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I did a DIY a portable monitor from a salvaged laptop LCD screen using a controller board I ordered online. Managed to make it work with a power adapter and an HDMI cable. I want to use it with my laptop but here's the thing. My laptop has a USB type C charging port with a 12V 3A brick. I'm trying to figure out a way to power both laptop and DIY monitor with just that one brick.

My initial plan was to look for a Y-cable with USB-C female split into USB-C male and USB-A male (or micro), and splice the DC jack to the USB-A for the DIY monitor, but I cannot find such a cable anywhere online or in stores. So I thought maybe I can use a short USB-C male to USB-C female extension cable and splice the DC jack to the wires in the extension cable.

Before I do any purchasing, I'd like to know would it be possible to accomplish this? Will there be enough current to power both devices? Is there another or better way to go about this issue?

Thanks!

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    \$\begingroup\$ USB type-C uses a power negotiation protocol to figure out what voltage to output and how much current can be drawn. Attempting to make a USB type-C Y-splitter cable is almost certainly going to end in tears when a charger inevitably sends 12V down to your 5V device. \$\endgroup\$
    – tangrs
    Commented Jul 2, 2018 at 14:59
  • \$\begingroup\$ How do we suppose to answer your question about "enough current" if you don't say how much power your devices consume? \$\endgroup\$
    – Ale..chenski
    Commented Jul 2, 2018 at 15:38

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There is no such power splitter on the market that I have seen, and if it did, would violate the USB spec and defeat the purpose of Type C PD spec.

But technically, you could do this on your own but may not be worth it. You could just tap the VBUS and GND lines of the power adapter and tap power from that.

You will not get any voltage unless you plug the adapter into the laptop. And because the laptop is the device in control, it could ask for 5V, 9V, or 12v (or even 20V if a 20V charger was plugged in), so you might not get the voltage you were expecting. Also, in the standard, the device knows the capabilities of the power adapter, but your additional load throws that off and the total load can overload your adapter.

USB Type-C has complicated power negotiation. If the laptop or other device is not plugged in, it will turn off its output for safety. The default voltage is 5 Volts. Higher voltages require special signalling between the device and power brick. And the laptop could tell the brick it no longer needed power and voltage would fall to 0.

Anyway, that should give you some idea on the ramifications of what you are asking for. Here's some technical info on it:
https://www.ti.com/lit/pdf/slyy109

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