USB-C cable polarity

[JulianLights]

Hi everyone

I’m looking to connect my Raspberry Pi to one of my power supplies instead of using the ac plug. When I stripped the USB-C cable that came supplied with the Pi it had an inner sheathed cable and a bare wire wrapped around the inner sheathed cable (see pictures). Is there a standard polarity for these types of cables or a way for me to test it through the USB-C connection?

 

[darylc]

can you measure the polarity from other end thats still connected to the plug pack?

 

[JulianLights]

Thanks KateKate and Daryl. Not sure how to test from the USB-C end as I have cut it from the plug back end. Might be easier to buy another cable with clearly identified negative and positive cables

 

[TerryK]

If you can do a continuity check, there are two ways. The first is to check from the shield to either (or both) outer connector bars inside the USB-C connector. Because the USB-C is reversable either end and top or bottom will work. USB-C connector bar IDs are in the graphic below. As Kate mentioned the shield is almost certainly the Ground or V-. It would be very unusual if it is not.
A second method is to plug the cable into a RPi and do a continuity check from the cable end to the correct RPi GPIO pin. Again, GPIO IDs are in the graphic below for RPi models 3 and 4.

I would test using the shield as some DVMs put out voltages some of the RPi components are not going to appreciate.

 

[David_AVD]

I take it that he's wanting to plug the USB C into the pi and connect the bare ends to a PSU with screw terminals.

You could plug the USB C into the pi, then measure for continuity between the cable shield and the metal part of one of the USB sockets.

You should also be to measure continuity between the cable inner wire and one of the pins marked 5V PWR in the diagram above.

 

[JulianLights]

Thanks for the advice everyone. Ended up testing the continuity using the diagram from TerryK and you were right, the shield was ground. Hooked the Pi up to the PSU this morning and all is working fine. Thanks again!

 

[jamieh]

Could you plug the end with the USB-C connector in a USB-C power adapter and then measure the voltage on the cables you have exposed?

 

[TerryK]

While it would seem logical that one could, I am going to say no. But it takes a bit of explaining so bear with me.

First, not all USB-C cables are the same and thus the reason I say no. The USB Type C connector is more of a form factor than a standard. There are the typical data and power cables as well as power only. USB 3.1 and 3.2 both use the type C connector. And there is a USB 4 on the drawing board which appears will use the type C too. The USB-C connector arrived after USB 3.0 but there are USB 3.0 Type C cables available as well.
From the posted graphics, the cable is a power only cable so the method you ask should work, a 3rd method although powered USB-C ports are not all that common. Most USB powered ports are of a type A (the typical rectangular connector). I also did not mention a 4th method which would have been to plug in the cable to the RPi, apply power to the RPi using the GPIO connector, and then measure the voltage at the cut/stripped cable end.

There are always pros/cons to methods and these two put power on exposed wires. Granted 5 volts can hardly be considered lethal unless going subdermal or having small 'spitballs' of copper thrown at you. Primary hazard would be possible damage to the RPi.

 

[Dreamin]

Personally I would just solder the power wires directly to the bottom of the Pi. That's what I did, saves stuffing around. Plenty of info on the web about where and how for different Pis. They have points designed for hardwiring.