Author Topic: earth loop through usb  (Read 4909 times)

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Offline V_King

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earth loop through usb
« on: July 10, 2013, 12:48:20 pm »
Hello,

been following the website for a while, there is a lot of useful information. Got a question, which can not find an answer for.
I am a mechanical engineering student, and try to tinker with elecronics on my spare time.

I am trying to automate the old mini mill into cnc machine using arduino mega and have a dilema with the earth loop through the USB cable, used to pass data to the PC.
uC is housed in steel box, which is earthed. All the cables to the linear displacement transducers have single earth to the same steel box. I draw a rough diagram to ilustrate the situation.

Should I unsolder the USB ground casing from the uC, to prevent the earth loop? Any common practices in such situations?

Cheers
« Last Edit: July 10, 2013, 12:52:19 pm by V_King »
 

Offline bilko

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Re: earth loop through usb
« Reply #1 on: July 10, 2013, 01:45:16 pm »
To prevent earth loops you need star point earthing. Make sure that all of your earth cables terminate at the same earth stud, this will go a long way in reducing earth loops.

 

Offline ivan747

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Re: earth loop through usb
« Reply #2 on: July 10, 2013, 01:51:28 pm »
Remember the grounded USB shield must only be grounded at the host. It is left disconnected on the device. Of course, the grounds must be joined (you must connect the GND pin, the black wire on USB cables).
 

Offline bilko

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Re: earth loop through usb
« Reply #3 on: July 10, 2013, 02:03:02 pm »
Remember the grounded USB shield must only be grounded at the host. It is left disconnected on the device. Of course, the grounds must be joined (you must connect the GND pin, the black wire on USB cables).
So instead of the shield carrying the earth loop, it now travels through the black wire (which is still connected to ground)  ?
If both chassis are star earthed and connected with substantial earth cable, they will be at the same earth potential so no current will flow through the USB ground. Ohms law, impedance of the ground cable should be substantially less than that of the USB cable shield. Disconnecting the USB ground will lead to other EMI problems.
« Last Edit: July 10, 2013, 02:04:40 pm by bilko »
 

Offline V_King

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Re: earth loop through usb
« Reply #4 on: July 10, 2013, 08:17:46 pm »
thanks for answers everybody.

I want to achieve star point earthing - transducers and communications to the motors inverters cable shields are connected to the steel casing (electric box). The whole casing is mounted to the mill chassis and grounded to the earth to single point.

That's were the USB cable confusion arose - as if you check the cable shield and the PC mains power earth connection, they have direct connection. So if I would connect the USB cable to uC in the steel box, I would have a second earth.

Would it be a good idea to buy an isolated usb panel socket, mount it to the steel box and inside only run the data wires, isolating the uC from the shielding? As I understand the steel box, I mounted all the electronics into, works as shield (I am using a typical steel box for electronics, which are used in industrial machinery).
 

Online IanB

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Re: earth loop through usb
« Reply #5 on: July 10, 2013, 08:27:27 pm »
You said the steel box is earthed--is that the mains protective earth? If so, it should be common with the PC mains earth connection. As long as both the PC and the steel box are earthed at the same local mains supply, then they should share a common ground reference and excess voltage differentials should be avoided. It is common to make USB connections between PCs and peripherals like printers with separate mains cords, so there is no strong reason to expect trouble as long as you follow normal wiring conventions.
I'm not an EE--what am I doing here?
 

Offline bilko

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Re: earth loop through usb
« Reply #6 on: July 10, 2013, 08:40:49 pm »

Would it be a good idea to buy an isolated usb panel socket, mount it to the steel box and inside only run the data wires, isolating the uC from the shielding? As I understand the steel box, I mounted all the electronics into, works as shield (I am using a typical steel box for electronics, which are used in industrial machinery).

It doesn't sound like you have a star earthed system.
All earth connections should go to the same earth stud/busbar which is isolated from the machine.
Machine earths should connect to this same earth stud/busbar.
Only one earth lead from the busbar to the mains earth connection.
Anything else will introduce ground noise.

USB connections are quite critical, especially anything faster than USB1. Check the specifications, but if you isolate the shield,  how are you going to stop the shield from behaving as an antenna, injecting noise into the data wires ?

 

Offline V_King

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Re: earth loop through usb
« Reply #7 on: July 10, 2013, 10:15:07 pm »
Am I over-engineering here? The mill is tiny, run of the single phase mains. Mill and pc are connected to the same mains socket. I don't exactly know where the main earth connection goes behind the mains socket.

In one of the IET local meetings, one old-timer was sharing his experience with most common mistakes in control engineering, and he was very serious with ground loops. But he comes from designing control systems for big plants, where you have big distances for data signal lines etc.

There are two reasons I am interested in this. Firstly, I want to learn the correct way of approach for the future projects. Secondly, the mill is at the moment at mates workshop in industrial estate with plenty of noise sources, so it would be good to remove at least one variable as I have some minor noise issues with analog signals.
 

Online Monkeh

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Re: earth loop through usb
« Reply #8 on: July 10, 2013, 10:27:45 pm »
You're over-thinking. You won't have any ground loop issues there.
 

Online IanB

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Re: earth loop through usb
« Reply #9 on: July 10, 2013, 10:30:46 pm »
USB is used all the time to connect between two devices with separate power supplies. For example between PC and printer, or between PC and USB disk drive.

Just make sure you read up on the conventions for USB device and cable wiring and follow the documented standards for what ground connections should be made where and you should be OK. If the mill and PC are connected to the same mains socket they will share the same earth reference potential. Unless you are unlucky or do something daft you won't have any problems.
I'm not an EE--what am I doing here?
 

Offline V_King

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Re: earth loop through usb
« Reply #10 on: July 10, 2013, 11:51:30 pm »
thanks very much for the replies.  I do sometimes over-think  :phew:

Any recommendations for good reads about wiring conventions?
 

Online IanB

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Re: earth loop through usb
« Reply #11 on: July 11, 2013, 12:40:23 am »
thanks very much for the replies.  I do sometimes over-think  :phew:

Any recommendations for good reads about wiring conventions?

http://www.usb.org/developers/docs/
I'm not an EE--what am I doing here?
 


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