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| Powering uC with USB from PC while connecting ground with external supply |
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| AQUAMAN:
Hello I have an external high current power supply, where the power supply is 'isolated', but the negative terminal is earthed for safety reasons. See first page of this doc: http://www.delta-elektronika.nl/upload/APP_SAFE%20OPERATION%20POWER%20SUPPLY_V201705.pdf The configuration is literally what you see here. I plan on using an LPC1768 to control and monitor the voltages of a circuit powered by this power supply, and stream the data via the USB to a PC. It's about 20V and 100A. So I will connect the ground of the LPC1768 to the negative earthed terminal of the power supply. But at the same time the LPC1768 will be connected to my PC via USB and also 'earthed' through that (I think). If I do this, will I create a ground loop and blow up my microcontroler or PC? Or should it be OK because this is basically the same as an oscilloscope right, and there are no problems using the oscilloscope in this way? Just as long as I don't connected my mbed ground to anywhere else in the system? |
| JS:
Not recommended, it could go fine for some time but you are exposed. Serial ports are not hard to insulate, usb is much harder (there are ICs for that but expensive and not so common) So, what I'd say is to use a serial out to the µC, optoisolate the serial bus, serial to USB converter. That's one way to do it if high bandwidth is not required. If you are constrained with space or something you could use the USB isolator, Dave has mentioned it on a teardown not long ago, I can't remember which. JS |
| rstofer:
There are USB isolators around, Adafruit has one: https://www.adafruit.com/product/2107 Even with that, I might still run the USB through a powered hub. |
| AQUAMAN:
--- Quote from: rstofer on May 17, 2018, 11:32:50 pm ---There are USB isolators around, Adafruit has one: https://www.adafruit.com/product/2107 Even with that, I might still run the USB through a powered hub. --- End quote --- Thanks for your reply Had no idea these things existed. So if I buy one of these powered hubs, I will have an isolated power supply to my mbed, and it won't affect any of the speed? I'm streaming the data at 921600 baud I have a feeling this will work even without any precautions, I don't see any difference between this and an oscilloscope probe which works fine, but I can't risk breaking anything right now. although my circuit is only about 15V max, it is 100A+ |
| David Hess:
--- Quote from: AQUAMAN on May 17, 2018, 05:58:48 pm ---If I do this, will I create a ground loop and blow up my microcontroler or PC? --- End quote --- That is a risk. I have blown up a couple of computers that way. --- Quote ---Or should it be OK because this is basically the same as an oscilloscope right, and there are no problems using the oscilloscope in this way? Just as long as I don't connected my mbed ground to anywhere else in the system? --- End quote --- Oscilloscopes and most other test instruments have the same problem and the same risk. This is why differential and isolated probes are used with oscilloscopes and why some test instruments have isolated outputs and isolated USB ports. Ethernet has a decisive advantage over USB with its built in galvanic isolation. Get an isolated USB hub or bridge to place between your computer and device under test. |
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