In many cases we can manage with a suitable
fixed voltage regulator such as 5v or 12v, but occasionally we
need a non-standard voltage. In these circumstances the LM317
range of regulators proves to be very useful.
The output voltage is selected using two
resistors. Normally R1 is chosen to be around 220R or 240R. The
formula for calculating the value of R2 is V = 1.25(1+(R2/R1))
or to put it another way R2 = R1((V/1.25)-1).
For example, for an output voltage of 8.5v.
R2 = 220((8.5/1.25)-1) = 1276 ohms. The nearest preferred
value to this would be 1K3.
Setting R2 to zero, ie. grounding the Adjust
pin, will cause the output voltage to drop to 1.25v.
R2 may be replaced by a pot to give an
adjustable output voltage range. The maximum voltage is approx.
37v although a higher voltage variant is available.