The discarded computer power supplies which I modified, ( see January 26th and March 7th ), are very useful items of equiment to have on the bench in the workshop; 12V for my frequency synthesiser project and 5V for microcontrollers. They are, of course, fixed voltage supplies. There will be times when other voltages will be needed. I am thinking in particular about 9V and 3V. So I made an adjustable series voltage regulator to connect externally to the 12V power supply. The regulator uses a linear voltage regulator IC, RCA type CA723CE, or National Semiconductor type uA723CN; both have identical pin assignments, and are directly interchangeable, ( same package outline 14pin DIP ). Unfortunately two different, separate circuits are needed to provide 3V and 9V; the difference being the connections between the resistive potential divider and the error, ( "long-tailed pair" differential ), amplifier. For the time being at least, I decided to make just a 9V version for permanent use, ( the output voltage is adjustable between 8.43 - 9.81V ); but I also built a 3V version on experimenting board to check it, ( the output voltage range was 2.8 - 3.6V ).
The xA723Cx IC on its own can provide a maximum of 150mA output load current. This is insufficient for general purpose use on the work-bench; more current should be available. So the IC drives a 'pass' transistor rather than the load directly; but the output will be 0.6V less. Just about any power switching transistor capable of passing a few amps is suitable. From my stock I pressed into service an ancient and hitherto unused Motorola type BU326A, rated at maximum 8A, and fitted it to a small 2.9degC/W heatsink.
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