Basically I’m trying to de-clutter the cables to my charging station, and was hoping to use a 3-way cable for my phone, watch, and headphones.
My main concern is if one device is able to successfully negotiate QuickCharge or PD, would that send 9-20 volts to the other devices?
e.g. if I grabbed the wrong cord and hooked it into my laptop, the laptop requests 20v, would that PD negotiation succeed and also send 20v to my 5v devices on the other two leads?
I’ve only used these kinds of cables with USB-A and chargers that can only output 5V. Most of my chargers now are QC/PD so I’m curious if I should avoid those or take any particular precautions.
That’s assuming this cable forwards PD signals at all.
No. In QC it is the main voltage that is changed. No extra signal lines.
USB-PD changes thr main voltage in response to a handshake protocol on the C1 and C2 data lines, which this cable likely doesn’t connect for precisely this reason. Qualcomm QC still uses D+ and D- to negotiate the higher voltage, which this cable likely wouldn’t connect either.
That would actually be the safer scenario, as a failure in the PD handshake simply defaults to 5v which every USB device can accept. It would be more dangerous to only have the PD negotiation be on one of the three ports, because then you could potentially burn out devices on the other two by sending them too much voltage.
I guess the best case scenario would be to have the PD signal run on all three, then default to low voltage if any of them fail. But that’s likely not what they’re doing, because then things would only fast charge if all three ports are plugged into fast charge devices. Because an unplugged tail would likely register as a failed PD handshake.