By converting the cheap (free) ATX power supplies that can be found in any discarded computer, you can get a phenomenal lab power supply with huge current outputs, short circuit protection, and very tight voltage regulation.
Things You'll Need
An obsolete computer with an ATX 250W, 300W or 400W power supply.
Wire cutters, needle nose pliers, drill, reamer, soldering wire, soldering iron, electrical tape, heat shrink tubing
Binding posts for output terminals, LED, current limiting resistor for the LED, power resistor to load the power supply, a low wattage switch.
Unplug the power cord from the back of the computer. "Harvest" a power supply from a computer by opening up the case of the computer, locating the gray box that is the power supply unit, tracing the wires from the power supply to the boards and devices and disconnecting all the cables by unplugging them.
Remove the screws (typically 4) that attach the power supply to the computer case and remove the power supply.
Cut off the connectors (leave a few inches of wire on the connectors so that you can use them later on for other projects).
Discharge the power supply by stripping the insulation of the ends of a black and a red wire and connecting them together for a few seconds.
Gather the parts you need: binding posts (terminals), a LED with a current-limiting resistor, a switch (optional), a power resistor (10 ohm, 10W or greater wattage, see Tips), and heat shrink tubing.