**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

resistor.

To make any fixed regulator adjustable, use the following

formula:

**V**out = **V**fixed + **R**2(**V**fixed/**R**1

+ **I**stndby)

**V**out

=

Desired output voltage

**V**fixed

=

Fixed voltage of IC regulator (5 volts for 7805 or

1.25 volts for LM317T)

**R**1

=

Assume any value from about 470 to 1K for best

results

**I**stndby

=

Standby current of regulator (use 2.5MA for 7805 or

zero for LM317T)

Common Resistor Combinations for the 7805 regulator:

Vout

(Approx.)

R1

R2

6 Volts

470

100

8 Volts

470

220

9 Volts

470

330

12 Volts

470

510

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.

DE