[rektide]

That's not really how electricity works. Wall power is a stable supply. 120v going up & down in a sine wave.

If someone is drawing 120w - 1 amp - your only control as a power source is to try to lower the voltage (voltage potential). Problem one: that will go out of spec with what devices expect, if you try to provide 80 vac instead. Problem two: switch mode power supplies that can handle sagged voltages will just try to draw more amps to get the same net power.

There's really not a good way to "cap out" power. The device just sees voltage potential (volts) and doesn't know to go lower. Blowing a fuse is the only real indicator we have.

[spookthesunset]

To be ultra pedantic, 120VAC refers to the root mean square (RMS) voltage of the sinusoidal waveform present in household electrical systems. The RMS value represents the effective voltage that produces the same amount of power as a DC voltage of the same value. In the case of a 120VAC circuit, the peak voltage, which represents the maximum value of the voltage waveform, is approximately 169.7 volts. This peak voltage is achieved when the voltage swings between +169.7V and -169.7V in a full sine wave pattern.

[ilyt]

There is, just not for "AC wall voltage". It could simply drop the voltage and expect the other device to say start limiting current when it goes 5% or more below nominal voltage. Akin to how MPPT works on solar chargers

It "just" requires both sides to support that. Which won't happen as we already have USB-C if you want to negotiate the power usage.

[rektide]

We could invent other signaling strategies too. Maybe we lower the voltage by 5% as a "low" signal. Now we can yell at the device in binary, and more explicitly spell out the conditions.

Maybe the source sends S-O-S in Morse code. Maybe it sends a requested max power.

We can invent signalling strategies to communicate yes. EV chargers use HomePlug Green for example to commitcate. This could be consumerized & put into all devices, used to perform negotiations. https://en.m.wikipedia.org/wiki/HomePlug

[ilyt]

If we wanted to go entirely analog just simple CV/CC on the power source side (meaning "keep constant voltage up till max amps, then start dropping voltage) + MPPT-like controller on the sink is enough. That's easy enough to be done on off-the shelf chips.

>We can invent signalling strategies to communicate yes. EV chargers use HomePlug Green for example to commitcate. This could be consumerized & put into all devices, used to perform negotiations. https://en.m.wikipedia.org/wiki/HomePlug

I'm kinda surprised that's relatively rare approach, in-band communication like that saves 2 wires so would technically allow USB-C to get extra 20% power boost over same connector. I think I saw it used in some solar stuff to coordinated various devices of same manufacturer.