Well said Chris. I worked on a system once where the 24V system bus shared a common ground with a 12V instrument bus. The system bus could have as much as 20A flowing through it as a Servo motor moved one section. There was also other equipment that sporadically drew more and less current. They used one common ground from the battery/charger systm to all the devices tapping onto this ground wire where needed.
On a small panel that approach had worked for them. When the batteries/chargers and 12V power supply along with one instrument were 30m away from the rest of the instrumentation they had all sorts of induced noise issues. They argued that they had measured the noise on the 24V system in the control box with a scope and it was minimal. I demonstrated that if they ran the scope ground 30m back to the battery and measured the ground at the control box they would see lots of noise. Splitting the 12V and 24V grounds made the problem go away. The 30 meter distance was still an issue since it wasn't physically possible to run a star ground for ever device back 30m to the battery. Add that some of the 24V devices had a non-isolated CAN bus and shared their CAN bus ground with the 24V system the potential for the 24V return path to take the 12V return path. But the major 24V ground separation from the 12V was a big step in the right direction. John > Gene is right. Many people don't understand grounds. The star ground is > not magic. It is based on two basic laws of physics. Just two things to > remember > > 1) It is a fundamental law of electricity the current ONLY flows if there > is a loop. Take a straight piece of wire and connect one end to a battery > and leave the other end free and absolutely zero current flows and the > wires has EXACTLY the same voltage all along it's length. THIS is why > you use a star ground. No current will flow in the ground wire if ONLY one > end of each wire is connected to your ground reference. Make a loop and > you have current. > > 2) Mr. Ohm has a law that says if there is current you have voltage. In > fact the volts are proportional to the current. So if you make a loop > with your ground wires there will be current and hence different parts of > the loop will have different voltages on them. It will not be zero > everywhere as you might think. > > Basically the combination of Kichoff's and Ohm's laws. People try can > get around this by using real heavy and thick ground wires. But they > forget (or never knew) that impedance has an imaginary component and the > thick wire only effects the real component. This noise that is > transmitted is high frequency AC not DC. you have to make the path "low > impedance" if you want to get around Ohm's law. This noise is basically > in the radio spectrum, big wires don't help much there. > > Just make sure ALL you grounds are "dead ends" There are places where > where loops sneak in. One of then is shielded cables where the ground > shield is connected to each end of the cable. > > There are more sophisticated systems where they shunt the AC to ground > using capacitive coupling but don't even try unless you are a specialist > engineer. You do see this in some equipment. They use a capacitor in > parallel with a high value resister and they connect signal and protective > grounds this way. What they are doing is making a low impedance path for > AC to ground. > > The other thing you can and should do is try to stop this electrical noise > at the course. The best way to is again remember Kichoff. Current flows > in a loop. So any time to have a wire with current, say going to a motor, > some place there is another wire with that same current flowing in the > opposite direction. Always route those to wires as close as possible > Best way to do that is to twist them together. The opposing directs will > cancel any electric field. At about 8 wire diameters, from a twisted pair > the field will be nearly zero. So if you d this the noise will only > radiate for less then about an inch. Wire routing and layout matters. > > > But this brings up a second recommendation, which is to establish a > > single bolt as a common ground point, with the commons of everything > > else connected ONLY to this bolt. This is known as a star ground system. > > Stuff connected to the nearest ground often results in having more than > > one ground and that constitutes a ground loop, which acts as an antenna, > > picking up noise from anything radiating it, and this noise can easily > > blow gates in the fpga on the 7i90. That bolt should also be the only > > place the 3rd rounded in the US pin in the power cord is connected, cut > > them off till you've only one left to get rid of ground loops, just > > don't cut the last one! > > ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
