Jun 052018
 

LM317 variable voltage regulator circuit diagram

LM117 / LM217 / LM317 variable voltage regulator characteristics

The LM317 is a 3-terminal positive-voltage regulator with a 1.25 to 37 volts output voltage range. The LM317 regulator terminals are: IN (input), OUT (output), ADJ (adjust).

To achieve this voltage variation, only 2 external resistors are needed (one of them is a variable resistor). Among its main characteristics of the LM317 voltage regulator have current limitation and thermal protection against overloads.

How the LM317 Variable Voltage Regulator circuit works?

The voltage between the ADJ and OUT terminals is always 1.25 volts (voltage established internally by the regulator) and consequently the current flowing through the resistor R1 is: IR1 = V/R1 = 1.25/R1. (1)

LM317 variable voltage regulator circuit diagram

The current through resistor R1 is IR1  and it is the same current that circulates through resistor R2 (the IADJ current is neglected). Then the voltage on resistor R2 is: VR2 = IR1 x R2 (2)

If IR1 (1) is substituted into the last formula (2), the following equation is obtained: VR2 = 1.25 x R2/R1.

Since the output voltage is: Vout = VR1 + VR2, then …

Vout = 1.25 V + (1.25 x R2/R1) V.  ….. simplifying (common factor)
Vout = 1.25 V (1 + R2/R1) V.

From the last formula it is clear that if you modify the R2 resistor value (variable resistor), the Vout voltage is modified.

  • R1 = 240 ohms
  • R2 = 5K or 3K (potentiometer)

LM317 voltage regulator pinout

The IADJ current was neglected, since it has a maximum value of 100 uA and it remains constant with load and input voltage variations.

To improve the regulation, resistor R1 must be placed as close as possible to the regulator, while the R2 resistor ground terminal must be connected  as close as possible to the ground connection of the load.

LM317 voltage regulator circuit operation improvement

In order to optimize the  LM317 variable voltage regulator circuit operation, some additional elements can be incorporated into the design. See the diagram below.

Improved LM317 variable voltage regulator circuit diagram

LM317 / LM117 / LM217 variable voltage regulator using protection diodes

  • A 0.1uF capacitor (C1) is placed at the input terminal (IN), if the voltage regulator is away from the rectification components.
  • A 1uF tantalum capacitor or a 25 uF electrolytic capacitor (C3) is placed at the output terminal (OUT) to improve the transients response.
  • A 10 uF electrolytic capacitor (C2) is placed in parallel with resistor R2, to improve rejection of the ripple voltage.
  • A 1N4001 diode (D1) is placed to protect the regulator against possible short circuits at the regulator input.
  • A 1N4001 diode (D2) is placed to protect the regulator against possible short circuits in its output when capacitors are discharged.
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