Adjustable Regulated Power Supply

    Variable voltage regulated power supply circuit

    Circuit Description

    This adjustable regulated power supply circuit provides a variable DC output voltage from 1.25V to 30V at currents up to 1A. Built around the versatile LM317 adjustable voltage regulator, it's perfect for electronics lab use, testing circuits, and powering projects that require different supply voltages.

    How It Works

    The circuit operates in several stages:

    1. Transformer: Steps down the mains AC voltage (120V/240V) to a lower AC voltage (typically 24V).
    2. Rectification: A bridge rectifier converts AC to pulsating DC.
    3. Filtering: Large electrolytic capacitors smooth the pulsating DC to a relatively steady voltage.
    4. Regulation: The LM317 regulator provides a stable, adjustable output voltage. The output voltage is determined by the voltage divider formed by the fixed resistor (R1) and the variable resistor (potentiometer).

    The output voltage is given by the formula: Vout = 1.25 × (1 + R2/R1) where R2 is the total resistance of the potentiometer circuit.

    Circuit Schematic

    ACInTransformerBridgeRectifierC1C22200μF2200μFLM317RegulatorINADJOUTR1240Ω10kΩC310μFC40.1μF+OUT-OUT

    Components List

    • 1 × Transformer (230V/120V AC primary, 24V AC secondary, 1-2A rating)
    • 1 × Bridge rectifier (2A or higher)
    • 2 × 2200μF 50V electrolytic capacitors (C1, C2)
    • 1 × 10μF 50V electrolytic capacitor (C3)
    • 1 × 0.1μF ceramic capacitor (C4)
    • 1 × LM317T voltage regulator
    • 1 × 240Ω resistor (R1)
    • 1 × 10kΩ potentiometer
    • 1 × Heat sink for LM317
    • 1 × Suitable enclosure
    • 2 × Output binding posts or banana jacks
    • 1 × Power switch
    • 1 × Fuse holder with appropriate fuse (0.5-1A)
    • 1 × Power cord
    • 1 × PCB or prototyping board
    • Optional: Digital voltmeter module

    Installation Instructions

    1. Prepare a suitable enclosure for the power supply, ensuring adequate ventilation.
    2. Mount the transformer securely in the enclosure, ensuring it's properly isolated from the case.
    3. Install the power switch and fuse holder in the enclosure, connecting them to the primary side of the transformer.
    4. Wire the power cord to the fuse holder and power switch.
    5. Mount the bridge rectifier on a heat sink if needed (for currents above 1A).
    6. Connect the secondary winding of the transformer to the input of the bridge rectifier.
    7. Connect the output of the bridge rectifier to the filter capacitors (C1, C2), observing correct polarity for the electrolytic capacitors.
    8. Mount the LM317T on a heat sink using thermal compound and an insulating washer.
    9. Connect the positive rail from the filter capacitors to the input pin of the LM317T.
    10. Connect the negative rail to ground.
    11. Connect the output pin of the LM317T to the output positive binding post and to C3.
    12. Connect the 240Ω resistor (R1) between the output and adjust pins of the LM317T.
    13. Connect one end of the potentiometer to the adjust pin of the LM317T and the other end to ground.
    14. Connect C4 across the output binding posts.
    15. Double-check all connections carefully before powering up.
    16. Test the power supply with a multimeter, adjusting the potentiometer to verify the output voltage range.

    Usage Notes

    • Voltage adjustment: Turn the potentiometer clockwise to increase the output voltage, counterclockwise to decrease it.
    • Current rating: The basic circuit provides up to 1A output current. For higher currents, add an external pass transistor.
    • Heat considerations: The LM317 will get hot, especially at higher current loads or when there is a large difference between input and output voltage. Ensure adequate heat sinking.
    • Protection: Consider adding a diode from output to input to protect the regulator during sudden input voltage drops.
    • Calibration: Use a multimeter to mark voltage points on the enclosure for quick reference.

    Enhancement Options

    • Digital display: Add a digital voltmeter module to show the exact output voltage.
    • Current limiting: Add a current limiting circuit using another LM317 configured as a constant current source.
    • Short circuit protection: Add components for automatic shutdown in case of a short circuit.
    • Dual output: Create a positive and negative supply by adding a second LM337-based circuit.

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