How to build Variable DC Power Supply

October 12, 2010 - category: DC power supply

Voltage range: 0.7 - 24V

Current limiting range: 50mA - 2A

Circuit diagram

Variable DC Power Supply-Circuit diagram


  • P1 500R Linear Potentiometer
  • P2 10K Log. Potentiometer
  • R1,R2 2K2 1/2W Resistors
  • R3 330R 1/4W Resistor
  • R4 150R 1/4W Resistor
  • R5 1R 5W Resistor
  • C1 3300µF 35V Electrolytic Capacitor (see Notes)
  • C2 1µF 63V Polyester Capacitor
  • D1,D2 1N5402 200V 3A Diodes
  • D3 5mm. Red LED
  • Q1 BC182 50V 100mA NPN Transistor
  • Q2 BD139 80V 1.5A NPN Transistor
  • Q3 BC212 50V 100mA PNP Transistor
  • Q4 2N3055 60V 15A NPN Transistor
  • T1 220V Primary, 36V Center-tapped Secondary 50VA Mains transformer (see Notes)
  • PL1 Male Mains plug
  • SW1 SPST Mains switch

Device purpose:

A Variable DC Power Supply is one of the most useful tools on the electronics hobbyist's workbench. This circuit is not an absolute novelty, but it's simple, reliable, "rugged" and short-proof, featuring variable voltage up to 24V and variable current limiting up to 2A. It's well suited to supply the circuits shown in this website. You can adapt it to your own requirements as explained in the notes below.


  • P1 sets the maximum output current you want to be delivered by the power supply at a given output voltage.
  • P2 sets the output voltage and must be a logarithmic taper type, in order to obtain a more linear scale voltage indication.
  • You can choose the Transformer on the grounds of maximum voltage and current output needed.
  • Best choices are: 36, 40 or 48V center-tapped and 50, 75, 80 or 100VA.
  • Capacitor C1 can be 2200 to 6800µF, 35 to 50V.
  • Q4 must be mounted on a good heatsink in order to withstand sustained output short-circuit.
  • In some cases the rear panel of the metal box in which you will enclose the circuit can do the job.
  • The 2N3055 transistor (Q4) can be replaced with the slightly less powerful TIP3055 type.
  • Excellent quality-price ratio: enjoy!

author: RED Free Circuit Designs
circuit from http://www.redcircuits.com/

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