Hi Morgen,
The datasheet for the LM117/317 shows a 240 ohm reference current setting resistor R1 on its 1st page, for the premium version LM117. The resistor's max value is determined by the IC's max idle current, that flows through its output. Without a load, the only thing holding the output voltage down is the current in the current-setting resistor and pot. If the resistor and pot aren't the required low enough value to carry the IC's idle current then the output voltage will rise and good-bye voltage regulation.
The datasheet spec's the max idle current for the LM117 to be 5mA. Therefore the max value for R1 is 1.25V/5mA= 250 ohms. They use 240 ohms which is the nearest standard value.
The datasheet spec's the max idle current for the cheaper LM317 to be 10mA. Therefore the max value for R1 is 1.25V/10mA= 125 ohms. You should use 120 ohms which is the nearest standard value.
Using 120 ohms for R1, its current is 10.42mA and the voltage across a 1k pot is 10.42V. Therefore the circuit's regulated output voltage is 1.25V + 10.42V = 11.7V. A 2k pot would make the output voltage 22.1V if the IC's input voltage is high enough. A 500k pot is rediculous since the circuit's output voltage will be max over more than 99% of its adjustment. :lol:
The datasheet for the LM117/317 shows a 240 ohm reference current setting resistor R1 on its 1st page, for the premium version LM117. The resistor's max value is determined by the IC's max idle current, that flows through its output. Without a load, the only thing holding the output voltage down is the current in the current-setting resistor and pot. If the resistor and pot aren't the required low enough value to carry the IC's idle current then the output voltage will rise and good-bye voltage regulation.
The datasheet spec's the max idle current for the LM117 to be 5mA. Therefore the max value for R1 is 1.25V/5mA= 250 ohms. They use 240 ohms which is the nearest standard value.
The datasheet spec's the max idle current for the cheaper LM317 to be 10mA. Therefore the max value for R1 is 1.25V/10mA= 125 ohms. You should use 120 ohms which is the nearest standard value.
Using 120 ohms for R1, its current is 10.42mA and the voltage across a 1k pot is 10.42V. Therefore the circuit's regulated output voltage is 1.25V + 10.42V = 11.7V. A 2k pot would make the output voltage 22.1V if the IC's input voltage is high enough. A 500k pot is rediculous since the circuit's output voltage will be max over more than 99% of its adjustment. :lol: