Experiment 13 - The Adjustable Voltage Regulator LM317 II

By replacing a resistance by a potentiometer in the LM317 circuit, you can freely adjust the output voltage...

Hence replace the resistor R2 by the potentiometer P1.

Circuit diagram free adjustable voltage regulator LM317.

Freely adjustable voltage regulator circuit LM317. (Enlarge)

Now you can freely adjust the output voltage between...

• By turning the potentiometer to one side, P1 = 0 Ω...

V_out = 1.25 × (1 + ⁰/₁₀₀₀) + (0.00005 × 0)

        = 1.25 Volt

• By turning the potentiometer to the other side P1 = 5 kΩ...

V_out = 1.25 × (1 + ⁵⁰⁰⁰/₁₀₀₀) + (0.00005 × 5000)

        = 7.75 Volt

However, the LM317 has a drop-out voltage V_DO = 2 Volt. With an input voltage V_in = 9 V, as in this circuit, the output voltage can be adjusted between a minimum of 1.25 V and a maximum 7.0 V.

But what about the heat generation of the adjustable voltage regulator...?