How to build Dual-rail Variable DC Power Supply

September 30, 2010 - category: DC power supply


This design was conceived as an add-on for the Variable DC Power Supply, a very successful circuit posted to this website. This simple unit provides a dual-rail variable output ranging from ±2.5V to ±15Vdc with precise tracking of the positive and negative output voltages, still retaining the current limiting and short-proof capabilities of the "master" circuit. As the purpose of such a dual-rail design is to supply experimental or under-repair circuits, the maximum current output delivered was deliberately kept to about 500 - 600mA per rail, thus avoiding the use of expensive power transistors and complex circuitry.

Circuit Diagram:

Dual-rail Variable DC Power Supply-Circuit Diagram


  • R1 = 4.7K-1/2W
  • R1 = 4.7K-1/2W
  • C1 = 100nF-63V
  • C2 = 220µF-25V
  • C3 = 220µF-25V
  • C4 = 100nF-63V
  • C5 = 100nF-63V
  • Q1 = BD437
  • Q2 = BD438
  • IC1 = LM358


  • The circuit can be placed into the existing Variable DC Power Supply metal cabinet.
  • Q1 and Q2 must be mounted on heatsinks. Usually, bolting them to the metal case (through insulating washers etc.) proved effective.
  • The full ±15V output can be achieved only if the secondary winding of the supply Transformer used in the Variable DC Power Supply is rated at 48V minimum (center tapped).
  • When using this circuit, please set the Current-limit control (P1) of the Variable DC Power Supply to any value comprised in the 50mA - 1A range but not higher.
  • The second Op-amp (IC1B) contained in the LM358 chip was not used, but its input pins were tied to the negative supply and the output was left open.

circuit from http://www.redcircuits.com

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