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I think USB-PD is still fairly new (I don't know anyone who owns a device that has it) but reading wikipedia and info sites does not make it clear what happens when you use a cable that is out of spec in a USB-PD charging setup.

So if I have a host and device that are USB-PD compliant, and my device wants to use say a 12V profile, can I get away with using a normal USB cable? Or will it not charge or charge at a lower profile instead?

What about the edge case where I use the 5V profile at 2A. Technically that is out of spec also since USB BC1.2 is spec'd to 7.5W (1.5A), but many normal cables work fine with 2A in practice.

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USB-PD only uses two wires (GND and VBUS), but the standard has the whole section (4.4 in 'USB_PD_R2_0 V1.0 - 20140807.pdf') which describes how the cable detection is implemented. In short, regular USB-A to USB-B cables need to have the special mechanical mark to be considered for >5V/>1.5A, while micro-usb send special signal on "ID" pin, and determines capabilities from that.

In theory, if you do not use special PD cable, then the devices will silently limit the maximum power to pre-PD levels, and all will be safe (at most 5V, 1.5A current).

In practice, I bet there will be manufacturers which produce very cheap cables which still claim to support maximum power, so we might still read about cheap cables catching fire.

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  • \$\begingroup\$ So makers of devices like smartphones that charge with 5V have no incentive to make their device PD-compliant? As the older BC1.2 spec doesn't throttle the current if they want to draw more on a normal cable... \$\endgroup\$ – jiggunjer Apr 21 '15 at 7:48
  • \$\begingroup\$ Well, the charger is either BC compliant or USB-PD compliant. In the latter case, device has to implement negotiation, or it will be limited to 1.5A. So the question is 'which charger will be more common'. \$\endgroup\$ – theamk Apr 22 '15 at 1:30
  • \$\begingroup\$ Actually USB-PD is designed to co-exist with BC compliance. So the device can always make a charger default to BC if they don't feel like implementing PD; thus getting even more than 2A if they want... I guess this means only devices using higher voltages will bother to implement PD. \$\endgroup\$ – jiggunjer Apr 22 '15 at 14:47
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USB power delivery has cable detection mecahnisms at the host end of the cable and will refuse to raise the voltage if a power delivery cable is not detected.

However with the number of noncompliant cables and adaptors on the market it is certainly possible to assemble a combination of cables and adaptors that will fry stuff. For example one could plug a power delivery cable into a power delivery capable host, connect the power delivery cable to an A female to B female coupler, and then connect a "power combiner" cable with it's host plug connected to the coupler, it's device plug connected to a power delivery capable device and it's power only plug connected to the item you want to fry.

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USB-PD no more works on VBus. It uses USB Type-C connector signals for power negotiations.

Yes cable plays an important role in this ecosystem. Not all cables will support 100W. A power source will detect cable capabilities and ten advertises its power capabilities

A standalone USB Type-C cable can now support upto 15W. With these new specifications I believe BC spec may obsolete soon.

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