SDG #263 How to modify the output voltage of a SMPSU power supply

Off the shelf switch mode supplies rarely land on exactly the rail you need, and buying a bespoke unit is often expensive or slow. This build walks through the practical middle ground: understanding the feedback network inside a typical isolated SMPSU and changing the divider so the supply regulates at a different voltage.

The starting point is the adjustment trimmer found on quality bricks such as the Traco Power 24V units, which typically only offer a few percent of trim around the nominal figure. That is fine for compensating cable drop, but useless if you want a substantially different rail. A teardown of a cheaper supply shows why: the secondary side reference, usually a TL431 shunt regulator driving an optocoupler back to the primary controller, sets the output through a simple resistor divider tied to the 2.5V reference pin. Move that divider and the whole loop settles somewhere else.

The maths is straightforward once the divider is identified, but the video is useful because it flags the things that catch people out. Output capacitor voltage ratings, secondary rectifier reverse voltage, transformer turns ratio headroom and the controller’s duty cycle limits all constrain how far you can push a supply before it becomes unreliable or unsafe. Modest changes are realistic; doubling the output generally is not.

After fitting the recalculated resistors, the modified supply is loaded and measured to confirm regulation holds. For anyone repurposing surplus bricks or tuning a rail for a specific project, this is a repeatable technique rather than a one off hack, provided you respect the component ratings you are working around.