Unfortunately our hobby circuits 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 six throw switch, it is possible to get 3, 6, 9, 12, 15, 18 V. Switches and multi-tapped transformers are difficult to procure; cumbersome and even unavailable at some places.
Such schematics are not shown here as there are better ways to build circuits to get variable voltages. Also these power supplies suffer from a major draw back; poor voltage regulation, the voltage falls as more and more current is drawn and voltage also changes with mains voltage fluctuations.
A 12 V power supply shown in Schematic 3 may show as much as 16.8 V without load and it may go down to less than 12 V as the load current is increased. This still can be and is safely used for most of the non-critical circuits, but it is an unstabilised supply.
Why not build a regulated power supply, which doesn’t cost an earth but still gives very good results. Now for the time being, without going into details and complications of integrated circuits, we will use them for the pleasure and ease of using them and build a regulated power supply as a starter.
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