A laptop docking station has a dead output rail. The technician looks for the single failed component on that rail, as if the switching supply were a resistive divider with one bad part. What does the section teach?

Select one answer.

• Nothing — a switching supply's output rail is set by a divider like any other, so a dead rail means one failed part in that divider chain, and finding and replacing that single component is the complete and correct repair. A rail is a divider, so the output voltage is fixed by a resistive ratio and a dead rail is a broken link in that ratio, making the search for one failed resistor the right and sufficient approach, because the supply passes voltage through a chain and a dead output means one element of the chain has simply opened somewhere.
• A switching supply is an energy-transfer engine, not a resistive divider — a switch banks energy in the inductor and releases it to the output, regulated by a control loop — so a dead rail is reasoned about as a system of switch, inductor, rectifier, and controller, asking which stage of the cycle has stopped, not hunted as one bad part in a divider that does not exist.
• Only the output capacitor — a dead rail always means a shorted output capacitor, so replacing that one capacitor restores the rail without considering the rest of the supply.
• The input — a dead output rail means the input is missing, so confirming input voltage is the entire diagnosis and the supply itself needs no examination.