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3A adjustable power supply

3A Adjustable Power Supply

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.

3 Ampere power supply circuit schematic

3A adjustable power supply

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.

Adjustable Power supply PCB

3a adjustable power supply pcb

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).

4 Comments

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  • Jim Keith

    My guess is that the LM350 is connected reverse –the schem and layout appear to be OK in this regard. Try wiring the input as full-wave center-tapped (center tap grounded) to reduce the voltage –then when working, rewire as a full-wave bridge.

    Also put a 470Ω resistor in series with the voltage adjust pot –it will prevent the voltage from turning below approx 5V, but will protect the pot under fault conditions when it is set for zero resistance.

    • Roland

      Thank you, Jim Keith. I will try that. This project has been frustrating for me, to say the least.

  • roland

    Hi. I have built the above circuit with all new parts exactly as shown three times on new boards. Each time the adj. potentiometer smoked and burned out when I turned the unit on. I am using the LM350 with a 24volt AC center tapped transformer with 12volts each side wired to put 24 volts into the rectifier. It shows 34volts after the 4700uF cap which I understand to be peak voltage. I have not found out much info on what to do. I have seen other sites with the same circuit and parts. I found one site that mentions adding a switch to use 12 or 24volts for lower or higher voltages, but prefer to run as shown in your diagram. Please help. Thank you!

  • KROKKENOSTER

    Guys hi! we must make sure that we do not overload the WATT RATING of the TO220 package AS ON A HEAT SINK IS ONLY FIFTEEN WATTS I had problems with LM 317 T in this package this ***********internally protected for power overload is misleading and leads to failure or shut down prematurely I would rather use LM338Ksteel TO3 package if you want a supply that can supply for argument 5 volt output and the input is 30 volts You have 3X 25 volts and that is 75 watts and that makes an ideal hotplate to keep your tea warm !! I am not aware that the TO 220 is increased in the meantime

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