Notes that do not fit on a calculator page. Mostly about the parasitic capacitance and leakage current that the equivalent circuit leaves out, and what they look like once the thing is built.
2026-09-10
If 20 V appeared the moment you plugged in, every 5 V-only device would die on first contact. USB PD avoids that by splitting the job in two: a part settled by resistors, and a part where nothing moves until the sink asks.
2026-09-10
Plenty of things that are not PoE devices plug into an RJ45 port. That is why a PSE does not just put 48 V out: it checks who is there with a few volts, asks how much power they need, and only then switches on.
2026-09-07
It gets you an indicator LED without a 3PDT, which is why it turns up so often in home-built pedals. The circuit is simple, but it creates one node at a very high impedance, and the quirks all follow from that.