I wanted to reverse this, using the barrel jack side to get 12v/3a into usb-c and I got schooled by kind but firm wiser heads: these cables are highly specialised and directional.

So to anyone looking to leverage ATX power supply like me for USB-C specific voltage and current be warned: you cannot reliably invert a cable aiming to get USB-C into a barrel jack. It's not reversible.

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baobun 9 hours ago | parent [-]

Sounds like what you are looking for is a "PD trigger". They sell for cents online and you can DIY a cable from that.

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ggm 9 hours ago | parent [-]

Yes, that's where I was pointed. A simple example would be welcome because Ali express has too many choices. I backed off and used board level 12v pins but for my hardware this bypasses the current limiting circuits the USB-C input sits in front of.

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baobun 8 hours ago | parent [-]

Just make sure you get specifically for 12v (variable voltage sounds overkill if you know you just need 12v). There's like hundreds of vendors selling the same stuff under different names. There's not that much to it.

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ggm 8 hours ago | parent [-]

The problem is the directionality. It's trivially easy to be led to usb-c TO 12v where what's needed is USB-C output (powered device not power source) and a question I couldn't get answered is: what is the specific words which specify this directionality -because "to" doesn't always do it.

Virtually all "pd trigger" matches are to take usb-c in from a power brick, and wire to a dc input. They're all the complete reverse of what I want. I begin to think the word "trigger" here means "trigger a PSU to send 12v" and I want to satisfy a client asking for 12v to be sent.

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baobun 8 hours ago | parent [-]

oh. Still hard to figure out exactly what your goal and problem are between "using the barrel jack side" and ATX supplies... If you already have 12v3a and that's what you want can't you just wire a normal 12v3a charger straight to the usb...? maybe this helps?

https://www.pretzellogix.net/2016/11/14/a-review-of-four-swi...

	▲	ggm 8 hours ago \| parent [-]
		I have an ATX supply with 12v dc output leads. I have a usb-c powered device which can only consume 12v dc coming in on usb-c and needs to be signalled, or attached to a power source which matches the signals. I do not need a buck conversion. I need the supply side signalling suitable to act as a 12v usb-c output, to connect to the devices usb-c input. A pd trigger appears to be a chipset which acts as a device needing power and accepts voltage in on usb-c to put out, offering the client side signals to trigger 12v to be emitted. I want the other direction: the chipset and logic which would be inside a 12v power source emitting usb-c. It doesn't help that 12v was the voltage emitted from the final specification, it's optional.