What would happen if I plugged a power source into a USB-C port on a desktop?

[RenaKry]

This is one of those really silly questions, and I am unsure what category this falls in.

 

I'm sitting here looking at my case which has a USB-C port which is connected to my motherboard using an adapter. Whether the adapter has onboard logic or not is unknown to me, but considering its size, I would assume not, and I suspect it isn't quite up to spec. Next to it is my UPS with two USB-A ports that provide power, and connected to one of the ports is a USB-A to USB-C cable used for charging a phone.

 

I wonder what would happen if I plugged the USB-C end into my desktop computer while still having the Type-A side connected to a power source. Would it fry my motherboard's USB 3.0 controller or would nothing happen?

 

I also have a Type-C to Lightning cable, and I wasn't able to get a voltage reading with a multimeter, but licking the exposed pads does make my tongue curl slightly, like licking a 9V battery but much less intense. Perhaps I'm just imagining it. I did not attempt to probe the USB-A end of the cable because my probes are too thick and I do not want to short the cable.

[da na]

Don't. Best case, it trips your circuit breaker. Worst case, it fries your USB port, motherboard, or even PSU.

[the gamer that is bad]

21 minutes ago, Sesame said:

This is one of those really silly questions, and I am unsure what category this falls in.

 

I'm sitting here looking at my case which has a USB-C port which is connected to my motherboard using an adapter. Whether the adapter has onboard logic or not is unknown to me, but considering its size, I would assume not, and I suspect it isn't quite up to spec. Next to it is my UPS with two USB-A ports that provide power, and connected to one of the ports is a USB-A to USB-C cable used for charging a phone.

 

I wonder what would happen if I plugged the USB-C end into my desktop computer while still having the Type-A side connected to a power source. Would it fry my motherboard's USB 3.0 controller or would nothing happen?

 

I also have a Type-C to Lightning cable, and I wasn't able to get a voltage reading with a multimeter, but licking the exposed pads does make my tongue curl slightly, like licking a 9V battery but much less intense. Perhaps I'm just imagining it. I did not attempt to probe the USB-A end of the cable because my probes are too thick and I do not want to short the cable.

pop goes the computer

[Mling]

The USB charger is 5V DC. That 5V is the potential difference between the -ve/ground and +ve wires. I'm wondering what would the difference be between the +ve on your charger and the +ve in your USB port? Their grounds are isolated from each other. When I did first year physics, if my memory is correct, I asked what happens if I connect two logic gate outputs and my lab teacher said "Boom".

[dizmo]

I mean. I can see why you're curious. But at the same time, all I can think is that you know you shouldn't do it, so why does it matter what happens when you do?

[RenaKry]

The general consensus seems to be "it is a very bad idea" so I will refrain from experimenting with this question on my own personal computer. The temptation is still strong; perhaps I may sacrifice an old motherboard for an answer sometime in the future. As for why the question had to be asked in the first place, I was a dumb child (now I am a dumb adult). I had a male-to-male USB-A cable for some reason and I connected two computers together with it. If memory serves, nothing happened.

[Blue4130]

5 hours ago, Sesame said:

 

I also have a Type-C to Lightning cable, and I wasn't able to get a voltage reading with a multimeter, but licking the exposed pads does make my tongue curl slightly, like licking a 9V battery but much less intense.

 

5 hours ago, Sesame said:

, I was a dumb child (now I am a dumb adult). 

Hmmm, case proven I guess. 

[Kronoton]

6 hours ago, Mel0nMan said:

Best case, it trips your circuit breaker.

 

If something small like that trips the breaker either the USB-walwart or the computer involved were very sketchy to begin with.

 

5 hours ago, Caroline said:

 

IMO nothing happens, it's like wiring 2 chargers together

 

 

Which can work, but also can end bad... Bridging 2 power sources is always a bad idea even more if the ain't "smart" (or worse the wrong kind of "smart").

 

Now USB does have a protocol and if both sides adhere to it they should both see the other side and shutdown any power delivery pretty quick. Not that I would try it with HW that wasn't on it's way to the recycler anyways.

[akio123008]

6 hours ago, Caroline said:

IMO nothing happens, it's like wiring 2 chargers together

 

6 hours ago, Sesame said:

The general consensus seems to be "it is a very bad idea"

These seem to be the main two conclusions. Here's how it works;

 

In theory, when you wire two voltage sources of identical voltage together, nothing happens. I have 5V on wire 1, and 5V on wire 2, so connecting them in parallel just creates a 5V source with more current capacity, like hooking up a couple of batteries in parallel.

 

In practice, the voltages of the two sources isn't always exactly the same. For instance with batteries, say one battery is at 12.5V and the other one at 13V. If I connect them in parallel, a surge of current will flow because we've got 0.5V over an almost zero resistance load (just a wire). That surge won't last long though, because the batteries' voltages will rapidly change to equalise, so they'll both end up at 12.75V and current stops flowing.

 

The same would happen with USB ports; they won't have exactly the same voltage to start with, so one might be at 5.01 and the other at 4.92 or whatever, and so some current would flow, depending on how big the difference would be.

 

If that current is particularly large it could destroy one of the ports, but I suspect because USB is quite low power, and because the voltage regulation is quite good (aka both will be pretty damn close to 5V) that won't happen. So you'd probably be fine, but it's not an ideal solution by any means.

 

If you want to connect multiple sources to each other, a better way of doing it is by putting some resistors in series with the sources. Usually quite small resistors, because the voltage differences are small too (say <1 ohm). This way the current is limited and your sources are prevented from affecting each other too much.

[mariushm]

You'll probably trip the fuse for the group of usb connectors.

If that fails, you'll probably damage the traces going to the usb connectors, as the traces may act as fuses.

 

It's complicated because it's same voltage, and both power supplies will "push" 5v one way but the voltages are never the exact same value, and there's gonna be losses in the cable at higher currents, so the voltages will drift, and you get into the scenario explained by akio above me and that can cause fuses to temporarily trip (motherboards often use resettable fuses, which reset after a shutdown or after they cool down) or traces on motherboard could break.

[Tikitiuamba]

Don't mean to revive this old topic, but it seems the discussion never left the theoretical/"better not" stages, so I thought I'd chime in with my experience. I just accidentally plugged a USB-C cable to a charger (both from my Meta Quest 2) to a 220v line and the other end to my USB-C 3.2 Gen 2 port on my Asus Tuf Gaming B550M-plus motherboard. It took me about 3-5 seconds until I noticed the mistake and plugged it out. Nothing happened, no boom at all. The port is still working normally.

[Kisai]

On 6/26/2021 at 6:34 PM, Sesame said:

This is one of those really silly questions, and I am unsure what category this falls in.

 

I'm sitting here looking at my case which has a USB-C port which is connected to my motherboard using an adapter.

If everything is wired correctly, the USB-C port on the UPS is USB-C PD-only. And the Motherboard likely doesn't accept USB-C PD, so the result would be Windows detecting an unknown USB device at best, and nothing at worst.

 

The USB-C port is designed for a mobile phone, and will not be sufficient to power anything.

https://www.apc.com/ca/en/product/BR1000MS/apc-backups-pro-1000s-1000va-120v-sinewave-avr-lcd-2-usb-charging-ports-10-nema-outlets-4-surge/

Quote

USB charging ports: The 2 USB ports provide a total of 15W of DC power, and will provide power even when the UPS is on battery

Like only phones, and smart-watch chargers would even work off this port.

[Jorgemeister]

I guess there is only one way to find out. who is risking their system for science?