BUILDING THE POWER SUPPLY

For safety reasons, you must enclose the power supply in a plastic or
metal chassis (plastic is better because there is less chance of a short circuit). Use a perforated
board to secure the components, and solder them together using 18- or 16-gauge
insulated wire. Alternatively, you can make your own circuit board using a home etching
kit. Before constructing the board, collect all the parts and design the board to fit the specific
parts you have. There is little size standardization when it comes to power supply
components and large value electrolytic capacitors, so presizing is a must.
HOW THE CIRCUIT WORKS
Here’s how the circuit works. The incoming AC is routed to the AC terminals of the transformer.
The “hot” side of the AC is connected through a 2-amp slow-blow fuse and a single-
pole, single-throw (SPST) toggle switch. With the switch in the off (open) position, the
transformer receives no power so the supply is off.
The 117 VAC is stepped down to the secondary voltage of the transformer (12 to 18
volts, depending on the exact voltage of the transformer you use). The transformer specified
here is rated at 2 amps, which is sufficient for the task at hand. Remember that the
power supply is limited to delivering the capacity of the transformer (and later the voltage
regulator), no more. A bridge rectifier, BR1 (shown schematically in the box in Fig. 15.18),
converts the AC to DC. You can also construct the rectifier using discrete diodes, and connect
them as shown within the dotted box.
When using the bridge rectifier, be sure to connect the leads to the proper terminals.
The two terminals marked with a “~” connect to the transformer. The “ ” and “ ” terminals
are the output and must connect as shown in the schematic in Fig. 15.18. Use a
5-volt, 1-amp regulator—a 7805—to maintain the voltage output at a steady
5 volts.