This is a Variable Voltage Regulator Circuit, which in this circuit is controlled by IC LM317T. Which is a LM317T adjustable 3 terminal positive voltage regulator capable of providing more than 1.5 amps through a variety of output volts 1,25-37. This device also has built in current and thermal shutdown limit which makes it essentially blow-out proof. This circuit can be used to create a stable power supply. On the working principles of circuit output voltage is set by two resistors R1 and R2 connected as shown below. Voltage through the R1 is a constant 1.25 volts and the adjustment terminal current is less than 100uA. Output voltage can be closely approximated from Vout = 1.25 * (1 + (R2/R1)) which ignores the adjustment terminal current but will be close if the current through R1 and R2 is several times larger. A minimum load of 10mA is required, so that the value of R1 can be selected down to 1.25 volts at 10mA or 120 ohms. Something less than 120 ohms can be used to ensure a minimum greater than 10mA current. Here is a picture of the circuit diagram:
In the picture above shows the LM317 used as 13.6 volt regulator. The 988 ohm resistor for R2 can be obtained with the standard 75 and 910 ohm in series. When the power is shut off to the regulator’s output voltage should fall faster than predicted. If not, a diode can be connected across the input / output terminals to protect against reverse voltage regulator. A 1uF tantalum or electrolytic capacitor 25uF entire output increase response and a small 0.1uF tantalum capacitor is recommended across the input if the regulator is located appreciable distance from power supply filter. Power transformer must be large enough so that the input voltage regulator will remain in the top 3 volts output at full load, or 16.6 volts to 13.6 volt output.
