[zettabomb]

That would be true if both sides were exactly 5.0V, but they're not. There's a 5% tolerance, from 4.75V to 5.25V, and in practice you will see chargers often run "5V" at 5.1V intentionally, to account for resistive loss in the charging cable. If you accidentally connect your "5V" device to the host's "5V" you may find that the host simply disables the port, which has happened to me more than once. So no, you can't just blindly connect them together.

[f1shy]

Yes you can. They will not blow. Nothing will happen.

[zettabomb]

What do you mean by "blow"? There's often a polyfuse which will trip, and needs to cool down to reset. I haven't seen a normal fuse but I believe it's possible. Efuses are also common, to allow the system to automatically reset as soon as the fault condition is removed.

It's unlikely that anything will be damaged, but the device likely will not work until the issue is resolved.

[f1shy]

No it will not. Have done it thousands of times. No no no. You can downvote all that you want, but you will still be wrong. It will happen nothing. Period. If you do not know about it, educate yourself before downvoting and commenting about fuses.

Because of all these supplies work with transistors they do not act as a load to the other. Is like if the 2 had a diode in the output (in fact they do have one, but not directly in the output).

This is my typical experience in HN lately, is getting full of people with absolutely no idea what they talk about, and are constantly downvoting good comments.

[zettabomb]

>Because of all these supplies work with transistors they do not act as a load to the other.

This is utter nonsense, Ohm's law doesn't magically stop working with a transistor. I do know about this, I've designed power supplies and USB devices, and I've destroyed more than a few components accidentally by connecting two switching supplies together. Yes, there will be current flowing, and yes, sometimes a fuse or breaker will trip, I have experienced this many times, and just because you haven't doesn't mean it doesn't happen.

>Is like if the 2 had a diode in the output (in fact they do have one, but not directly in the output)

Sounds like you're referring to either ESD protection diodes, or flyback diodes, neither of which do anything in the case of two similar but unmatched power supplies.

I'd advise you to get a degree in engineering (as I have), or do some serious studying, as this kind of uninformed discussion is not productive or helpful to anyone, it's just noise.

[f1shy]

Wow. You really are interesting person. You obviously have no idea what you talk about, but keep insisting…

Man… you are really a nice case.

Let me make a last attempt, even when I know it will fail:

1) “Ohm's law doesn't magically stop working with a transistor”

Ohm’s law works only with linear components, is a linear relation. So NO it does not work in a transistor or a diode. No it doesn’t. No because of your magic ignorance, but because they are not frigging linear! Go study some physics.

2) No, I was certainly not referring to ESD diodes, but the rectifier, at the end of any SMPS. Some may have a last stage linear regulator, in that case the diode is part of the juncture of the output transistor. At any rate, ANY wall mounted power supply, and 99% of all supplies in the world, when the output is higher that the target voltage will just shut down. GO TEST IT AND STOP with your nonsensical replies.

BTW: the 1% of supplies that do regulate down are called “4-cuadrant-supply” are much more complicated, and expensive, and makes no sense to use in a USB charger.

I don’t care which degree you have, if you really do, and was expensive, ask for your money back. In case is not a degree in prompt engineering…

[zettabomb]

I know what I'm talking about, I am a professional working in electrical engineering. Please don't insinuate that I don't, it's unnecessarily insulting.

If you have two power supplies at different voltages and connect them together, there will be a finite resistance through the cable and Ohm's Law applies. Current will flow. With a low resistance and big enough voltage difference, there will be a significant current, and it can trip the supply. This is not difficult to achieve.

The rectifier you're referring to is the flyback diode in that case. But now as you've said yourself, the power supply will shut down if the voltage coming in is too high, which is frequently due to either a polyfuse or an efuse tripping. So it sounds like you're just arguing to argue, while actually agreeing with my point. You said "nothing will happen", but if one shuts off, something has happened.

I don't need to test this, I have done it. I also have quite literally thousands upon thousands of other engineers, books, universities etc backing me up, and you do not. Connecting two USB supplies together is a bad idea and will likely result in one switching off. Don't do it.

Either way, I'm done trying to convince an amateur. Feel free to do what you want.

[cowboylowrez]

akshually (hehe sorry couldn't resist) ohms law keeps working, its just that the resistive component in the formula varies according to the applied voltages because of active components but ohms law continues to obey the math that results. very trivial observation tho, so downvote if you must lol