Adjustable Voltage Regulator Using LM317

The circuit intends to produce an adjustable voltage regulator in the range of 0V to 3V with the use of the three-terminal adjustable regulator LM317.

Without applying any methodology, the normal value of the voltage from LM317 cannot be less than 1.25V which is the inner reference voltage of the device. The voltage output can only be less than this value if proper biasing is done.

The use of a reference-voltage source based on two diodes is one way to solve this problem. But this approach will only be suitable for a 1.2V to 15V or higher voltage regulator and it will not be fitted for extra-low, fixed-, or adjustable voltage regulator. This is because the needed potential bias of 1.2V is not employed by the two 1N4001 diodes. Furthermore, they contain additional instability of temperature that is around 2.5mV/K. There is an additional temperature drifting of the output voltage that is more than 6% for a 1.5V output voltage which results to around 100mV.

An inexpensive approach is using a simple temperature-stabilized constant-current source can provide the necessary potential bias. An alternative is using LM185 or AD589, but will be more costly.

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