La-c781p Schematic -

Open page 42 of LA-C781P. Locate PQ101 (AON6414). Measure voltage at Source (AC adapter input) – 19.5V present. At Drain (+PWR_SRC) – 0V. Result: Open MOSFET.

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Here is where the LA-C781P saves your sanity. Open page 42 of LA-C781P

Notice the B+ line (Main Power Rail). In older laptops, B+ just went straight to the CPU buck converter. On the LA-C781P, trace the B+ line to PQ100 and PQ101 (the isolation MOSFETs). Here is where the LA-C781P saves your sanity

The schematic cleverly shows a "Back Drive Protection" circuit. If you inject 19V directly onto the main cap without consulting the schematic, you might blow a fuse you didn't know existed. But worse, the schematic highlights that PD102 (a TVS diode) is designed to short to ground if voltage spikes above 22V.

Interesting takeaway: If you accidentally plug in a 20V Dell charger that has a faulty center pin, the schematic tells you exactly which diode will sacrifice itself to save the PCH.