Variable Dc Power Supply


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 is simple, reliable, "rugged" and short-proof, featuring variable voltage up to 24V and variable current limiting up to 2A. You can adapt it to your own requirements as explained in the notes below.

Circuit Diagram :

Circuit Diagram


  • P1 = 500R
  • P2 = 10K
  • R1 = 2.2K-1/2w
  • R2 = 2.2K-1/2w
  • R3 = 330R
  • R4 = 150R
  • R5 = 1R-5W
  • C1 = 35V-3300uF
  • D1 = 1N5402
  • D2 = 1N5402
  • D3 = 5mm Red Led
  • C2 = 63V-1uF
  • Q1 = BC182
  • Q2 = BD139
  • Q3 = BC212
  • Q4 = 2N3055
  • SW1 = SPST Mains Switch
  • T1 = 36VCT-Transformer


  1. P1 sets the maximum output current you want to be delivered by the power supply at a given output voltage.
  2. P2 sets the output voltage and must be a logarithmic taper type, in order to obtain a more linear scale voltage indication.
  3. 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.
  4. Capacitor C1 can be 2200 to 6800µF, 35 to 50V.
  5. 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.
  6. The 2N3055 transistor (Q4) can be replaced with TIP3055 type.