The Adjustable Voltage Regulator
Many amateurs have stopped by their local Radio Shack store
and have noticed the famous LM317T adjustable voltage regulator.
But, did you know that all voltage regulators are adjustable?
Yes, any IC voltage regulator can be adjusted to a higher voltage
than its fixed voltage by just adding a couple of resistors.
As an example, lets consider using the popular 7805 (5 volt)
voltage regulator as a 12 volt regulator. In figure 1, lets
assume 470 for R1 which means that a constant current of 10.6 mA
will be seen between terminals 2 and 3. This constant current
plus a regulator standby current of about 2.5mA will flow through
R2 to ground regardless of its value. Because of this constant
13.1 mA, R2 can now be set to a value which will give us a
constant 7 volts across this resistor. A resistor value of 533
ohms or 510 (standard value) will give us the necessary 7 volts.
With 5 volts across R1 and 7 volts across R2, a total regulated
value of about 12 volts will appear across terminal 2 and ground.
If a variable resistor is used for R2, then the output voltage
can be easily fine tuned to any value greater than 5 volts. The
regulator standby current will vary slightly in the 7805 but
2.5mA will yield good results in the calculations. If an exact
voltage (within .3 volts) is needed then R2 must be a variable
To make any fixed regulator adjustable, use the following
Vout = Vfixed + R2(Vfixed/R1
Desired output voltage
Fixed voltage of IC regulator (5 volts for 7805 or
1.25 volts for LM317T)
Assume any value from about 470 to 1K for best
Standby current of regulator (use 2.5MA for 7805 or
zero for LM317T)
Common Resistor Combinations for the 7805 regulator:
Incidentally, the famous LM317T adjustable regulator is really
nothing more than a fixed regulator with an output voltage of
1.25 volts. Amateurs seldom need voltages below 5 volts so the
7805 regulator is a good choice and it even costs a little bit
less than the LM317T.