Variable DC Power Supply with 2N3055

This is variable power supply for multi purpose usage and very useful to supply your electronic tools or your projects. Voltage range will be 0.7 - 24V and the urrent limiting range is 50mA - 2A.
Variable DC Power Supply with 2N3055 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


  • 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.

About Power Supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term of "power supply" is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy (e.g., mechanical, chemical, solar) to electrical energy.
A power supply may be implemented as a discrete, stand-alone device or as an integral device that is hardwired to its load. In the latter case, for example, low voltage DC power supplies are commonly integrated with their loads in devices such as computers and household electronics. More explanation about power supply can be found at

This is the tutorial about "How to build an AC to DC power supply ". The video tutorial covers the basics of diodes, bridge rectifiers, and how to build simple unregulated AC to DC power supplies than can handle a few mA up to several Amps.

Watch the video: