Variable Power Supply - 3A


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).

3A linear regulator circuit
3A variable power supply schematic

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.


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