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It still amazes me that it's almost the end of 2018 and still no one seems to have been able to, to my knowledge, find or create such a device. I did however gather some more information, so here I go.

To clarify what the adapter should be able to do:

The USB-C male port is for the host device, aka a phone.

The USB-C-PD female port is for power. So it could be connected to a wall adapter with a C-C cable and provide power to the host device (and maybe also for the second device).

The USB-C data female port is for USB devices that can be connected to the host device, aka a flashdrive, mouse or 3.5mm dongle (with DAC).

For reference, I own a USB-C hub that does all these things, however it's very large (especially for a phone) since besides these ports it also packs in an extra USB-C port and two USB-A 3.1 ports. I'm talking about the Aukey CB-C48 https://www.aukey.life/products/cbc48-usb-c-hub-2-port-usb-3-0-2-port-usb-c-1-port-usb-c-with-pd. This device existing, shows that the idea of a "splitter" is actually possible, just never done before (to my knowledge).

The size I'm looking for is around this size: this size (Don't be fooled by its appearance, the adapter in the picture only wires the lines regarding power one way, while it wires the lines for Analog-Audio-over-USB-C the other way, which is great, if you have a phone with a DAC and only want to use it for headphones but not for anything else.)

Another option would be this form factor: https://hackaday.io/project/26263-pd-buddy-wye. (This one works the other way around, allowing your phone to fast-charge over PD from a charger and connect to a PC via USB. It doesn't allow the phone to be the host.)

Finally I found another adapter of which I do not know whether it works: https://www.amazon.com/gp/product/B07KSZMG5J?pf_rd_p=1581d9f4-062f-453c-b69e-0f3e00ba2652&pf_rd_r=SMDQYBNA8X75H9EGVK16. These types of adapters have a DAC inside them,thus enabling phones without a DAC to connect to USB-C headphones (or dongles) without DAC. At least that's what I've gathered from the descriptions. However, I don't know if the port can also be used for other USB peripherals. The seller did tell me it didn't work with mice/flashdrives, but it did work with the 3.5mm dongle from Google, which is odd, because it does have a DAC inside and thus is a completely normal USB peripheral, just like a mouse for instance. I did order one to test what's true and what isn't about these. When mine arrives, I'll update the post.

Edit 1: Another item I found was the Cusby, a failed indiegogo campain: https://www.indiegogo.com/projects/cusby-first-usb-c-modular-expandable-adapter and http://www.cusby.com/. In particular their "power port" module does exactly what I'm looking for. Unfortunately they've been silent for 3 years now, so yeah...

So if anyone finds anything, please let me know and if anyone has the capacity to design and build such an adapter also let me know. I really hope it can be realized!

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These small splitters should be avoided at all costs. As Benson Leung's post explains

There are no active components inside. It bridges many pins (power, data, gnd, cc) among the three USB-C connectors). What this means is that your phone or laptop can negotiated much higher voltage than vSafe5V, and that will just magically show up on the other USB-C receptacle.

If the other USB-C receptacle has a headphone or a USB-C thumbdrive or a USB-C Yubikey that is only 5V tolerant, it will be destroyed.

A later post by someone much less famous but nonetheless correct:

A proper adapter would need a buck converter for 5V, a power switch on the downstream data port, and two USB PD interfaces with good firmware that implements proper derating. Essentially a full blown charge-through hub.

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  • Correct. However, the adapter I used to answer my own question is a full blown hub I believe. It's just small, but way bigger than the adapters I found in my post (which indeed should be avoided as they just rewire pins instead of interfacing with them). – Jolan Rensen May 27 at 9:48
  • Could you edit your answer noting you tested with a charger and it worked? – chx May 27 at 17:28
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Well let me be the one to answer my own question. I found a working adapter! It allows to pd charge my phone and have a USB c port left for other peripherals.

This adapter https://nl.aliexpress.com/item/2-in-1-Snelle-Lading-Headset-Adapter-Type-C-USB-C-3-5mm-Digitale-Audio-Kabel/32961645430.html?spm=a2g0s.9042311.0.0.4b6a4c4d82wJiw [1] from AliExpress is advertised as a USB C splitter just like the ones on Amazon, but on the contrary it never mentions having a DAC like the ones on Amazon. What it does mention is working with the Pixel phones and I can confirm that! Not only does it work for the Google 3.5mm dongle, but it also works for a mouse and USB flash drive I tried. I'm sure there are some on Amazon working the same way (as they often re-sell Chinese stock) but I haven't tried any of those as they were more expensive.

I also haven't tried it on other devices like a Chromebook but I assume it works the same for all USB 3.1 devices (like the pixel 2xl). Whether USB alternative mode is supported for displays I cannot test currently.

Edit: I just tried the adapter with a Oneplus 6T (USB 2.0 with analog audio over USB) and it only works one port at a time. And analog audio doesn't work. This is quite expected because of the broken and old spec Oneplus uses and the lack of audio over usb c wires in the adapter.

Edit: part number is 4NB900424-B if the link stops working

[1]: http://€ 8,92 27%OFF | 2 in 1 Fast Charge Headset Adapter Type-C USB-C 3.5mm Digital Audio Cable Converter For iPad Pro Google HTC Huawei Essential Mac https://nl.aliexpress.com/item/2-in-1-Snelle-Lading-Headset-Adapter-Type-C-USB-C-3-5mm-Digitale-Audio-Kabel/32961645430.html?spm=a2g0s.9042311.0.0.4b6a4c4d82wJiw

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