Electronics > Beginners

Probing usb powered circuit. Should I be scared?

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Moriambar:

--- Quote from: tooki on October 19, 2018, 10:45:22 am ---
--- Quote from: Moriambar on October 18, 2018, 06:11:12 pm ---
--- Quote from: tooki on October 18, 2018, 04:08:10 pm ---
--- Quote from: Kremmen on October 18, 2018, 01:35:55 pm ---OK to use the Arduino GND since the circuit is galvanically isolated from mains.

--- End quote ---
An absolutely wrong assumption. Computer USB sockets are almost universally grounded through to earth ground. The only major exception, really, is laptops that don't have an earth ground connection. (In the case of Mac laptops, they are grounded if using the 3-prong cord.)

--- End quote ---

Well I measured my mac laptop, and with the 3 pronged chord I get that usb GND is ~1k to mains earth. So I guess I will have to use the two pronged one when powering circuits that I have to test with the scope, I guess…

Thanks.

--- End quote ---
Literally the only thing to watch out for if the USB is grounded is to only ever connect the scope ground to the Arduino ground (which is the normal situation anyway). Done it many times with my Mac desktop, which is where my Arduino development is done. Sometimes there was various noise, which was cured by firmly tying the grounds together, and by having the scope ground as short as possible (i.e. standard good practice).

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So let me get this straight: you are saying that it is safe (both for the scope and the mac) to tie the grounds together (even with the current that will be ensured by the ~1k resistance), and that the only mistake I can make is to tie the scope ground to anything else than the arduino ground, right?

I'm sorry if I sound a bit confused: I am; both being a beginner and a non-native speaker is hindering me.

Thanks

Moriambar:

--- Quote from: Mr. Scram on October 19, 2018, 10:39:33 am ---As a rule of thumb I wouldn't power circuits under test from computers you care about. Sooner or later you'll blow at least one USB port from it, and possibly worse.

--- End quote ---

well the problem is that I only have that one computer and the circuit is programmed and powered by that... Apart from galvanically isolating the laptop from mains, and the alternative to unplug/restart the circuit powered from somewhere else (which will make me lose the computer-arduino serial communication) is there anything else you think I can do to use scope&laptop together?

Cheers!

Mr. Scram:

--- Quote from: Moriambar on October 19, 2018, 11:05:00 am ---well the problem is that I only have that one computer and the circuit is programmed and powered by that... Apart from galvanically isolating the laptop from mains, and the alternative to unplug/restart the circuit powered from somewhere else (which will make me lose the computer-arduino serial communication) is there anything else you think I can do to use scope&laptop together?

Cheers!

--- End quote ---
You could isolate the connection between the device and laptop. There are devices you place between the DUT and your laptop which allow you to isolate the connection. Another option would be to use a USB hub or card that acts as a sacrificial device in case things go wrong, although it isn't guaranteed that it will take the hit alone.

sokoloff:
I would recommend specifically a powered USB hub. Over the course of several years, I blew one USB port on a MacBook by drawing too much power from it. (In this case, it wasn’t a short, but was simply drawing far too much power from the port. A powered hub would have protected me there I’m fairly sure.)

Mr. Scram:

--- Quote from: sokoloff on October 19, 2018, 12:30:48 pm ---I would recommend specifically a powered USB hub. Over the course of several years, I blew one USB port on a MacBook by drawing too much power from it. (In this case, it wasn’t a short, but was simply drawing far too much power from the port. A powered hub would have protected me there I’m fairly sure.)

--- End quote ---
A powered hub would offer you better chances, but without verifying the exact construction there's no saying what would actually happen.

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