leaking current? shortage? static electricity?

TheMonkeyStink
Posts: 12
Joined: Mon Jun 02, 2014 3:41 am

Re: leaking current? shortage? static electricity?

Post by TheMonkeyStink » Tue Jul 01, 2014 10:45 pm

Okay smart people,

Take a look at the content below and let me know what I missed.

Background

Ground loops are almost always a problem when they arise. The most common type of ground loop encountered is when two pieces of audio equipment are plugged into separate AC outlets and are connected between each other with a signal wire. The ground loop will manifest itself as a hum at 60hz (in the US), or one of the harmonics of that frequency. Ground loops in this case are caused by the fact that the audio equipment is plugged into two separate outlets (that may or may not be on the same circuit). The two outlets will have different paths to earth ground, different either in distance travelled or route taken, and as such will have slightly different potential charges. Because the equipment has different electrical potentials on the ground plane the signal wire will now act as a conductor of the difference in potential (along with the signal). For audio equipment that interprets the difference between the ground and signal conductor an oscillating ground loop signal will alter the interpreted signal and cause hum.
It should be noted that the differences in earth ground potential is one source of ground loops, another source of problems is inductance from electrical fields. In the presence of an electrical field a piece of equipment will induct an electrical charge that can cause the ground plane to have a different potential than another piece of equipment connected via a signal wire, hence a ground loop.
The final source of ground loops is leakage from hot electrical sources to ground. You’ll see this in poorly designed circuits that don’t properly separate high and low voltage traces on a PCB. Also, if there is an outright failure in a device and voltage is sunk to ground, a ground loop will be created. However it should be noted that in the last scenario of an equipment failure, the ground plane is fulfilling its purpose of safely dissipating voltage to earth, and this situation should be immediately resolved.

Shapeoko Applications

Using the audio equipment example as a basis for understanding a ground loop, it becomes easier to understand how a ground loop can be applied to a Shapeoko 2 installation. If the computer ground plane has a different potential than the ground plane of the Arduino then current will flow along the shield of the USB cable and potentially cause damage or undesired operation.
In the case of the Shapeoko 2 both ends of the USB connection are attached to devices that are potentially very sensitive to voltage differences.

Solutions

The way to resolve ground loops depends on the source.
• If the ground loop is caused by inductance from an electrical field then the source of the field or the inducting equipment should be moved. For instance, if signal wire is running adjacent to a transformer, try moving the signal wire to another location. Also, if at all possible, use shielded twisted pair (STP) as opposed to unshielded twisted pair (UTP) or untwisted wire.
• Another highly effective way to resolve ground loops is to isolate the ground planes that are connecting through the signal wire. In the case of USB signal wire this is accomplished via a USB Ground Loop Isolator. A device of this type completely isolates the electrical system on either side of the device via the use of components called opto-isolators. Opto-isolators are devices that allow for the transmission of a signal without the conduction of current.
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Shapeoko 2 #5177: DW660 with nema 23. Acme Z axis (I believe these come from the same company that Wile E. Coyote gets his stuff)
# of failed cuts (a.k.a. Shapeoko Ohnos): 7
# of successful cuts: 12

Woodworker
Posts: 639
Joined: Tue Mar 11, 2014 1:37 am
Location: 5 miles north of Benson, NC

Re: leaking current? shortage? static electricity?

Post by Woodworker » Tue Jul 01, 2014 11:47 pm

I don't think you missed much. Problem is that most people are having problems with both the computer and the SO2 power brick plugged into the same power strip. Potentially there can be a problem with the brick if the cord is not polarized AND the power supply is poorly designed. What I want to test but can't, my meter walked away recently. Is the potential from the GRBL ground plan to earth ground while reversing the AC power cord. Other test would be the same if you are using a laptop, the laptop power brick could be the problem. The USB isolator is probably a good idea to have in place, just in case there is an issue with either.
BRuce - SO2 #4798 - IC's Z axis upgrade, customized Z rail and Z motor mount, spindle Dewalt 611

veng1
Posts: 250
Joined: Fri Nov 30, 2012 12:09 pm

Re: leaking current? shortage? static electricity?

Post by veng1 » Thu Jul 03, 2014 3:45 am

Not all isolators are optical, some are capacitive and some are magnetic. I believe that the USB isolator that Catlin supplies is magnetic. You might consider that. Capacitive isolation works very well for data streams that have no DC component such as Manchester encoded.

People can feel voltages that may be harmless and some times they feel harmful voltages only once in their lives. However, feeling between two conductors is not an ideal method and, actually, a voltmeter is not ideal but is a good first start. If the potential between conductors is not too high then measure the current with an ammeter. If the current is fairly low, it may be possible to reconfigure the grounding system to lower the current or just put a wire or resistor between the two conductors. Be careful with this approach because you are not fixing the problem but hiding it. If the ammeter indicates more than a few milliamps, the problem needs to be fixed not hidden.

In the States, isolated low voltage supplies are designated UL Class 2. Even though these are isolated, there is often a high value resistor, in the mega ohm range, bridging the isolation which is almost always a transformer. It's purpose is to insure that the floating output doesn't drift to a high potential with respect to ground. Without that resistor, I've seen floating, isolated outputs get to over a hundred volts due to capacitive pumping. It was not AC leakage but it was also a fairly unique circuit. A large value resistor fixed the problem.

In inexpensive power supplies, it may be necessary to open it up and verify that the said resistor is connected to the AC neutral or preferably to the AC ground. I can assure you that on an inexpensive off-shore power supply a UL Mark or CE Mark does not guarantee that it was tested or built to any standard.

One of the lines in the USB cable is ground and one is Vcc. If the amount of current drawn from the Vcc line is high enough, it may cause ripple and it may be fixable by use of some bulk capacitance. Try a 330uF or 470uF cap (get the polarity correct). Sometimes threading the USB cable through a ferrite toroid multiple times will work better than the single clamp on ferrite beads.

If the amount of power required is low, a simple linear regulator circuit that reduces the voltage may work if the higher voltage supply is not noisy.

Sometimes nothing will work but a good USB isolator. I designed an interface that worked with a Dell tablet that had a touchscreen and the device it was connecting to was an industrial piece of equipment that couldn't be modified as there were tens of thousands in the field. The USB data was more or less reliable but the noise from the USB power line caused the touchscreen to just go crazy. Adding an Analog Devices isolator chip fixed after weeks of trying everything else.

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