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The USB standard is for the host to provide 5 V to peripherals. When I measure the voltage on an attached Arduino, I have 5.78 V. Is this much too high or within specification?

Update:

The measurement that gave rise to the confusion is in measuring the 5V line on the Arduino, which I understood was directly passed through from the USB port if that was the power source.

Final Update:

To end confusion, the issue was caused by a multimeter whose own battery was critically low, making the device unable to function accurately. Answers and discussion below add detail.

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    \$\begingroup\$ Which instrument are you using to measure this voltage? When was it last calibrated? \$\endgroup\$ – rdtsc Dec 18 '20 at 16:34
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    \$\begingroup\$ An exceptionally good question, and one that now answers a lot of questions for me. On a good day this device is within 0.01 V DC. Today was not a good day, the 9 V battery was putting out a blistering 6.4 V. It's a wonder it even switched itself on. Now my anticipated precise ATX power supply, USB power supply and everything else I've been measuring lately returns much more sensible values. \$\endgroup\$ – J Collins Dec 18 '20 at 18:35
  • \$\begingroup\$ @JCollins, that thingy didn't even have a low battery warning? I recall getting one from my multimeter, and then wondering why apparently dead batteries measured like new ones... Until I realized to take the warning seriously. Actually, with that failure mode, it would have been better if the meter just declined to show any results. \$\endgroup\$ – ilkkachu Dec 19 '20 at 13:28
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    \$\begingroup\$ So add an answer and specify that your voltmeter has low battery and is giving incorrect readings. \$\endgroup\$ – Nelson Dec 19 '20 at 15:33
  • \$\begingroup\$ @ikkachu It in fact does have an indicator but I had neglected it since typcally the LCD screen becomes slow/laggy/dull when there's a noticeable problem, so have taken it to mean that it must just be a bit overly sensitive. Will recalibrate my expectations..! \$\endgroup\$ – J Collins Dec 20 '20 at 12:52
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The USB specification says 4.4 V or 4.45 V to 5.25 V for VBus. Some non-standard USB supplies will be higher to combat the voltage drop over a long wire and high current, but it is not within normal specifications.

Are you measuring under load? That would be important. Most devices will regulate down and even 5 V ICs tend to tolerate up to 6 V, so it's not a great problem.

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    \$\begingroup\$ Engineering Change USB Specifications do increase the standard VBUS level to 5.5V \$\endgroup\$ – Ale..chenski Dec 19 '20 at 7:24
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As it turns out, my multimeter battery was under-voltage, and this was the source of seeing the high reading on the USB power. With a fresh battery I now see 5.01 V across the 5V terminals.

On a new ATX power supply I have -12.01 V, -5.01 V, 5.01 V and 12.30 V on the nominal -12, -5, 5 and 12 volt terminals.

The low battery also affected readings across precision resistors. Check your batteries.

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  • \$\begingroup\$ Please can you edit this into your question and delete this answer, rather than post it as an answer. Otherwise, readers have to go hunting for this definitive answer amongst all the speculative ones. Thanks. \$\endgroup\$ – TonyM Feb 25 at 9:29
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    \$\begingroup\$ Typically the meta advice is to keep the question as-is and keep the answers in the answer section, rather than to confuse the two. I have however added the multimeter battery detail to the question. \$\endgroup\$ – J Collins Feb 25 at 15:09
  • \$\begingroup\$ Don't know why the site advises to do that, makes it harder for the reader for the reasons stated. Also encourages OPs to post questions knowing they may answer them and 'earn' votes (some chase them!). But I'm not going to follow it through meta, far too busy at the moment. In any event, I'm very sure you're acting in good faith here and thanks for your edits :-) \$\endgroup\$ – TonyM Feb 25 at 16:26
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If it's a cheap power circuit, it might need a tiny minimum load to get within the specified voltage. Would expect to see this on dollar-store USB-chargers, not from a computer!

Put a 5k resistor on it and see what it does.

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No, it is not in tolerance according to specifications. It is too high.

Edit: Based on comments by OP, the summary is that most likely the issue was not that actual voltage was too high on the USB port, but simply that the multimeter was low on batteries and it gave too high readings because of this.

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    \$\begingroup\$ This answer could be improved with minimal effort by simply citing the applicable specifications, and ideally giving a source. \$\endgroup\$ – Cody Gray Dec 19 '20 at 4:43
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    \$\begingroup\$ @codygray You are correct, the answer could be better. You can also edit it with minimal effort. But the question could be better too. As it is now, the question expects a yes/no answer, which I provided. It could also be voted to close due to being a potential duplicate (it has been asked but in a form what is the tolerance) or due to lack of own research before asking (the tolerance has been asked before and it even reads on Wikipedia page about USB). \$\endgroup\$ – Justme Dec 19 '20 at 12:04
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    \$\begingroup\$ Not-great questions aren't a good excuse to post minimal answers. If you think the question is poor, downvote and/or comment, perhaps motivating the OP to rephrase into something possibly worth asking (like what's going on here, aren't computer's supposed to be better than this?) \$\endgroup\$ – Peter Cordes Dec 19 '20 at 20:16
  • \$\begingroup\$ @PeterCordes Thank you I am aware this was brief. I sometimes make a placeholder answer and improve it later. Then again, the question already has a better answer to the original question and it has been accepted. Besides the problem was not the laptop USB port outputing out of spec voltages to begin with, as the problem was simply using a multimeter which had a low battery warning on screen, which was only revealed on comments below. So all answers are wrong anyway, and I am unsure how I should edit my answer then, as per the original question, or the actual problem behind original question. \$\endgroup\$ – Justme Dec 19 '20 at 20:41
  • \$\begingroup\$ This might be a case where deleting your answer makes the most sense at this point. You were the first to answer, but within minutes a higher quality answer came along. So if you don't have anything to add to it, you could just tidy up the question by deleting. In this case, your answer isn't adding anything future readers can't get from the other answers. Or you could edit to summarize comments; the "check your batteries" answer could certainly be useful so it belongs in an answer somewhere. Was hoping the OP would post that, but they haven't yet. \$\endgroup\$ – Peter Cordes Dec 19 '20 at 21:03

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