Here's another linear power supply circuit. Although it is a mini power supply unit, it is able to deliver up to 3 A at an output voltage of 1.25 to 25 V. It is based on LM350, which is a popular linear regulator. The regulator has on-chip overload protection that comes into operation when the dissipation in the device reaches 30 W. The output voltage, Uo, is calculated from
Uo = [1.25 (1 + P1/R1)] V
where P1 and R1 are in ohms (the value of P1 is measured between the wiper and the junction with R1, i.e., 0...2.5 KΩ).
C1 is a conventional filter capacitor, while C2 and C3 improve the regulation. Protection diodes D1 and D2 ensure that at switch-off the potential at the output of IC1, is more positive than that at its input.
It is essential that IC1, is mounted on a large heat sink rated at about 1 K/W .
When only low output voltages are needed, it makes sense to use a mains transformer with a lower secondary voltage (for Uo = 5 V, the secondary voltage should be 9 V). When a 24 V secondary is used, and the required output voltage is 1.25 V, the maximum output current is 1 A, otherwise the maximum dissipation of the LM350 is exceeded, and the internal protection will switch off the regulator. When the secondary voltage is 9 V, and Uo = 1.25 V, the maximum load current amounts to 2.5 A.