Building a Dual-Supply Bench Regulator

A symmetric ±15V power supply is essential for op-amp circuit prototyping. Here's the design I settled on after three revisions.

The transformer is a 30VA toroidal with a center-tapped secondary: 0V, +21V, and -21V after rectification and filtering. Two independent LM337/LM317 regulator pairs with adjustment potentiometers give ±1.25 to ±18V output. The output capacitors are low-ESR polymer types for stability with the adjustable regulator feedback loop.

Key design decisions: thermal coupling the LM317 and LM337 to a common heatsink ensures both rails track temperature equally — important for op-amp circuits where supply asymmetry affects headroom. Current limiting uses sense resistors and a 2N3904 transistor that shunts the adjustment pin when the sense voltage exceeds 0.6V.