5V?

I apologize for my abject ignorance!

Ilan

the Vout equation for the LM317 is
Vout = 1.25v[1+(R2/R1)]+100uA*R2
so when R1 = 390 you get 3.32V
and when it is 390+330 or 720 you get 5.07v

 

[ilan1]

Did you figure this out from the LM317 data sheet or is there
some general principle here?

Ilan

 

[spindrah]

Did you figure this out from the LM317 data sheet or is there
some general principle here?

Ilan

its in the datasheet