RE: Adjustable Power Supply HELP

The schematic is incorrect.  As wired the switch will not affect the voltage in any way.

The correct verison would be as follows
      _______
     |       |
     |       |
     |_1_2_3_|
       | | |_____From transformer/rectifier
       | |
       | |_______Output
       | |    |
       | /   ---
       | \   ---
       | /390 |
       | \    |
       |_|    |
         |
         /
         \ 10R
         /
         \
         |
         /
         \ 10R
         /
         \
         |  To switch and variable resistor

Note that a 2200uF capacitor on the output is really way too much.  This amount of capacitance does not help the regulator work better, it slugs the transient response.  A single 100nF will suffice in most cases with maybe a 10uF cap if you are expecting big load spikes.

Also note that when the switch is inbetween positions, i.e. when you switch it, the output voltage will briefly rise to almost the unregulated voltage, probably destroying any sensitive equipement on the output.  You can use a make before break switch to stop this, or you could change the whole scheme and switch the 390R resistor to other values.  This way the voltage will fall to 1.25 volts during switching.

Regards

Mike Rigby-Jones