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After connecting a PD charger, I see 15V on the power lines. That means, the device asked for 15V and the charger switched from 5V to 15V. Can I just put two wires into the USB-C port on the device, giving it 15V without any warnings, or it must somehow prepare it's inner power system first?

If not, how can I tell the device it's OK to take 3A from the 5V charger (instead of non-PD 1A)? I've heard something about connecting resistors to proper pins of the USB-C.

I want to use either 15V 1A charger or 5V 3A charger, but both of them are none-PD. From running the device with a PD-compatible charger, I know it can use both of them.

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    \$\begingroup\$ I believe you need to negotiate for anything other than USB 2.0 voltages and current compliances, which are 5 V and at most 500 mA if memory serves. But I'm not positive (haven't read the spec yet) about what USB-C supports when there's no negotiation available. So that's only my guess and nothing more. \$\endgroup\$
    – jonk
    Sep 23, 2020 at 22:26
  • \$\begingroup\$ I can answer a portion of your question regarding the sense resistors. A legacy USB 2 through 3.1 device can connect to a USB-C connector using a 5.1K pull-down resistor on each of the D+ and D- data lines. These 5.1K pulldown resistors act to negotiate 5V at 2A (e.g 10W) but to my knowledge, the sense resistor method is backward compatible and will not get you above 5V, 2A, 10W. As for negotiating with USB-C, look for chips or boards known as USB-C "trigger". There are cute and plentiful little adapters online. These adapters have the trigger chips you need to get 15W. \$\endgroup\$ Sep 23, 2020 at 23:01

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Can I just put two wires into the USB-C port on the device, giving it 15V without any warnings

Nope. That is a good way to kill devices.

Voltages above 5V require active USB PD negotiation. And yes, you can expect that the receiving device needs to switch the proper power path on.

how can I tell the device it's OK to take 3A from the 5V charger

Read USB-C specs: There is a pullup resistor, and one value is reserved for 5V/3A chargers. This allows drawing up to 3A by just measuring a voltage instead of the more complex USB PD protocol.

Warning: There are USB-A-to-C cables with this resistor value in the wild. These are not unlikely to cause damage if your device tries to pull 3A out of a PC or 1A charger.

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  • \$\begingroup\$ So, >5V device inputs are like switching PSUs, not like multi-voltage "traveller" PSUs. That's all I needed to know. And thanks for the warning, those cables should be soldered to a 3A charger instead of type A connector. \$\endgroup\$
    – NickDoom
    Sep 28, 2020 at 11:13
  • \$\begingroup\$ UPD: I've found a device which operates like multi-voltage (non-switching) PSU. It's Rock Pi X. wiki.radxa.com/RockpiX/getting_started As I can see, the ability to operate in such non-std. conditions has been explicitly mentioned, and I should not expect it from any device. \$\endgroup\$
    – NickDoom
    Oct 8, 2020 at 19:20

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