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7 years, 6 months ago.
Powering LPC1768 with USB from PC, and connecting ground with external power supply
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 and also 'earthed' through that (I think). So If I do this will I create a ground loop and blow up my mbed 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?
2 Answers
7 years, 6 months ago.
Hello Nick,
According to the schematic the mbed LPC1768 board is equipped with diodes (see below D1 and D2) to prevent reverse power supply currents. So in my opinion no "ground" loop can be established over the power supply rails. However, keep in mind that the maximum voltage which can be connected to the LPC1768s input pins ( Analog In included) is 3.3V!
The LPC1768 board is quite expensive. That's why I would recommend you to first experiment and test your design with this cheap one. Although it isn't equipped with protective diodes, it wouldn't be a big pain when destroying one by mistake. To protect the PC connected over USB I would remove the power supply regulator chip U1 from the board (see on the schematic here) and use only the +3V input to supply the board with power.
posted by 18 May 2018
7 years, 6 months ago.
Yes, you will have a ground loop. The return current from the equipment to the power supply can travel through the mbed, down the USB cable to the PC, through the PC ground connection to the mains power and from there return to your power supply. No amount of diodes on the mbed voltage rails would have any impact on that since they aren't on the ground connections.
However assuming your power supply is connected as described and as shown in fig 1 of the document you linked then you should be OK. I'm guessing that the wires for the power supply are nice solid cables with good connectors, the resistance of the direct return path will be far smaller than the indirect path and so the currents on the indirect path will be insignificant. It may add a small amount of RF noise and a tiny amount of noise to any measurements you are taking but not enough to matter.
As you say this is no different to connecting an oscilloscope ground to a system. There are some situations where this can be an issue (I keep a power cord with the earth wire removed in my desk draw for when this is the case) but they are relatively rare.
The only time you could have an issue is if you didn't connect the ground wire from your equipment to your power supply, the the only path left is through the mbed, That's easy to check for, make sure things power up and work before you connect the mbed to the PC.