Build an Experimenters Power Supply

Posted on Feb 6, 2014

Note the incoming ac routed to the primary terminals on a 12. 6-volt transformer. The hot  side of the ac is connected through a 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 about 12. 6 volts. The transformer specified here is rated at 2 amps, sufficient for the task at hand.

Build an Experimenters Power Supply
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Remember that the power supply is limited to delivering the capacity of the transformer (and later, the voltage regulator). A bridge rectifier, BR1, converts the ac to dc (shown schematically in the dotted box). You can also construct the rectifier using discrete diodes (connect them as shown within the 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. A 5-volt, 1-amp regulator, a 7805, is used to maintain the voltage output at a steady 5 volts. Note that the transformer supplies a great deal more voltage than is necessary. This is for two reasons. First, lower-voltage 6. 3- or 9-volt transformers are available, but most don`t deliver more than 0. 5 amp. It is far easier to find 12- or 15-volt transformers that deliver sufficient power. Second, the regulator requires a few extra volts as overhead  to operate properly. The 12. 6-volt transformer specified here delivers the minimum voltage requirement, and then some. Capacitors C1 and C2 filter the ripple inherent in the rectified dc at the outputs of the bridge rectifier. With the capacitors installed as shown (note the polarity), the ripple at the output of the power supply is negligible. LED1 and R1 form a simple indicator. The LED glows when the power supply is on. Remember the 270-ohm...

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