Unfortunately our hobby circuit work on different voltages and a power supply with single voltage will not be sufficient. Simplest way would be to buy a multi-tapped transformer and connect the voltage taps with a suitable switch. These transformers are available with taps of 1.5, 3, 4.5, 6, 9, 12v. We need a single multi throw switch ( as many throws as there are taps). With 9-6-0-6-9 transformer and with a double pole double throw switch, you can get 6 and 9V dc. With the same transformer , and with a double pole double throw switch it is possible to get 3, 6, 9, 12, 15, 18V. Switches and multi – taped transformers are difficult to procure; cumbersome and even unavailable at some places.
Such schematics are not shown here as there are better way to build circuits to get variable voltages. Also these power supplies suffer from a major draw back ; poor valtage regulation, voltage fluctuations.
A12V power supply shown in schematic 3 may show as much as 16.8V without load and it may go down to less than 12V as the load current is increased. This still can be and is safely circuits, we will user them for the pleasure and ease of using them and build regulated power supply as a starter.
We have some wonderful devices in three terminal regulators, just three terminals, IN- GROUND – OUT