Eurorack power runs on three rails (+12V, -12V, +5V) distributed from a PSU through busboards to modules

Eurorack power is split into three sections — +12V, -12V, and +5V — commonly called ‘rails.’ A power supply creates all three types of power and distributes them to your modules via busboards. Each module generally ships with a power cable: you plug it into the busboard, screw the module in, and turn it on. Crucially, a power supply can only output a finite amount of each type of power, measured in milliamps (mA), and it must be able to output more than your modules draw on each of the three rails independently. The three rails are separate supplies — headroom on one rail does not cover a shortfall on another, so each is budgeted on its own.

Examples

A PSU rated +12V/1500mA, -12V/300mA, +5V/500mA can power modules that collectively draw up to those milliamp values on each rail — the budgets are per-rail and not interchangeable.

Assessment

Name the three Eurorack power rails and the unit their capacity is measured in. Explain why a PSU with plenty of spare +12V current can still fail to power a system.