Is that even normal?

[NienorGT]

Hello,

I'm currently working on my computer case so I put my PC parts into an old case and as I was plugging stuff, I noticed something weird that shouldn't happen...

My USB devices (like my USB sound card) was powered up (lights on) and I was still in the process of replugging stuff and the PSU was not even plugged to the wall yet.

I was like, WTF, why my USB is getting power, and even allowing me to charge my smartwatch, without even my PSU being plugged to the wall?

And then I remembered that my USB drawing monitor was connected to a USB wall plug using a Y cable...

Basically: the drawing monitor is connected to the PC with a USB-C> HDMI/USB cable. My PC is able to power the monitor with the USB cable without external help, but I still used the included Y cable to plug the monitor into a USB wallplug so I can use the monitor as a normal HDMI monitor when my PC is turned off (using a basic unpowered HDMI switch). 

 

But why the heck powering the usb monitor via an external power source is able to transmit power to my other USB ports on my motherboard even if the PC is turned off and unplugged?

Isn't that dangerous/problematic or it is a normal "feature" of a motherboard? (my motherboard is a MSI B360M Motar)

 

Here how it is connected:

[GDRRiley]

it shouldn't work like that but USB is often so badly done weird stuff will happen

[Jurrunio]

It is possible that all USB ports of the system share the same power plane so they get powered up together, after all you're just charging but not trying to transmit data.

[Vishera]

3 minutes ago, Jurrunio said:

It is possible that all USB ports of the system share the same power plane so they get powered up together, after all you're just charging but not trying to transmit data.

Aren't USB devices connected in series through the bus?,but the question is if there are powerlines that go with the bus or is it only for data?

[Jurrunio]

4 minutes ago, Vishera said:

Aren't USB devices connected in series through the bus?,but the question is if there are powerlines that go with the bus or is it only for data?

data and power use different wires/contact points. Otherwise the USB device wont be getting any power if you give it "0" as signals constantly.

 

Of course OP is basically letting a single USB port handle the power draw of a ton of more stuff which strains it more than usual, I hope other components arent doing anywhere near their full load power draw since the system itself is still turned off and isnt giving or responding to commands.

[Vishera]

Just now, Jurrunio said:

data and power use different wires/contact points. Otherwise the USB device wont be getting any power if you give it "0" as signals constantly.

I know that,it seems i didn't use the right words.

What i meant to ask is if the powerlines of USB are in series (and go along the bus),or if the powerlines are in parallel.

I guess that in series managing and achieving up to spec voltage is a challenge,but parallel makes more sense to me.

 

And am talking about powerlines per port.

[NienorGT]

1 hour ago, Jurrunio said:

Of course OP is basically letting a single USB port handle the power draw of a ton of more stuff which strains it more than usual, I hope other components arent doing anywhere near their full load power draw since the system itself is still turned off and isnt giving or responding to commands.

Well, I don't think I overdraw power from my USB =/

 

 

The biggest power draw would be the monitor at 2000mA if I would directly plug it, but as I said, I use the Y cable that comes with the USB monitor and is normal to use to power the monitor.

The only thing: I use my own Samsung wall charger instead of the included *chinese* wall wart. 

See the user manual:

 

EDIT:

Guys, here's a video:

I plugged the monitor straight to the PC, powered it off, unplugged the PSU (sorry, I forgot to put additional light before filming, but trust me, it's unplugged) and look what happen when I plug the Y cable in:

 

Edited June 2, 2020 by NienorGT
Uploaded a video

[Jurrunio]

59 minutes ago, Vishera said:

I know that,it seems i didn't use the right words.

What i meant to ask is if the powerlines of USB are in series (and go along the bus),or if the powerlines are in parallel.

I guess that in series managing and achieving up to spec voltage is a challenge,but parallel makes more sense to me.

 

And am talking about powerlines per port.

Power lines are in parallel. Otherwise as you said, voltage management will be stupid difficult (with a ton of regulators everywhere).