12.2) Adding an IC regulator to a wall adapter or battery
From Sci.Electronics.Repair FAQ: Various Schematics and Diagrams
http://www.repairfaq.org/REPAIR/F_samschem.html


For many applications, it is desirable to have a well regulated source
of DC power.  This may be the case when running equipment from
batteries as well as from a wall adapter that outputs a DC voltage or
the enhanced adapter described in the section: "Converting an AC
output wall adapter to DC".

The following is a very basic introduction to the construction of a
circuit with appropriate modifications will work for outputs in the
range of about 1.25 to 35 V and currents up 1 A.  This can also be
used as the basis for a small general purpose power supply for use
with electronics experiments.

What you want is an IC called an 'adjustable voltage regulator'.
LM317 is one example - Radio Shack should have it along with a
schematic.  The LM317 looks like a power transistor but is a complete
regulator on a chip.

Here is a sample circuit:

                    I +-------+ O                       
 Vin (+) o-----+---| LM317 |---+--------------+-----o Vout (+)
               |   +-------+   |              |
               |       | A     /              |
               |       |       \ R1 = 240     |
               |       |       /              |           ___
              _|_ C1   |       |            +_|_ C2      |_0_|  LM317
              --- .01  +-------+             --- 1 uF    |   |  1 - Adjust
               |  uF   |                    - |          |___|  2 - Output
               |       \                      |           |||   3 - Input
               |       / R2                   |           123
               |       \                      |
               |       |                      |
 Vin(-) o------+-------+----------------------+-----o Vout (-)

Note: Not all voltage regulator ICs use this pinout.  If you are not
using an LM317, double check its pinout - as well as all the other
specifications.

For the LM317:

1. R2 = (192 x Vout) - 240, where R2 in ohms, Vout is in volts and
   must be at between 1.2 V and 35 V.

2. Vin should be at least 2.5V greater than Vout.  Select a wall
   adapter with a voltage at least 2.5 V greater than your regulated
   output at full load.

   However, note that a typical adapter's voltage may vary quite a bit
   depending on manufacturer and load.  You will have to select one
   that isn't too much greater than what you really want since this
   will add unnecessary wasted power in the device and additional heat
   dissipation.

3. Maximum output current is 1 A.  Your adapter must be capable of
   supplying the maximum current safely and without its voltage
   drooping below the requirement in (2) above.

4. Additional filter capacitance (across C1) on the adapter's output
   may help (or be required) to reduce its ripple and thus the swing
   of its input. This may allow you to use an adapter with a lower
   output voltage and reduce the power dissipation in the regulator as
   well.

   Using 10,000 uF per *amp* of output current will result in less
   than 1 V p-p ripple on the input to the regulator.  As long as the
   input is always greater than your desired output voltage plus 2.5
   V, the regulator will totally remove this ripple resulting in a
   constant DC output independent of line voltage and load current
   fluctuations.  (For you purists, the regulator isn't quite perfect
   but is good enough for most applications.)

   Make sure you select a capacitor with a voltage rating at least
   25% greater than the adapter's *unloaded* peak output voltage
   and observe the polarity!

   Note: wall adapters designed as battery chargers may not have
   any filter capacitors so this will definitely be needed with 
   this type.  Quick check: If The Voltage On The Adapter's output 
   drops to zero as soon as it is pulled from the wall - even with
   no load - it does not have a filter capacitor.

5. The tab of the LM317 is connected to the center pin - keep this in
   mind because the chip will have to be on a heat sink if it will be
   dissipating more than a watt or so.  P = (Vout - Vin) * Iout.  

6. There are other considerations - check the datasheet for the LM317
   particularly if you are running near the limits of 35 V and/or 1 A.