Ground Loops, Noise, and USB Communication Errors
There is a lot of buzz around the grounding of the Chinese clones. It is generally in reference to earth ground and the safety of the machines. I have been down many rabbit holes about why or why not to add an additional earth ground to the machines and I want to touch briefly on it to provide some basis for the main topic which is the dropout of device communications which causes jobs to stop in progress and other erratic behavior. The bottom line is IF all of the below conditions are met THEN you do not need an additional earth ground and adding one may, in fact, cause many more problems. If all of these apply to your laser engraver then you do not need additional earth grounds:
- The electrical systems in the location of your laser have 3 prong plugs that are compliant with current electrical codes.
- The ground wires and lugs in the laser engraver have good continuity to the ground prong of the 3 conductor power cord.
Luckily, Thunder Lasers do not suffer from these grounding issues! This article is simply for informational purposes about the various anomalies that can occur with serial communications.
Electrical Load Panels and Branch Circuits
First, let’s look at your typical home or shop wiring. If you are in a primary residential/small commercial structure you will have a main breaker panel where there is a ground bus, which is connected with a #4 AWG solid copper ground to an 8′ earth grounded rod nearby. There is also a neutral bus. The neutral bus and ground bus are bound in the main panel so they have the same potential difference. In an electrical system, a ground loop or earth loop occurs when two points of a circuit both intended to be at ground reference potential have a potential between them. This can be caused, for example, in a signal circuit referenced to ground, if enough current is flowing in the ground to cause two points to be at different potentials. You also have L1 and L2 which are each 120V and 180 degrees out of phase from one another. If you measure the potential difference between L1 and neutral or ground you will get a reading of 120VAC. The same will happen if you measure L2 to neutral and ground. If you measure between L1 and L2 you get 240VAC. L1 and L2 alternate in the breaker columns to help balance the loads and allow 240v (this is why 240-volt breakers are two combined breakers).
Earlier we noted that ground AND neutral reference the same EARTH ground. This is significant because this means the laser engraver will reference the main earth ground and so does any other device you have plugged into an outlet that is on the main panel. If you go adding additional grounds to your laser engraver you will cause the laser to reference a different ground which causes a potential difference between neutral and ground. This means that your computer has a different ground reference than the laser. The potential difference could be as high as 2V or more! Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. Wiring practices that protect against ground loops include ensuring that all vulnerable signal circuits are referenced to one point as ground. The use of differential connections can provide rejections of ground-induced interference. Removal of safety ground connections to equipment in an effort to eliminate ground loops also eliminates the protection the safety ground connection is intended to provide.
Ground loops are hardly the only thing that causes electrical noise; pretty much any device with a motor (hair dryers and blenders, for instance), as well as light dimmers and failing fluorescent fixtures will create this type of interference. Capacitor start motors in refrigerators, AC units, etc…. are even worse. The obvious solution to this type of noise is to not use those types of devices while you’re watching TV or listening to music. You might be able to make that work—if you live alone. If there are other people under the same roof, perhaps not. If you’re willing to part with a few Benjamins, you can assure yourself of pristine AC without ground-loop noise by using an online UPS (uninterruptible power supply) or an isolation transformer. An online UPS is a battery-backup system whose battery is always engaged (online) between the input AC and the output AC. This requires the electrical power to go through a conversion to DC (direct current) and then back to AC, which will remove all the noise. This is also known as dual conversion. You will also want to make sure it produces a full sine wave.
Now let’s explore the USB cable streaming information to your laser. USB is a differential bus – that is, it has two wires, and the logic level depends on the difference between them, not an absolute voltage. A USB logic ‘1’ is the state when the D+ line is 200mV or higher (more positive) than the D- line, and a logic ‘0’ is the state when the D- line is 200mV or higher (more positive) than the D+ line. The actual DC voltage on the data lines conveys other information, such as whether a device is plugged in, but the only thing that determines the data is the difference between them. So 200mV is all it takes to scramble a USB data stream.
Diagnostics & Troubleshooting
I have been in tech service for almost 30 years. From being a certified autosound installer, RF communications and land mobile radio bench tech to senior level network engineer and color digital imaging systems technician. I have had to deal with countless ground issues and other electrical line condition anomalies. I use fairly sophisticated instrumentation to identify ground issues and other power line faults. The main 2 tools are an Ideal SureTest for quick identification of common issues and a Fluke VR1710 line event recorder for more intensive diagnostics. Many of these faults, when presented to the affected electricians and utility engineers, were foreign to them and reasonable solutions could not immediately be implemented. I can tell you these odd problems are so vague that collaboration between the participating entities was difficult and a resolution was sometimes taxing to all.
The best way to handle this is to verify that your power distribution systems meet or exceed the current NEC, state, and local codes and that your outlets are 3 prong outlets. Then make sure your laser engraver wiring and terminations are adequate. If you still have power quality problems the test equipment mentioned above will help diagnose the issues so you can mediate the anomalies.