A laptop motherboard turns one input — the adapter or the battery — into dozens of supply voltages. Each feeds a different part of the board and appears at a different moment during start-up. Board-level repair is largely the work of finding which of those rails is missing, wrong or shorted. To do that you need a map: which rail comes from which, what each one powers, and when it should be present. This guide provides that map.
The rail hierarchy
Power flows downwards through a chain of converters. Each level depends on the one above it.
| Level | Typical names (vary by maker) | Comes from | Feeds |
|---|---|---|---|
| Adapter input | DCIN, VADP, ADP_IN, PPDCIN | Charger plug or USB-C | Input protection, charger IC |
| System rail | B+, VIN, PPBUS, VSYS | Charger IC (or battery) | All main regulators |
| Always-on low-power rail | 3VLP, +3VL, VL, PP3V42 | Small LDO or charger IC | EC, power button logic |
| Always-on main rails | 3VALW, 5VALW, +3V_S5, PP3V3_G3H | A dual-output controller | EC, PCH deep-sleep well, BIOS chip, USB charging |
| Suspend rails | +3VS3, +1.2V_DDR, S3 rails | Regulators enabled in sleep | Memory, wake circuits |
| Run rails | +3VS, +5VS, +1.8VS, +1.05V, S0 rails | Regulators or load switches | Most chips when the laptop is on |
| Processor and graphics rails | VCore, VCCGT, VCCSA, VCCIO, GPU core | Multi-phase and single-phase regulators | Processor, GPU |
The names change between board designers. Compal and LCFC boards often use +3VALW and +3VS; Quanta and Wistron boards use their own conventions; Apple boards name rails with PP prefixes and power-state suffixes. The schematic's power map page is the reference for each board. The glossary entries for power rail and b plus rail cover the general ideas.
Power states and when rails appear
Laptops follow standard power states. Which rails are present depends on the state:
| State | Meaning | Rails present |
|---|---|---|
| G3 | Mechanical off — no adapter, no battery | None |
| S5 | Soft off — adapter or battery connected, laptop off | Adapter/system rail, always-on rails |
| S3 | Sleep | Always-on, memory and suspend rails |
| S0 | Running | Everything, including processor rails |
The progression from S5 to S0 is the power sequence, controlled by the EC and the chipset with enable and power-good signals. Reading those pages in a schematic is explained in Power Sequence Pages in Laptop Schematics Explained, and the states are described under sleep states.
Adapter input: DCIN
The adapter's voltage enters through the DC jack or the USB-C power path, then passes through protection — a fuse, a TVS diode, input MOSFETs that block reverse current and disconnect bad adapters. A shorted input rail is the classic cause of a charger LED that goes off when plugged in. The adapter rail should be present as soon as a charger is connected, regardless of the power button.
The system rail: B+
The charger IC turns the adapter or battery into the system rail — the main supply for all other regulators. Depending on the design, the system rail follows the adapter voltage or sits close to the battery voltage. If B+ is missing, nothing else on the board can start. If B+ is shorted, the charger shuts down to protect itself.
Always-on rails
The always-on rails are the first rails made from B+. They power the EC and the parts of the chipset that must work while the laptop is off, so the power button can wake the system. On many boards a tiny low-power rail appears first, from an LDO or the charger IC, to power the EC; then the EC or a fixed enable starts the main 3 V and 5 V always-on rails. Their details are in Always-On Rails (3VALW, 5VALW): What They Do.
A laptop with no always-on rails is completely dead: no lights, no response to the power button.
Suspend and run rails
When the power button is pressed, the EC asks the chipset to wake, and the chipset raises sleep-state signals. Those signals enable regulators and load switches for the memory rails, the run rails and finally the processor. Run rails are often made from the always-on rails through load switches rather than separate regulators — the same voltage, switched on only when needed.
Processor and graphics rails
The processor needs several rails, the largest of which is VCore — a low voltage at very high current, made by a multi-phase regulator whose output voltage is set digitally by the processor itself. The graphics chip, if present, has its own core regulator. Because these rails carry the largest currents, they are where the biggest regulators, inductors and banks of capacitors sit. The processor core circuit is explained in The Laptop CPU VCore Circuit Explained.
Regulated rails and switched rails
Not every rail on the map has its own regulator. Two kinds appear side by side:
- Regulated rails are made by a converter — a buck regulator or an LDO — with its own inductor or pass transistor, feedback divider and enable pin. Their voltage is set on the board.
- Switched rails are copies of another rail, connected through a MOSFET or load switch when a control signal asks for them. A switched 3.3 V run rail is simply the always-on 3.3 V rail turned on for the running state.
The difference matters when a rail is missing. For a regulated rail, check the converter's input, enable, feedback and output. For a switched rail, check the source rail, the switch's control signal and the switch itself. The schematic's power map usually shows both kinds, often with the switch drawn as a single MOSFET between two net names. Knowing which kind you are looking at saves a lot of searching for a regulator that does not exist.
Using the map in diagnosis
- Identify the state the laptop should be in: off with charger connected (S5), or after pressing the power button (S0).
- Measure from the top down: adapter rail, system rail, always-on rails, then suspend and run rails, then processor rails.
- Stop at the first missing or wrong rail. That is where the problem lives — the regulator for that rail, its enable, its input, or a short on its output.
- Do not chase rails that should not be present yet. A missing VCore in S5 is normal.
Example: a board with only the adapter rail
A laptop shows a charger LED but no other sign of life. Measured from the top down: adapter rail present, system rail present, the always-on low-power rail present, but both main always-on rails at zero. The controller that makes them has its input and its enable, yet its outputs stay off. With power removed, the 3 V always-on rail reads close to zero ohms to ground — a short. The controller is refusing to start into it. Following the short leads to a ceramic capacitor near the EC; once it is replaced, both always-on rails come up and the power button works again.
FAQ
What is B+?
The main system rail, made by the charger IC from the adapter or battery, from which all other regulators draw power.
Why do rail names differ between laptops?
Each board designer uses its own naming convention. The schematic's power map shows the names for each board.
Which rails should be present with the laptop off?
The adapter rail, the system rail and the always-on rails. Run and processor rails appear only after the power button.
What is the difference between a suspend rail and a run rail?
Suspend rails stay on in sleep (for memory and wake functions); run rails are on only while the laptop is fully running.
Is VCore always present when the laptop is on?
It is present while the processor runs, but its voltage changes constantly with load, and it may drop very low in idle states.
Bottom line
Think of power as a tree: adapter, system rail, always-on rails, suspend and run rails, then processor rails. Measure from the top, stop at the first missing or wrong rail, and remember which rails belong to which power state. That single habit turns a dead laptop into a specific, findable fault.