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LM317 - LM338

Voltage Regulator

Schematic for  a minimally configured LM337 voltage regulator.

    To find the value for R2 using the typical values of R1 (240Ω), Iadj (50µA), and Iadj max (100 µA), enter a value for Vout. To find Vout, enter either R2 or both R1 and R2.

    If desired, you can enter values for R1, Iadj typ, or Iadj max. For each of the resistor tolerances shown in the result, the minimum and maximum voltages shown include the effects of the resistors tolerances as well as adjustment pin currents up to Iadj max. In addition, a tolerance of 1% for the LM317/338’s internal voltage reference is included in the indicated voltage span.

    The LM317 (and higher current variant LM338) are well established and versatile adjustable voltage regulators. Although not strictly required, C1 (0.1µF typical) should be included if the regulator is more than a few inches from Vin’s filter capacitors. Likewise, including C2 (minimally a 1µF tantalum or 33 µF aluminum electrolytic) can be used to ensure stability.

    When C2 is included, it is a good idea to include the protection diode shown below if Vin can appear like a low resistance when power is removed. This is particularly true for large values of C2. The diode allows current to flow around the LM317/338 from C2 to Vin. This protects the regulator from a potentially damaging transient current flow. A “garden variety” diode such as the 1N4002 is sufficient for this application.

Schematic for  LM317 voltage regulator with optional protection diode.

  Although not needed for most applications, you can increase the ripple rejection, particularly at higher frequencies, by including a capacitor in parallel with R2 as shown in the following schematic.

Schematic for LM317 voltage regulator with optional protection diode and ripple reduction capacitor.

    A 10µF tantalum capacitor is sufficient for the 120Hz (or 100 Hz) ripple that is likely to be encountered when Vin is derived from a line powered AC source.

  The following equation is used to find the values presented by this solver:

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