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Measuring max output current of a USB port.
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alm:
Unless the scope uses an isolated DC-DC converter. There are off the shelf parts for this.
py-bb:
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.
Rick Law:
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...
magic:

--- Quote from: alm 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.

--- End quote ---
Fair enough, that's an option and hopefully the way they are doing it.


--- Quote from: Nominal Animal 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.
--- End quote ---
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.
pcprogrammer:
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
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