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The USB Power Delivery (USB-PD) spec allows devices to negotiate up to 20V - the key word, however being "negotiate", means this is no different than how older

by snuxoll 10y ago

The USB Power Delivery (USB-PD) spec allows devices to negotiate up to 20V - the key word, however being "negotiate", means this is no different than how older USB charging solutions work (start at 5V+100mA, negotiate to whatever the client device can handle and what the host will allow). I'm always amused by how uninformed many people are about USB and power supply, if devices can't negotiate something that works for both ends they just stop. It's a shitty situation to be in if your device fails to charge, but devices that follow the USB-IF rules will never draw more current than the power source can supply, and the power source will never supply more than the device has requested.

Waterluvian 10y ago
So if I plug a dumb device into it that only makes electrical contact, it won't make that negotiation. So, so long as Apple's MacBook USB charger is compliant, it won't destroy anything USB that I plug into it, so long as those devices are also compliant. Right?
- snuxoll 10y ago
  A "dumb" device that doesn't negotiate will get the minimum 5V@100mA that the USB spec allows, dedicated chargers often decide to not drop the power after the negotiation window is over but a proper host device (laptop, etc.) will drop all power if a device fails to negotiate. So yeah, if you plug something into the Macbook charger it will either not charge (dunno if it supports traditional USB power specs since it's a type-C only charger) or charge as any other charger already does.
  - deleted 10y ago
    [deleted]
  - kalleboo 10y ago
    Re: "dumb" devices, this is how it's supposed to work, yes, but I don't think I've ever seen a charger that won't happily dump 1A into a resistor wired over the power pins - including brand name (Apple) chargers and yes, the USB ports on a MacBook Pro. I've never seen a modern device supply only 100 mA. For devices that DO attempt to negotiate, yes in those cases usually you'll see it work according to spec. I haven't tried USB-C yet, it would be interesting to see if devices have gotten stricter.
nicky0 10y ago
Isn't there also a spec for passive power supply over USB, used by chargers. Something about combinations of resistances across the pins to signal current requirements.

by snuxoll 10y ago

Waterluvian 10y ago

So if I plug a dumb device into it that only makes electrical contact, it won't make that negotiation. So, so long as Apple's MacBook USB charger is compliant, it won't destroy anything USB that I plug into it, so long as those devices are also compliant. Right?

snuxoll 10y ago
A "dumb" device that doesn't negotiate will get the minimum 5V@100mA that the USB spec allows, dedicated chargers often decide to not drop the power after the negotiation window is over but a proper host device (laptop, etc.) will drop all power if a device fails to negotiate. So yeah, if you plug something into the Macbook charger it will either not charge (dunno if it supports traditional USB power specs since it's a type-C only charger) or charge as any other charger already does.
- deleted 10y ago
  [deleted]
- kalleboo 10y ago
  Re: "dumb" devices, this is how it's supposed to work, yes, but I don't think I've ever seen a charger that won't happily dump 1A into a resistor wired over the power pins - including brand name (Apple) chargers and yes, the USB ports on a MacBook Pro. I've never seen a modern device supply only 100 mA. For devices that DO attempt to negotiate, yes in those cases usually you'll see it work according to spec. I haven't tried USB-C yet, it would be interesting to see if devices have gotten stricter.

nicky0 10y ago

Isn't there also a spec for passive power supply over USB, used by chargers. Something about combinations of resistances across the pins to signal current requirements.