| Electronics > Beginners |
| Probing usb powered circuit. Should I be scared? |
| << < (5/7) > >> |
| tooki:
--- Quote from: Moriambar on October 19, 2018, 11:02:38 am ---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? --- End quote --- Yes. Tying the grounds together actually reduces the chances of problems. I routinely tie together scope ground and Arduino ground when scoping Arduino circuits. I'm sure many more mistakes are theoretically possible, but yes, the main one is just to remember that the scope ground is earth ground (and thus Arduino ground, USB ground, etc), so don't connect the scope ground to anywhere else in the circuit but ground. Of course, this is all assuming you are working with normal low voltage stuff. Don't even think of probing mains or other high voltage stuff until you're much, much more experienced and know how to do it properly. (I would not consider myself to be properly educated on how to do that.) Finally, if you are powering your circuit (such as an Arduino) from a battery or isolated power supply (like most DC power supplies are), and USB is not connected (or the Macbook is not plugged in), then your circuit is "floating" and you can put the scope ground anywhere you like. --- Quote from: Moriambar on October 19, 2018, 11:02:38 am ---I'm sorry if I sound a bit confused: I am; both being a beginner and a non-native speaker is hindering me. --- End quote --- It can be confusing for beginners regardless. And your English is excellent! So don't worry. :) Part of the confusion is that the discussion has forked in two directions: one is the scope ground question, the other is protecting the computer's USB socket from damage. --- Quote from: Moriambar on October 19, 2018, 11:05:00 am --- --- 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? --- End quote --- What I do, like many people, is to use an AC adapter (or lab power supply) to provide 7-12V DC to the Arduino's DC input jack. (9V is typical, but lower will let the Arduino's voltage regulator run cooler.) When you do this, the Arduino automatically stops pulling power from USB — but still retains the data connection. The suggestion others have made, of using a powered USB hub in between, is an effective and simple way to protect the computer's USB ports in case you make a mistake. So it's certainly not a bad idea at all! |
| tooki:
--- 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 --- Out of curiosity, do you know how much current you drew?? Macs made since 2007 can provide 1A (or more) per USB port, above the 500mA maximum of standard USB. (Of course, this is only supposed to be drawn after power negotiation.) Both overcurrent and dead shorting them should cause the port to go into overcurrent protection and turn off, with a message to that effect on screen. Thinking aloud: wouldn't it also work to make a USB protector of sorts, that simply adds an inline fuse that would blow in a fault condition? |
| GeoffreyF:
Grounding is covered in section 8.16 of the third edition Chapter 9 also has a lot of useful information. "Practical Electronics for Inventors" by Paul Scherz and Simon Monk is also good (and perhaps better for this discussion) 2.10 talks about Grounds 2.22 talks about Mains power Figure 2.90 is really great for this discussion. Going back for finding these references, I am reminded that there is much overlap between the two books but Practical Electronics starts at a somewhat more basic level in some aspects. The art of electronics is more for people who are looking to create projects that support college and graduate school experiments while practical electronics is more oriented towards people who want to invent something for it's own sake or as a business opportunity. Geoff |
| Moriambar:
--- Quote from: tooki on October 19, 2018, 02:34:40 pm ---Yes. Tying the grounds together actually reduces the chances of problems. I routinely tie together scope ground and Arduino ground when scoping Arduino circuits. I'm sure many more mistakes are theoretically possible, but yes, the main one is just to remember that the scope ground is earth ground (and thus Arduino ground, USB ground, etc), so don't connect the scope ground to anywhere else in the circuit but ground. --- End quote --- Did measure a 0.076V stable difference between the usb ground and the usb gnd. I connected the scope gnd to it and nothing blew up. Great! --- Quote ---Of course, this is all assuming you are working with normal low voltage stuff. Don't even think of probing mains or other high voltage stuff until you're much, much more experienced and know how to do it properly. (I would not consider myself to be properly educated on how to do that.) Finally, if you are powering your circuit (such as an Arduino) from a battery or isolated power supply (like most DC power supplies are), and USB is not connected (or the Macbook is not plugged in), then your circuit is "floating" and you can put the scope ground anywhere you like. --- End quote --- Yes, the only project I have that goes up over 5V is a motor one featuring a 12V DC motor, so I think I'll be safe. And, although I understand about "putting the grounding anywhere when you're floating", I feel like for now it's best for me to grow a habit to use the normal gnd, and maybe be extra careful and change it only when needed and floating --- Quote --- It can be confusing for beginners regardless. And your English is excellent! So don't worry. :) Part of the confusion is that the discussion has forked in two directions: one is the scope ground question, the other is protecting the computer's USB socket from damage. --- End quote --- Thanks. Yes, both issues are important here… --- Quote ---What I do, like many people, is to use an AC adapter (or lab power supply) to provide 7-12V DC to the Arduino's DC input jack. (9V is typical, but lower will let the Arduino's voltage regulator run cooler.) When you do this, the Arduino automatically stops pulling power from USB — but still retains the data connection. --- End quote --- Well I did not know this. Great! I have a PSU albeit old and crusty (but it was free!) and I can use that one! Thanks --- Quote ---The suggestion others have made, of using a powered USB hub in between, is an effective and simple way to protect the computer's USB ports in case you make a mistake. So it's certainly not a bad idea at all! --- End quote --- One can never bee too safe. Thanks a lot, now all is way clearer. |
| Moriambar:
--- Quote from: GeoffreyF on October 19, 2018, 02:58:14 pm ---Grounding is covered in section 8.16 of the third edition Chapter 9 also has a lot of useful information. "Practical Electronics for Inventors" by Paul Scherz and Simon Monk is also good (and perhaps better for this discussion) 2.10 talks about Grounds 2.22 talks about Mains power Figure 2.90 is really great for this discussion. Going back for finding these references, I am reminded that there is much overlap between the two books but Practical Electronics starts at a somewhat more basic level in some aspects. The art of electronics is more for people who are looking to create projects that support college and graduate school experiments while practical electronics is more oriented towards people who want to invent something for it's own sake or as a business opportunity. Geoff --- End quote --- Great thanks, also for the book recommendation! |
| Navigation |
| Message Index |
| Next page |
| Previous page |