Adjustable DC Bench Power Supply with Digital Voltage Display

Every electronics lab runs on its bench power supplies, yet most students only ever turn the knobs — they never learn why a linear supply is heavy, why it gets hot at low voltage and high current, or what the current-limit knob actually does to the control loop. Building one answers all of it. This project is the canonical student linear supply: a mains transformer and bridge rectifier feed unregulated DC to a series-pass power transistor driven by an op-amp error amplifier, which holds the output at the knob-set voltage against load changes. A second loop watches the output current and folds the voltage back when the set limit is reached — that is the CC/CV behavior, and it makes the supply inherently short-circuit proof. Digital panel meters show volts and amps, and the enclosure, binding posts and heatsink make it a genuinely usable bench instrument, not a breadboard demo.

1. The mains transformer steps 230 V AC down to the secondary voltage; a bridge rectifier and reservoir capacitor produce unregulated DC with ripple.
2. The series-pass transistor sits between the unregulated DC and the output; the op-amp error amplifier compares a divided-down sample of the output against the knob-set reference.
3. Any output deviation drives the pass transistor harder or softer, holding the set voltage against load and line changes — the voltage-regulation loop.
4. A second op-amp loop senses output current across a shunt resistor; when current reaches the knob-set limit, this loop takes over and pulls the voltage down to hold current constant.
5. The CC/CV LEDs show which loop is active: green in normal voltage regulation, red when the current limit is engaged.
6. The digital panel meters continuously display output voltage and current, powered from an auxiliary winding.
7. Under a short circuit, the current loop clamps the output at the set limit while the pass transistor dissipates the full power — which the heatsink is sized to survive.

Tech stack:

• 230 V to ~24 V mains transformer
• Bridge rectifier + reservoir capacitor
• Series-pass power transistor (2N3055 class) on heatsink
• Op-amp error amplifier (voltage loop)
• Op-amp current-limit loop with shunt sensing
• Digital voltmeter + ammeter panel modules
• Multi-turn potentiometers for V/I setting
• Metal enclosure, binding posts, mains fuse + switch

• [0–30 V adjustable output] Single-turn or multi-turn voltage knob sweeps the full range via the op-amp reference divider.
• [0–3 A adjustable current limit] Independent current knob sets the limit; exceeding it folds the supply into constant-current mode automatically.
• [CC/CV indicator LEDs] Green LED for constant-voltage, red for constant-current — the supply always tells you which loop is in control.
• [Digital panel meters] 3-digit voltmeter and ammeter modules show output voltage and current simultaneously.
• [Series-pass linear regulation] Power transistor with op-amp error amplifier — the classic topology, fully documented stage by stage.
• [Short-circuit proof] The current-limit loop holds a shorted output at the set current indefinitely; the design is thermally rated for it.
• [Thermal design] Heatsink sized by calculation for worst-case dissipation (low output voltage at full current), with the sizing math in the report.
• [Binding-post outputs] Standard 4 mm safety binding posts (red/black) plus a chassis ground terminal on a labeled front panel.

• Complete bench supply unit in a ventilated metal enclosure, wired and tested
• Transformer, rectifier, pass transistor on heatsink, control PCB, panel meters
• Heatsink thermal-sizing calculation sheet
• Stage-by-stage circuit explanation (rectifier, regulator, current limit)
• Wiring diagram, front-panel layout and enclosure drawings
• Project report PDF, PPT presentation and viva Q&A document

• Linear topology: it is heavy and inefficient by design — at 5 V / 3 A output the pass transistor dissipates roughly 75 W as heat, which is exactly why the thermal design is part of the project.
• No programmable/digital interface — voltage and current are set by manual knobs; remote control is future scope.
• Ripple and regulation figures are design targets; the report includes the measurement procedure (multimeter + the module's own meters) for the student to verify their own build.
• Single output only; dual/tracking supplies are a documented extension, not included.
• Mains wiring inside: the kit is supplied assembled and tested, with a safety note on never opening the enclosure while powered.