Here’s a rather heated online discussion.Weigh the merits of the two sides and which one is more likely to be correct or incorrect, based on electrical engineering, USB specifications, and other relevant subject matter. Specifically, analyze the discussion about connecting two 5V USB supplies together.
  
<table style="min-width: 25px"><colgroup><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>derkades 3 days ago | root | parent | prev | next [–]</p><p><br></p><p>Except that connecting 5V to 5V does not cause a short circuit. No current will flow without a voltage difference. If there is a difference, the capacitors in one of the power supplies will charge up to the same voltage and then current stops flowing again.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>zettabomb 3 days ago | root | parent | next [–]</p><p><br></p><p>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.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>f1shy 2 days ago | root | parent | next [–]</p><p><br></p><p>Yes you can. They will not blow. Nothing will happen.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>zettabomb 2 days ago | root | parent | next [–]</p><p><br></p><p>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.</p><p>It's unlikely that anything will be damaged, but the device likely will not work until the issue is resolved.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>f1shy 2 days ago | root | parent | next [–]</p><p><br></p><p>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.</p><p>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).</p><p>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.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>zettabomb 2 days ago | root | parent | next [–]</p><p><br></p><p>&gt;Because of all these supplies work with transistors they do not act as a load to the other.</p><p>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.</p><p>&gt;Is like if the 2 had a diode in the output (in fact they do have one, but not directly in the output)</p><p>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.</p><p>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.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>f1shy 1 day ago | root | parent | next [–]</p><p><br></p><p>Wow. You really are interesting person. You obviously have no idea what you talk about, but keep insisting…</p><p>Man… you are really a nice case.</p><p>Let me make a last attempt, even when I know it will fail:</p><p>1) “Ohm's law doesn't magically stop working with a transistor”</p><p>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.</p><p>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.</p><p>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.</p><p>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…</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>zettabomb 1 day ago | root | parent | next [–]</p><p><br></p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>Either way, I'm done trying to convince an amateur. Feel free to do what you want.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>f1shy 1 day ago | root | parent | next [–]</p><p><br></p><p>I don’t know at this point if you have fun trolling around or what.</p><p>“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.”</p><p>That is an error that only a person with minimal knowledge from TV shows can do. That is absolutely bot true for regulated power supplies, SMPS or Linear, becauae (even if the later is called linear) they are not linear. So no, you cannot apply Ohm’s law.</p><p>“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”</p><p>Noooo I’m talking about the diode at thr end of any SMPS, buck, boost, fly-back or whatever type, or even a linear regulator. Is about how a regulated power supply works: as the voltage goes up in the output, it shuts down, stops the output current, in an intent to liwer the voltage. There is no fuse in 99% of supplies out there, because they have active protection. Is called a transistor, not a polyfuse.</p><p>“I also have quite literally thousands upon thousands of other engineers, books, universities etc backing me up”</p><p>That is what you think, but you are wrong. Sorry mate. If you would just go to a lab and put 2 power supplies in parallel, like I do pretty much daily, you will see you are wrong.</p><p>You are the most ignorant, obstinate and arrogant person I’ve seen in HN… and here is full of it. I’m 100% sure you studied CS in a medicre university, work as sw dev, and think you can talk with an EE. You say you work with electrical stuff, but your other posts reveal you are a SW dev. Obviously you are a liar, and trying to be right when you are sooo obviously wrong.</p><p>I do care what you do, because with such an arrogance and incompetence together, you are going to get somebody hurt or killed. Please start studying and stop being so stubborn when you are just wrong!</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>cowboylowrez 1 day ago | root | parent | prev | next [–]</p><p><br></p><p>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</p><p><u>reply</u></p></td></tr></tbody></table></td></tr><tr><td colspan="1" rowspan="1"><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><br></td><td colspan="1" rowspan="1"><p>f1shy 1 day ago | root | parent | next [–]</p><p><br></p><p>You are right. ohm’s law does not stop working. Never does. Is just you cannot apply it to any circuit blindly.</p><p><u>reply</u></p></td></tr></tbody></table></td></tr></tbody></table>