Making your own AC/DC power adapter is a good project for an intermediate-level electronics hobbyist. This will give you the fun of building a practical circuit you can have for general bench use or to power small electronic gadgets. The straightforward design uses the time-tested 7800 series voltage regulator, providing clean, steady voltage at up to 1 amp of current. You can buy the parts inexpensively at any electronics store. The power adapter can go together in less than two hours.
Determine the voltage you want. The 7800 voltage regulators come in fixed values ranging from 5 to 24 volts. For example, if you need 5 volts, choose a 7805 regulator. Select a step-down transformer whose secondary is a few volts higher than the voltage you need. If you use a 7805 regulator, a 7-volt transformer would work well. Select capacitors with voltage ratings at least 20 percent greater than the voltage you’ll be using.
Build the circuit on the perf board, laying the parts out from left to right, starting with the transformer. Align the transformer so its 110-volt primary side is on the left edge, with the secondary facing right. Using the sharp end of a knife, bore mounting holes for the transformer in the perf board, then use short #6 screws and nuts to hold the transformer in place. Strip the ends of the AC line cord, slip 1-inch pieces of heat-shrink tubing past the bare wire, then solder the line cord to the two transformer primary terminals. When the solder cools, slip the tubing over the bare solder joints and shrink the tubing.
Insert the bridge rectifier through the perfboard. Wire and solder the alternating current (AC) inputs, marked with a tilde (~) symbol, to the transformer secondaries. Insert a 200-microfarad (uF) electrolytic capacitor so its positive side matches up with the rectifier’s positive (+) output and likewise line up the negative (-) side of the rectifier and capacitor. Wire and solder the positive and negative outputs of the rectifier to the capacitor. Insert a .33 uF capacitor and wire and solder it in parallel with the 200 uF capacitor. Insert the voltage regulator and connect and solder its input to the positive side of the two capacitors. Connect the regulator’s common pin to the capacitors’ negative side. Insert a .1 uF capacitor. Connect one of its leads to the regulator’s output and the other lead to the regulator’s common pin. Solder these connections.
Bore holes in the right side of the perf board to fit the two banana jacks, and insert the jacks. Wire and solder the red jack to the regulator’s output and the black jack to the regulator’s common.
Check all connections. Plug the line cord into an AC receptacle and use the adapter.