Description & Features of LM317 Voltage Regulator:
Although the properties of the LM317 voltage regulator are excellent. They can be improved by cascading, two LM317s. There is then a constant difference between the output voltages of the two regulators and, consequently, a constant voltage across the input and output of 1C2. This arrangement results in an appreciable improvement of the regulating characteristics of IC2. Moreover, its dissipation is reduced so that the stability of the output voltage with temperature is improved. Other properties, such as the maximum output current of 1.5 A do not change, of course. The output voltage, Uo, of the circuit depends on the ratio R3:P1 as follows:
Uo= 1.25(1+R3/P1) [V].
With values of these components as shown, the output voltage may be varied over the range 1.25-11.5 V. The voltage drop, Ud, across IC2 depends on the operating point of IC-1 and may be calculated from:
Ud = 1.25(1+R2/R1)
With values of these components as shown, Ud = 3.5 V.
LM317 Voltage Regulator Circuit Diagram:
It should be noted that this voltage must not drop below 3.0 V. Moreover, the value of R2 must be about twice that of R1, and the minimum drop across the entire circuit must not be lower than Ud + 3 V.
LM317 Voltage Regulator Applications:
The circuit is particularly suitable for use as a 5-V power supply. It is, however, important that the direct voltage at the input is not lower than 12 V. This means that the secondary voltage of the mains transformer must be 12 V instead of the usual 9 V.