Variable as the output voltage LM317 three-terminal regulator integrated block, is a convenient, widely used integrated voltage regulator block. Model 317 series regulators block a lot: for example LM317HVH, W317L and so on. E-enthusiasts often make the output voltage regulator block 317, a variable power supply (the basic form of the circuit as shown below). Power supply's output voltage can be calculated, Vo = 1.25 (1 + R2/R1). Just look at itself from the formula, R1, R2 resistor value can be arbitrarily set.

However, the output voltage of power supply as the formula, R1 and R2 is the resistance can not be arbitrarily set. First 317 block of output voltage regulation range is Vo = 1.25V-37V (high-output voltage regulator block 317, such as LM317HVA, LM317HVK so, the output voltage range is Vo = 1.25V-45V), so the R2/R1 only the ratio of the range 0-28.6. Followed by the regulator block 317 has a minimum stable operating current, some information is called the minimum output current, and some information is called the minimum discharge current. Stable value of the minimum operating current is generally 1.5mA. The regulator block 317 different manufacturers, different models, the minimum stable operating current is not the same, but generally not more than 5mA. When the regulator block 317, the output current is less than the minimum stable operating current when the regulator block 317 will not work properly. When the regulator block 317, the output current stability is greater than the minimum current, the regulator block 317 can output a stable DC voltage. If the regulator block produced by 317 when the power supply (as shown), did not pay attention to the smallest stability regulator block 317, operating current, making the power supply then you may experience the following anomalies: power supply output on-load voltage and load voltage varied.
To solve the regulator block 317, the minimum stable operating current problem, you can set the size of R1 and R2 resistance, leaving the 317 block of no-load voltage regulator output current is greater than or equal to its minimum stable operating current, thus ensuring 317 In the no-load voltage regulator to stabilize the block to work. At this point, as long as Vo / (R1 + R2) ≥ 1.5mA, regulators can ensure that 317 blocks in the no-load to stabilize the work. 1.5mA on the type of voltage regulator block for the 317 minimum stable current. Of course, As long as the regulator block 317 to stabilize at the no-load work, Vo / (R1 + R2) value can be set to any value greater than 1.5mA.
The calculation shows that the maximum value of R1 to R1 ≈ 0.83KΩ. And because the maximum value of R2/R1 28.6. Therefore, the maximum value of R2 is R2 ≈ 23.74KΩ. 317 blocks in the use of the output voltage regulator formula to calculate the output voltage, must ensure that R1 ≥ 0.83KΩ, R2 ≤ 23.74KΩ the same time the two inequalities established in order to ensure that 317 blocks in the no-load voltage regulator to stabilize the work. Of course, in the 317 block of output voltage in parallel discharge resistor R (as shown), it can block 317 to provide minimum stable voltage current. However, due to the discharge resistor in parallel with the output voltage can not change, if the regulator block 317 to ensure that the output voltage is 1.25V, the output current is greater than the minimum stable operating current, voltage regulator in the 317 block When the output voltage is 37V, the current flowing through the discharge resistor too big, not only a waste of energy, and increased the burden on regulator block 317, is not an appropriate approach.