Electronic – How to two 5 V sources be connected to provide more current than a single can

There are many different things that can happen when you pull too much power and this will be dependent on the technology being used. The typical things you will see is the voltage dropping below the spec'd output or cutting out completely. Some systems might have a fuse that trips when you pull too much power. Or the worst case is you exceed the safe rating of some components that result in something over heating and potentially causes lots of problems (fire, etc).

The circumstances that this can happen pretty much all fall on to the board designer. Many times it is cheaper to build a power supply that can provide less current, so the designer will minimize the cost as much as possible while still being safe. If there is some case that the board designer hasn't considered, then they might pull too much power. These cases might be things like running an RF module while doing some DSP work, while the designer originally intended for them to only operate at separate times.

The other time that you might pull too much power is under an actual fault condition. This is when something truly goes wrong, like an IC fails into a short condition, or someone accidentally shorts two connections together, or something along these lines.

As to what happens to the device, this also all depends on the device itself and the PSU being used. If the PSU drops its voltage, potentially the device might fail if the system can't handle the voltage.

The problem with this approach is, as with every other "connecting two power supplies", that the voltages are not the same. The nominal voltage is 5V - yes, but the actual voltage is different. One might be 5.05V and the other 4.8V. Both are in the spec for USB 5V.

But if you connect those two together, one will start to drive the other because it has a higher voltage.

Why the normal Y-Cables work? You plug them into the same device which has a single 5V source, so the voltage is really the same. Only each port is current limited.

And I see another possible question coming:

But there are USB hubs which use an external power supply, which is also a different voltage than the original port - how does that work?

Right, but those use either completely the external power supply or have some switching logic inside to switch from bus-powered to self-powered mode. For example the TPS2070 from TI.

So I would not connect two seperate power supplies to a single cable as long as I don't know the exact inner workings and can be sure that nothing will go wrong. It might work (if the things are reverse current protected for example) or it might not.

But I'm not an expert on USB power, so maybe I'm completely wrong.