How does USB A deliver power differently such that it works with devices which don’t support USB PD?
With USB C, is the power delivery on a pin these devices don’t have? Then the pin USB A would use to send power isn’t present with USB C (either physically or on purpose)?
USB-C allows a certain pin to be used for PD negotiation. USB-A doesn't have this pin. USB-A to USB-C cables have a fixed resistor pulling this pin to signal which power requirements they support.
Badly designed devices do not negotiate power requirements and just start drawing power, which doesn't work with proper USB-C cables (which expect to negotiate first) but obviously works with USB-A to USB-C cables (which always support the same power requirements)
Thanks for the explanation. This explains all the weird behaviour of devices I have which don’t charge (or charge poorly) over USB C. This was so confusing because I assumed that they should work fine given that they have USB C ports, but no… I’ll have to keep some USB A to C cords around for years still.
I just bought a CalDigit element hub and had some hesitation because I thought “how long will I really need the USB A ports?” Evidently they will be handy for quite a while.
> How does USB A deliver power differently such that it works with devices which don’t support USB PD?
USB A devices (and by extension, USB A to USB C cables) always delivers power. USB C devices only deliver power once they see a specific resistor to ground on a control pin. If the device is missing that resistor (actually a pair of resistors, one for each side of the cable), a USB C device on the end of a USB C to USB C cable won't provide power.
(Preemptive nitpicking: actually, USB C devices provide a bit of power in that control pin, which is how they detect that resistor to ground; but that's separate from the main power pins.)
To add more detail, USB-PD (power delivery) is a complex negotiated protocol. The charger is supposed to output some meager 5v power supply like a traditional USB A supply, and the device asks for some specific voltage, and the charger tells the device if it can do that, and then does it (up to 24v, iirc). PD Chargers are supposed to support any of the supported voltages, but many (eg. Apple MacBook Chargers) only support a subset of the voltages.
