Most electronic appliances, such as TVs, computers, clock radios and cellphone chargers to name a few, require a variety of direct current (DC) voltages to operate. The electricity available in your home conveniently at every wall outlet, however, is alternating current (AC), where the polarity changes at a rate of 60 times per second. These appliances typically also require the voltage to be reduced to something other than 110 volts, as is present in your home. The basics of such a power supply for converting AC voltage to DC voltage needed for today's electronic devices, consists of a step-down transformer to change 110 volts AC to 6, 9, 12 or whatever voltage the particular device requires, and four solid state diodes configured to form a "full wave rectifier" circuit to convert the AC voltage to DC.
Connect the two step-down transformer input wires to the two wires in the lamp cord and install an electrical plug on the other end.
Label one wire of the step-down transformer's output "X" and the other wire "Y."
Label four power supply diodes "1", "2", "3" and "4" respectively.
Use a soldering iron to attach the components.
Connect the cathode of diode "1" and the anode of diode "2" to wire "X" on the transformer.
Connect the cathode of diode "3" and the anode of diode "4" to wire "Y" on the transformer.
Connect the anodes of diodes "1" and "3" together. This junction will be the "ground" or negative output of your AC to DC converter circuit.
Connect the cathodes of diodes "2" and "4" together. This junction is the positive (B+) output. The negative and positive DC outputs are used to power the desired electronic circuit.