A bench power supply is the first thing worth building properly. Variable voltage, current limiting, clean regulation — a good one saves every project after it.

The spec

Target output: 1.25V to 25V adjustable, up to 1.5A continuous. Source: a salvaged 24V transformer from a dead printer. Regulation via LM317T in TO-220 package mounted to a small aluminium heatsink.

Schematic overview

The LM317 is configured in the standard adjustable output topology:

  • Input from rectified/filtered transformer secondary (~28V DC under load)
  • Output voltage set by resistor divider: R1 = 240Ω fixed, R2 = 5kΩ pot
  • 0.1µF ceramic on input, 1µF electrolytic on output for stability
  • 1N4007 protection diodes across input and output per the datasheet recommendation

Nothing exotic. The datasheet application circuit is good enough — no need to reinvent it.

What actually mattered during the build

Heatsink sizing. At 28V input and 1.25V output with 1.5A load, the LM317 dissipates about 40W. A TO-220 without a heatsink handles maybe 2W before thermal shutdown. The aluminium plate salvaged from the same printer (approximately 80 × 50 × 3mm) brought junction temperature down to a safe range. Thermal paste between package and heatsink matters — don’t skip it.

Capacitor placement. The output cap needs to be close to the adj and output pins, not at the load terminals. Learnt this the hard way — oscillation on the output rail at around 150kHz when the cap was at the far end of the wiring.

Current limiting. Not implemented in this first version. A proper bench supply needs current limiting (fold-back or constant-current). This one relies on the LM317’s internal thermal protection. Adequate for light use, not for debugging shorted circuits.

Measured results

Pot position Measured output Ripple (no load)
Minimum 1.27V < 5mV
Mid 12.4V < 8mV
Maximum 24.8V < 12mV

Regulation under 1A load: < 50mV drop across the range. Acceptable for most bench work.

What comes next

Current limiting circuit — either a sense resistor + op-amp comparator approach, or a dedicated IC like the LM723. Will cover that in a follow-up build log.