Adjustable Output Voltage Regulator using 7805

          I had already published my blog on 5V Fixed-Output Voltage Regulator using 7805. 78xx series ICs can also be used to create adjustable voltage regulator. This blog will give you some tips to create adjustable voltage regulator using 7805. Circuit is done as shown in the following diagram.

          7805 is capable of handling an input voltage from 7.5V to 25V. I gave an input voltage of 9V from an SMPS. A 0.33 micro Farad capacitor is connected across the input and a 0.1 micro Farad capacitor is connected across the output. R1 is fixed and R2 is variable. I kept R1 at 420 Ohms. Similarly a 0 - 200 Ohm variable resistor is used as R2. Output voltage is normally measured using a multimeter. Circuit can be summarised as given below.

7805                -  Voltage Regulator

Input Voltage  -  9V, 1A supply from an SMPS

R1                   -  420 Ohms

R2                   -  0 - 200 Ohm Variable  

Capacitors       -  0.33 micro Farad at input side and 0.1 micro Farad at output side.

       
          Output voltage varies with the R2 resistance value. Output voltage can be calculated using a simple formula.

Vo    -  Output Voltage
Vxx  -  Nominal output voltage ( Output to Common ) of fixed regulator. Vxx is 5V for 7805, 8V for 7808 and 12V for 7812.
Io      -  Output Current.

Procedure

          Initially, keep the resistance R2 at minimum position ( 0 Ohm ). Then Vo = Vxx. That is, output voltage will be equal to nominal output voltage ( 5V for 7805 ). Now put R2 at maximum position ( 200 Ohms ). Then measure the output voltage using a multimeter. I got an output voltage of 8.14V. Calculation can be summarised as given below.

Vo    =    8.14V
Vxx  =    5.07V
R1    =    420 Ohms
R2    =    200 Ohms


          Substituting these values in the above formula, we will get Io. Io will be same for a particular load, at different voltages. Using this value of  Io, Vxx and R1, we can calculate the value of  resistance R2 to be used for a desired value of Vo. Vo measured for various values of R2 using multimeter is given below.

Inference 

          From the table, it is clear that, Vo increase with increase in R2. So we will get a variable output voltage from 5.07V to 8.14V. An increase in the value of resistances above the values given in the table didn't give me an expected result.