This is a compact adjustable power supply that delivers a stable voltage and currents up to 3 amperes. The circuit is very conventional and its voltage output can be varied from 1.25 volts to 25 volts.
The main module is the LM350 IC which integrates a voltage regulator and a power stage. It also has a built-in overload protection which activates at 30 watts of power dissipation. The voltage output is set by connecting the “adj” pin of the IC to the voltage divider made of R1 and P1.
The output voltage can be calculated using the following formula:
1.25V x (1 + P1/R1)
where the P1 value is between 0 and 2.5 KΩ. Capacitor C1 is a common ripple filter while capacitors C2 and C3 improves the regulation. The diodes D1 and D2 serves as protection for the regulator IC when the IC output is turned off. Resistor R1 is 120Ω. This ensures that the minimal load current for the IC (around 3.4mA) is high enough to maintain good performance.
One thing that is most important in building the electronic circuit: provide adequate heatsink for the LM350 IC. The power dissipation at the IC is very high, around 85 watts.
Consider that the heat resistance of a TO-3 package is 1.5oC/W and the maximum allowable temperature is 150oC. If a heatsink is used with a heat resistance of 1.5oC/W, when the total heat resistance is 4oC/W. At 30 watts dissipation and 25oC outside temperature, the resulting internal IC temperature is 145oC. Once this dissipation level is reached, the internal protection activates shutting down the IC.
One way to avoid high dissipation levels is to use a lower voltage transformers when needed output voltages are low. To put it simply: if you are using the circuit to supply voltages around 9 volts, do not use a 25 volts transformer but use a lower voltage one instead (e.g. 12 or 15 volts).