Power Supply – Variable Voltage

When working with electronics, you always need one basic thing; power.
This power supply is great for powering all kinds of electronic
projects. It produces a well filtered, variable 1.2-30 volts at 5 amps.
It is easy to build and the parts are realitively easy to find.


This is a schematic of the Power Supply


Total Qty.
C1 1 14000uF or 10000uf 40 VDC Electrolytic Capacitor
C2 1 100uF 50Vdc Electrolytic Capacitor
C3 1 0.1uF Disc Capacitor
C4 1 0.01uF Disc Capacitor
R1 1 5K Pot
R2 1 240 Ohm 1/4 W Resistor See Notes
U1 1 LM338K 1.2 to 30 Volt 5 Amp Regulator
BR1 1 10 Amp 50 PIV Bridge Rectifier
T1 1 24 V 5 Amp Transformer
S1 1 SPST Toggle Switch
MISC 1 Wire, Line Cord, Case, Binding Posts (for output)


  1. The regulator comes in a TO-3 case and MUST be used with a LARGE heatsink. You may want to mount a small fan to blow air across the regulator (I did).
  2. The filter capacitor is large. It won’t fit on any board so bolt it to the case.
  3. You can, of course, add a volt and amp meter.
  4. Since this project operates from 120 VAC, you must include a fuse and build the project in a case.
  5. R2 may need to be decreased to 120 Ohm if you experience
    voltage drift at light loads. 240 Ohm may not load the output
    appropriately on some regulators. The datasheet for the LM338K
    does specify 120 Ohm (I suggest you use a 1/2W unit) so you may just
    want to use 120 Ohm and not bother with the 240 Ohm resistor showin the
    parts list. This has been discussed on the forum.

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