Half wave rectification is simplest, as it requires only one transformer, one diode and one capacitor, but we have added a LED to give power supply indication. A simple half wave rectifier for 12V is shown below

500mA transformers are generally used for these circuits. Connect the primary wires to the mains chord after carefully insulating the joints. This transformer has three wires on the secondary side. AC voltage across both end wires is 12 and voltage across any one end wire and center wire is 6. So if both end wires are connected as shown in the present schematic the output DC voltage will be 12 and if any one of the end wires and center tap are connected the voltage will be 6.

Take a Veroboard and solder all components on it except mains power supply. Solder IN4003 diode to one end of secondary winding making note of the cathode. Solder lOOOuf /25V capacitor. Please note the capacitor is polarized, which means that you should connect it one way only. Negative side of the pin is marked on the can. Add a light emitting diode to know that the power supply is on. Now use a 1K-1A watt resistor in series with it.

LEDs are also polarized and marked. LEDs will not light up if they are connected in reverse. Take two pen light cells and connect the ends of a Red LED to positive and negative and then turn the leads over and try. (Forget the cathodes and anodes for the time being.) You will know that it works only one way. That also explains how a diode works. Do not connect them directly to 12 V  LEDs cannot work beyond 5 V and their current capability is extremely limited. Although they are very rugged devices, they must always be used with a current limiting resistor.

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