Here I will explain to you how I made a Variable DC source whose output voltage can be varied from 1.2-30V and have a load current of 0.5A, This source is suitable for Electronics hobbyists.

The 230V-AC coming from the powercord is fed to the transformer TR1 via the on-off switch and the 500mA fuse. The 30v ac output (approximately) from the transformer is presented to  four general purpose 1N4004 diodes(The rectifier). The pulsating DC output is filtered via the 2200μF capacitor (to make it more manageable for the regulator) and fed to ‘IN’-put of the adjustable LM317 regulator (IC1). The output of this regulator is your adjustable voltage of 1.2 to 30volts varied via the ‘Adj’ pin and the 5K potmeter P1. The large value of C1 makes for a good, low ripple output voltage.
Why exactly 1.2V and not 0-volt? Very basic, the job of the regulator is two-fold; first, it compares the output voltage to an internal reference and controls the output voltage so that it remains constant, and second, it provides a method for adjusting the output voltage to the level you want by using a potentriometer.

Internally the regulator uses a zener diode to provide a fixed reference voltage of 1.2 volt across the external resistor R2. (This resistor is usually around 240 ohms, but 220 ohms will work fine without any problems). Because of this the voltage at the output can never decrease below 1.2 volts,(But there is a neat hack that can be used to achieve this connect two diodes in series and forward biased at output a total voltage drop of 1.4V is obtained thereby making the output voltage Zero) but as the potentiometer (P1) increases in resistance the voltage accross it, due to current from the regulator plus current from R2, its voltage increases. This increases the
output voltage. D1 is a general purpose 1N4001 diode, used as a feedback blocker. It steers any current that might be coming from the device under power around the regulator to prevent the regulator from being damaged. Such reverse currents usually occur when devices are powered down.
The ‘ON’ Led will be lit via the 18K resistor R1. The current through the led will be between 12 – 20mA @ 2V depending on the type
and color Led you are using. C2 is a 0.1μF (100nF) decoupler capacitor to filter out the transient noise which can be induced into the
supply by stray magnetic fields. Under normal conditions this capacitor is only required if the regulator is far away from the filter cap,
but I added it anyway. C3 improves transient response. This means that while the regulator may perform perfectly at DC and at low
frequencies, (regulating the voltage regardless of the load current), at higher frequencies it may be less effective. Adding this 1 μF
capacitor should improve the response at those frequencies.

Construction:

Because of the few components you can use a small case but use whatever you have available..
You can mount the LM317 regulator on a heatsink. Note that the metal tab of the LM317 is connected internally to the ‘Output’ pin. So it has to be insulated when mounting directly to the case.
Drill the holes for the banana jacks, on/off switch, and LED and make the cut-out for the meter. It is best to mount everything in such a way that you are able to trouble-shoot your circuit board with ease if needed. One more note about the on-off switch S1, this switch has 230VAC power to it. After soldering, insulate the bare spots

If all is well, and you are finished assembling and soldering everything, check all connections. Check capacitors C1 & C3 for proper
polarity (especially for C1, polarity reversal may cause explosion). Hookup a multimeter to the power supply output jacks. Set the
meter for DC volts. Switch on S1 (led will light, no smoke or sparks?) and watch the meter movement. Adjust the potentiometer until
it reads on your multimeter. Good luck and have fun building!