Magsafe Sleep Mode

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Magsafe has a "sleep" mode where it sends out 3V or 6V (Magsafe2/Magsafe1) when disconnected. It is unrelated to 1-wire. It measures the current going out and when you connect it to a Macbook, it will detect the small current consumption of the small resistor between PPDCIN_G3H and ground. (22k, 68k…, R7010 on 820-3437) If it detects a current consumption within the correct range, it'll send out the 16.5V (or 14.8V/18.5V/20V depending on Magsafe power and version). The Macbook will enable PP3V42_G3H circuit only when voltage is above a threshold, due to the zener diode (D7012 on 820-3437) + MOSFET (Q7010 on 820-3437). And charger IC (U7100 on 820-3437) will turn on DC-in MOSFET (Q7180 on 820-3437) when it's above a certain threshold as well (on the ACIN pin n°3). The knockoff Magsafe (or a PSU) don't do that and that's why you can get sparks with them.

When a genuine Magsafe doesn't send out the full voltage but the Macbook works properly with a knockoff or a PSU, it is related to those circuits Either the resistor between PPDCIN_G3H and ground, the threshold circuit for PP3V42_G3H, or the DC-in MOSFETs and their gate control (resistors and traces). Most common is a shorted DC-in MOSFET.

PPDCIN G3H resistor to ground and PP3V42 threshold on 820-3437
Charger IC Threshold on 820-3437
File:DC-in MOFET and gates on 820-3437