Measuring max output current of a USB port. - Page 1
Forum section: electronics / general

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 12:28:41 pm
content: I need to know how much current the usb ports on my computer can output.How can I put a high load upon them and then measure the current?

author: tunk
author_rank: Super Contributor
timestamp: October 21, 2022, 12:37:04 pm
content: Maybe something like these:https://www.ebay.com/itm/294572711871https://www.ebay.com/itm/394010597121Wouldn't know, but you may risk damaging the port.

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 21, 2022, 12:41:40 pm
content: Do you know the motherboard used in your computer. It might be able to find the specifications for it.

author: alm
author_rank: Super Contributor
timestamp: October 21, 2022, 12:44:22 pm
content: What is the host? The max power modern USB-C ports on a computer or charger will deliver depends on what the device negotiates with the host. Look up USB power delivery. There were also older standards for up to 5V/3A or so. Just connecting a DMM across the USB port, or something similar, won't get you the maximum current or power.I imagine there are devices to test USB power delivery, but I doubt they sell for $10.

author: wraper
author_rank: Unknown
timestamp: October 21, 2022, 12:50:20 pm
content: Quote from: balnazzar on October 21, 2022, 12:28:41 pmNo, if there is a proper overcurrent protection, it will turn VBUS off. If there isn't, something likely will fail.

author: Gyro
author_rank: Super Contributor
timestamp: October 21, 2022, 12:55:11 pm
content: Quote from: alm on October 21, 2022, 12:44:22 pmActually I think they do sell for around $10 from China, depending on type.

author: Eraldo
author_rank: Regular Contributor
timestamp: October 21, 2022, 12:57:32 pm
content: Just plug it into a phone and see how long it takes to charge it. In my phone 6+ hours means a 5w output and with 2 hour it means 15w.Or just take a usb cable with wires sticking out of one end and just short it with a multimeter in the Amp range. It won't break anything if you do it quickly (or put a power resistor in series with the multimeter with low enough resistance for 4 or 5 amps to pass just to be safe)

author: ledtester
author_rank: Super Contributor
timestamp: October 21, 2022, 01:02:07 pm
content: See this table:https://en.wikipedia.org/wiki/USB#Powerand this comment:QuoteSo you must perform some USB negotiation in order to draw more than 1 unit load.

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 21, 2022, 01:08:11 pm
content: Quote from: Eraldo on October 21, 2022, 12:57:32 pmHere is some bad adviceDo not do this unless you know what you can expect. It can either blow something up in your computer or in your multi meter.When trying something like this always start out with a low current and work your way up, but then still you can blow up something in your computer when it does not have a proper USB power implementation.

author: Ranayna
author_rank: Super Contributor
timestamp: October 21, 2022, 01:23:02 pm
content: BigClive sometimes uses an USB Powermeter, in combination with a USB load, to test the output capability of powerbanks and USB Powersupplies.Similar to these:https://www.amazon.com/RD-Voltage-Voltmeter-Multimeter-Resistance/dp/B09B211239https://www.amazon.com/DollaTek-Electronic-Adjustable-Intelligent-Temperature/dp/B07PLDMK5KI got a set like that, and they work great.As other have said though: Be careful with your supply. Not all ports are properly protected, and you do not want to fry a port by grossly overloading it.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:29:01 pm
content: Quote from: pcprogrammer on October 21, 2022, 12:41:40 pmAsus z690-P.But I didn't find anything in the general (cosumer oriented) specs.. How can they omit those informations??

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:34:27 pm
content: Quote from: tunk on October 21, 2022, 12:37:04 pmYou mean by putting too much load upon them? From usb 3.0 onwards they *should* be protected against that..

author: Ranayna
author_rank: Super Contributor
timestamp: October 21, 2022, 01:35:09 pm
content: Quote from: balnazzar on October 21, 2022, 01:29:01 pmLikely because they follow the USB Specification. 100 mA without negotioation, 500 mA with negotiation on USB 2, 900 mA with negotiation on USB 3.Beyond that and for higher voltages than 5 Volts, you need USB-PD. If USB-PD is not specified, don't expect to have it.These values are the minimum. More is quite likely, especially the 100 mA limit without negotiation is often not followed, but drawing more *may* overload the ports and cause them to shut down. Or, if ASUS cheaped out, gross overloads may even damage the ports, or the controller chip, permanently.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:36:31 pm
content: Quote from: alm on October 21, 2022, 12:44:22 pmThe host is a Asus Z690-P board, the device is my shiny new Owon VDS1022i usb scope. It needs 5V@2A, and I wouldn't risk damaging it by providing insufficient current...

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:37:50 pm
content: Quote from: Gyro on October 21, 2022, 12:55:11 pmDoh, I discovered they actually sell from amazon at 10$ too.. 24hrs delivery..I should still establish if these contraptions can damage the usb port, though...

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:39:49 pm
content: Quote from: Eraldo on October 21, 2022, 12:57:32 pmThat could be a good idea. Let me see if I can find a usb cable useless enough that I can sacrifice it upon the altar of science...Ok, maybe that's not the best course of action.

author: alm
author_rank: Super Contributor
timestamp: October 21, 2022, 01:40:25 pm
content: Quote from: balnazzar on October 21, 2022, 01:37:50 pmDo they really support USB-PD? I think a USB-PD IC from companies like FTDI would already take up a substantial part of that money, although no doubt there's cheaper solutions for sale in China.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:41:24 pm
content: Quote from: pcprogrammer on October 21, 2022, 01:08:11 pmAh, mh.I thought that proper modern mobos from a reputable (so-to-say) brand would have had overcurrent protection o__O

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:42:28 pm
content: Quote from: Ranayna on October 21, 2022, 01:35:09 pmGreat!

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 21, 2022, 01:52:38 pm
content: Quote from: balnazzar on October 21, 2022, 01:41:24 pmYes, but it could just be a fuse that blows when overloaded. Some will have reset-able fuses, that restore after disconnect. Some will have electronic current regulation.What kind of cable came with the scope you bought? Back in the days spliced cables were used with an extra USB connector to allow drawing current from two ports. Or does your scope have a separate power input for a power supply?

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 01:53:47 pm
content: Now I was thinking.. Even with one of those 'usb-testers', I don't think it will attempt any negotiation. It will simply measure the maximum unnegotiated current output...

author: Eraldo
author_rank: Regular Contributor
timestamp: October 21, 2022, 01:57:25 pm
content: Here is some bad adviceDo not do this unless you know what you can expect. It can either blow something up in your computer or in your multi meter.When trying something like this always start out with a low current and work your way up, but then still you can blow up something in your computer when it does not have a proper USB power implementation.Almost all motherboards have current limiting in their usb ports. But what you said is correct. Going for a short circuit the first thing is stupid. I was giving a cheap and dirty way of doing it quickly without any specific tool.Yeah, having a digital load would make it more simple.My first advice is still valid though. First check if you have a fast charging phone. Then use your charger as a reference and check the charging speed (eg 15w charger takes 1.5hours). Then plug it on the pc and compare. The bad thing is that if your pc can deliver more current than your phone needs you won't know. At that point try the power resistor thing with multimeter with a resistance of 1 ohm (for a max of 5 amp. But check the voltage first too. Also the power rating should be ideally 25w ). You can also put a fuse in there just in case (5 amp or less depending on your needs).

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 02:00:03 pm
content: Quote from: pcprogrammer on October 21, 2022, 01:52:38 pmNo separate power input, *and* the scope indeed comes with a cable with two usb plugs. It works with these two plugs inserted. Not tested with just one, lest I could damage it.I was trying to establish if I can actually power it up properly with only ONE plug inserted (and a quality cable). I don't like being dependent upon this cable, and wasting one more port.. That's one thing.The other is that my laptop only has one usb-a ports and few usb-c ports, and I'd rather not meddle with adaptors (these usb-c ports seem to be pretty beefy though, since one of them powers up an external 4K 17.3" monitor without needing a separate power supply...)

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 02:02:05 pm
content: Quote from: Eraldo on October 21, 2022, 01:57:25 pmAlmost all motherboards have current limiting in their usb ports. But what you said is correct. Going for a short circuit the first thing is stupid. I was giving a cheap and dirty way of doing it quickly without any specific tool.Yeah, having a digital load would make it more simple.My first advice is still valid though. First check if you have a fast charging phone. Then use your charger as a reference and check the charging speed (eg 15w charger takes 1.5hours). Then plug it on the pc and compare. The bad thing is that if your pc can deliver more current than your phone needs you won't know. At that point try the power resistor thing with multimeter with a resistance of 1 ohm (for a max of 5 amp. But check the voltage first too. Also the power rating should be ideally 25w ). You can also put a fuse in there just in case (5 amp or less depending on your needs).The pc fully charges a M1 ipad pro (11") in less than one hour. Seems to be pretty strong, now that I think about it...

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 21, 2022, 02:06:08 pm
content: Ah a modern laptop. Yes USB-C can handle a bit more. There are cables that on one end have USB-C and on the other end the USB connector needed to plug into the scope. Just a google search awayHave you searched this site for threads about your scope?Here is one:https://www.eevblog.com/forum/testgear/owon-vds-1022i-review/

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 21, 2022, 02:25:30 pm
content: Quote from: pcprogrammer on October 21, 2022, 02:06:08 pmOrdered the cable right nowAlso reading through the thread, thanks! You know, that little Owon thing is good for its price (119 eur with two probes) and has a good software, after all. Not good as Digilent Waveforms, but good nonetheless.I'm considering the 12/14 bit, 4 channels, 100 MHz, 1 GSa/s, 10M depth,VDS6104A. 375 bucks plus VAT. It's waaay less than a Pico 2000 with similar specs. Like 3X less. And the Pico is just 8 bit...Can't do Bodes, but I got the AD2 for that.

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 21, 2022, 04:14:11 pm
content: You can use it to check things while building your own scopeI have two proper scopes and five simple ones to play with. Used the proper ones to measure on the simple ones while doing reverse engineering.

author: hans
author_rank: Super Contributor
timestamp: October 21, 2022, 04:33:14 pm
content: Concerning the short..If the port is protected, it should switch off power. But how are you certain you can catch the correct max current in time? If a port has some output caps, you'll measure their short circuit current spike. If the port protection is anything 'fast' (which you force it into because of the a dead short), then the DMM won't respond in time.Likewise, a gross overload for various durations can also kill power. Many ports are protected with polyfuses. E.g. these might burst in 0,1s with 5A load, but take 1min to burst at 1A load, or 15min at 800mA. If the rating is 500mA indefinite then 800mA or 1A is still too much.Trial and error tests, in particular short test IMO is bad advice borderline malicious, as it is not going to give any valuable information besides anecdotal evidence, but has high risks damaging poorly protected ports. I would expect a modern Asus MB to have proper protection, but anyone finding this thread through google to test on their 15 year old system is going to blow it up.That Owon scope looks very weird. Why is it using an USB A plug for a downstream port? It should be USB B or C.. and if they need more power then either A) use a second USB A plug upstream or B) use a DC barrel jack of some sort.Those PSU to USB A, or USB A to USB A cables also look downright dangerous..

author: wraper
author_rank: Unknown
timestamp: October 21, 2022, 06:57:23 pm
content: You don't need any scope or other nonsense. Peak currents will tell nothing about current capability of the port. To figure out at what current protection will trip, you need to gradually increase the current until protection acts. Easiest to do with electronic load. But just using a multimeter and adding resistors in parallel will work too. Also current at which protection will act is not the same as current capability of the port, actual capability is somewhat lower.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 21, 2022, 07:06:51 pm
content: The question doesn't make much sense if you know USB.

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 21, 2022, 07:28:39 pm
content: Quote from: wraper on October 21, 2022, 06:57:23 pmNo one mentioned using a scope for measuring the currentbalnazzar just wanted to verify that his new Owon USB scope is getting enough current when connected to a single port. And I told him that he can use his new scope to check on things when building his own scope, you know, what is being discussed here:https://www.eevblog.com/forum/testgear/building-my-own-scope/

author: mariush
author_rank: Super Contributor
timestamp: October 21, 2022, 08:23:21 pm
content: Majority of motherboards will have resettable fuses on the USB ports ... for example a 2.4A fuse for each pair of type A ports.By default, USB 3 is supposed to give up to 0.9A, so this way you could one of the two ports give 2A of current to a device (like an external mechanical drive for example) or some other gadget that needs more power, without tripping the fuse.

author: wraper
author_rank: Unknown
timestamp: October 22, 2022, 08:09:36 am
content: Quote from: mariush on October 21, 2022, 08:23:21 pmBarely any if any at all modern motherboards use resettable fuses for that. They use MOSFET based power switch/current protection ICs. With PTC fuses it's unlikely you can keep VBUS voltage in spec while having meaningful protection triggering current.

author: Nominal Animal
author_rank: Super Contributor
timestamp: October 22, 2022, 08:59:02 am
content: Personally, I'd connect the Owon VDS1022i to a powered USB hub, and connect the hub to my computer, making sure the hub is powered using an isolated wall wart and can deliver at least the 2A specified. Since it is an 'i' model, it's supposed to have an isolated USB interface.

author: magic
author_rank: Super Contributor
timestamp: October 22, 2022, 02:49:27 pm
content: If the USB interface is isolated but there is ground and 5V connection with the computer, it kinda defeats half of the purpose of isolation.If something bad enough happens to the scope, like grounding clip connected to a "hot ground", your computer goesThe scope should be fixed in the first place

author: alm
author_rank: Super Contributor
timestamp: October 22, 2022, 03:35:47 pm
content: Unless the scope uses an isolated DC-DC converter. There are off the shelf parts for this.

author: py-bb
author_rank: Regular Contributor
timestamp: October 22, 2022, 03:44:11 pm
content: Some of the stuff on this thread is genuinely terrifying.Like the guy recommending to short it with an ammeter quickly - please don't.For USB 2.0 type stuff, the max you can draw is specified, you're supposed to negotiate it but you can get away with it.upstream is supposed to implement overcurrent protectionbut so are ATX PSUs and they're iffy.For USB C type stuff - this is a totally different issue and very complex. I agree it's annoying not knowing WTF a port can do but it requires parts. The cables are not dumb cables for example, they at least use resistors on certain wires to communicate abilities, it's really bad.You can assume 100mA @ 5v from any USB port, you negotiate up from there - for data ports.

author: Rick Law
author_rank: Super Contributor
timestamp: October 22, 2022, 04:09:08 pm
content: For actual (not max) current, I think the better option is to do a USB breakout and insert a DMM (set at max Amp) in between source and load. Or if you already have a 0.1ohm (or lower) resister is at-hand, insert that as current sense.Either will likely beat the "USB Doctor" type devices since even cheap DMM will likely be more accurate than a cheap USB doctor type devices. Added benefit: a break out would allow you to do other things when needed.Of course you can make your load variable to measure the max, but that could allow you draw more than the port can support and possibly damage the port. With AC-USB charging adapters so cheap, use those if high-current is your purpose...

author: magic
author_rank: Super Contributor
timestamp: October 22, 2022, 06:55:49 pm
content: Quote from: alm on October 22, 2022, 03:35:47 pmFair enough, that's an option and hopefully the way they are doing it.Quote from: Nominal Animal on October 22, 2022, 08:59:02 amThat's a possibility, but there is also a possibility that the hub will current-limit at 500mA per port as it is frankly supposed to and you are back to square one.Other possibility is to modify the cable or the device (and void warranty in the latter case) so that 5V is completely disconnected from USB and taken from a 5V/2A wall wart. The output negative of the wall wart of course must be connected to USB ground...You can always simply connect one plug to the computer and see if it works. I have overloaded USB ports a few times, no permament damage ever happened.

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 22, 2022, 07:26:05 pm
content: It is a bit strange that balnazzar mentioned that the specs are 5V@2A and the supplied cable, from what I understood of his reply, only has a splice to two connectors.At 500mA per port this only yields 1A.As for the USB isolation a lot of the cheap devices that can be found on Aliexpress are only for full speed (12Mbps) and I guess the scope needs high speed (480Mbps), when it not already has it build in.The prices run up quick on these devices. Especially when one wants USB 3 performance.I recall someone did a review of these USB scopes and that the PCB between the two versions are the same. But not sure if it was for the Owon's

author: Nominal Animal
author_rank: Super Contributor
timestamp: October 23, 2022, 11:00:53 am
content: Quote from: magic on October 22, 2022, 06:55:49 pmThat's a better idea than mine above.I can easily find a suitable ready-made USB power injector here: shared ground, but VUSB connected only to the scope and to the 5V/2A regulated power source; data lines only to the host computer (male data connector on the splitter).For example, on Delock 82769, the red male USB connector is for the 5V 2A USB power wall wart, the female is connected to the scope, and the black male to the computer. Here in Finland, Verkkokauppa.com sells Deltaco USB2-16 for 8€, where the female connector is connected to the scope, the primary black male to the computer, and the secondary male connector to the USB 2A+ wall wart. The wall wart should still be isolated, to avoid a ground loop with the computer USB port ground – hereabouts, any legitimate ones sold at computer stores are –, and 2.1A per port ones are easy to find.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 23, 2022, 12:31:27 pm
content: Quote from: pcprogrammer on October 22, 2022, 07:26:05 pmAfaik, the scope is just USB 2.0And, they explicitly wrote 5V - 2A upon the scope's USB port.I crossed my fingers and just connected ONE usb plug. It works. I operated it with both channels connected and tried to load it with maximum acquisition rate & stuff like that.

author: David Hess
author_rank: Super Contributor
timestamp: October 23, 2022, 01:40:18 pm
content: Quote from: ledtester on October 21, 2022, 01:02:07 pmMany devices now draw 0.5 or even 0.9 amps without negotiation, and most systems will provide it.

author: Nominal Animal
author_rank: Super Contributor
timestamp: October 23, 2022, 03:03:08 pm
content: Many USB wall warts here provide 2.1A on a single USB port. It seems to me the Owon has been designed to be powered by one of those.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2022, 03:18:20 pm
content: Quote from: Nominal Animal on October 23, 2022, 03:03:08 pmA limit of 1 or 2 amps is common. There is a standard forUSB chargerswhere 200 ohms is connected across the data signals. The problem I have seen is that the loads which are suppose to use this standard will also draw high current from a standard USB port.