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I'm not sure about the standard - but in my experience, if a PD trigger IC requests 12V, that will be supplied, no matter the current draw. I have used this to power several low power devices, for example a 12V computer fan, and I have also verified this with no load at all. There is also PD PPS, which could be used to deliver 14V or any other voltage.
Edit: I normally use this kind of trigger board, they are available everywhere online.
12V support is optional in USB PD. If the power supply doesn't support it, you will get 9V instead. PPS support is even less common.
That is not correct, you won't get a random lower voltage. You are correct that thw PD supply needs to support the voltage but PD always starts with 5V for backwards comparability then digitally its negotiates the PD voltage it wants, if it asks for 12v and its not available it with just stay at 5V. It won't spit out the closest value unless the device asks for it. Its a very strict and wonderful standard.
It must be the PD trigger chip that does that then. With every PD trigger module I've used, I get a lower voltage if the requested one is not supported. It doesn't stay at 5V unless that's all the power supply supports.
So, if I added this board to my device, I must be sure to only charge it with a power supply that supports PPS in the 12v range - is that correct?
There are plenty of chargers that support a fixed 12V output without needing PPS. Just check the specs before buying one.
If you use a CH224K USB PD trigger chip, it has a power good pin. It will go low if the power supply is providing the requested voltage. You can use it with a MOSFET and driver circuit to switch the output on only when it gets the correct voltage.
Thank you, this is a very elegant solution.
This might be useful to you. https://www.adafruit.com/product/5450
That is extremely useful, thank you.
Do I understand correctly that the PD unit is in the USB C plug i.e. can I change the barrel jack connection to the connection on my existing cable?
That's what I understand. They might have one that doesn't go to a barrel jack but I don't remember.
You can also buy 'trigger boards' and wire it uo yourself if you like. I replaced all my barrel jack devices with USB.