Author Topic: Measuring max output current of a USB port.  (Read 6514 times)

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Offline balnazzarTopic starter

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Re: Measuring max output current of a USB port.
« Reply #25 on: October 21, 2022, 02:25:30 pm »
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 away 8)

Have you searched this site for threads about your scope?

Here is one: https://www.eevblog.com/forum/testgear/owon-vds-1022i-review/

Ordered the cable right now :)

Also 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.
« Last Edit: October 21, 2022, 02:28:53 pm by balnazzar »
 

Offline pcprogrammer

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Re: Measuring max output current of a USB port.
« Reply #26 on: October 21, 2022, 04:14:11 pm »
You can use it to check things while building your own scope :)

I 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.

Offline hans

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Re: Measuring max output current of a USB port.
« Reply #27 on: October 21, 2022, 04:33:14 pm »
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..
 
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Online wraper

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Re: Measuring max output current of a USB port.
« Reply #28 on: October 21, 2022, 06:57:23 pm »
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.
 
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Offline SiliconWizard

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Re: Measuring max output current of a USB port.
« Reply #29 on: October 21, 2022, 07:06:51 pm »
The question doesn't make much sense if you know USB. ???
 

Offline pcprogrammer

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Re: Measuring max output current of a USB port.
« Reply #30 on: October 21, 2022, 07:28:39 pm »
You don't need any scope or other nonsense.

No one mentioned using a scope for measuring the current :)

balnazzar 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/

 :-DD :-DD

Offline mariush

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Re: Measuring max output current of a USB port.
« Reply #31 on: October 21, 2022, 08:23:21 pm »
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.

 
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Online wraper

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Re: Measuring max output current of a USB port.
« Reply #32 on: October 22, 2022, 08:09:36 am »
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.
Barely 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.
 

Online Nominal Animal

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Re: Measuring max output current of a USB port.
« Reply #33 on: October 22, 2022, 08:59:02 am »
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.
 
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Online magic

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Re: Measuring max output current of a USB port.
« Reply #34 on: October 22, 2022, 02:49:27 pm »
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 goes :-BROKE

The scope should be fixed in the first place :-/O
 

Offline alm

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Re: Measuring max output current of a USB port.
« Reply #35 on: October 22, 2022, 03:35:47 pm »
Unless the scope uses an isolated DC-DC converter. There are off the shelf parts for this.

Offline py-bb

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Re: Measuring max output current of a USB port.
« Reply #36 on: October 22, 2022, 03:44:11 pm »
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 protection but 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.
 

Offline Rick Law

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Re: Measuring max output current of a USB port.
« Reply #37 on: October 22, 2022, 04:09:08 pm »
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...
 

Online magic

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Re: Measuring max output current of a USB port.
« Reply #38 on: October 22, 2022, 06:55:49 pm »
Unless the scope uses an isolated DC-DC converter. There are off the shelf parts for this.
Fair enough, that's an option and hopefully the way they are doing it.

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.
That'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 :P) 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.
 

Offline pcprogrammer

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Re: Measuring max output current of a USB port.
« Reply #39 on: October 22, 2022, 07:26:05 pm »
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

Online Nominal Animal

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Re: Measuring max output current of a USB port.
« Reply #40 on: October 23, 2022, 11:00:53 am »
Other possibility is to modify the cable or the device (and void warranty in the latter case :P) 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...
That'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.
 

Offline balnazzarTopic starter

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Re: Measuring max output current of a USB port.
« Reply #41 on: October 23, 2022, 12:31:27 pm »
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

Afaik, the scope is just USB 2.0

And, 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.
 

Online David Hess

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Re: Measuring max output current of a USB port.
« Reply #42 on: October 23, 2022, 01:40:18 pm »
See this table:

https://en.wikipedia.org/wiki/USB#Power

and this comment:

Quote
Low-power devices may draw at most 1 unit load, and all devices must act as low-power devices when starting out as unconfigured. 1 unit load is 100 mA for USB devices up to USB 2.0, while USB 3.0 defines a unit load as 150 mA.

So you must perform some USB negotiation in order to draw more than 1 unit load.

Many devices now draw 0.5 or even 0.9 amps without negotiation, and most systems will provide it.
 

Online Nominal Animal

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Re: Measuring max output current of a USB port.
« Reply #43 on: October 23, 2022, 03:03:08 pm »
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.
 
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Online David Hess

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Re: Measuring max output current of a USB port.
« Reply #44 on: October 24, 2022, 03:18:20 pm »
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.

A limit of 1 or 2 amps is common.  There is a standard for USB chargers where 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.
 
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