In this episode of Atmel Edge, Analog Aficionado Paul Rako discusses the importance of understanding ground symbols for electronic schematics. As Rako notes, Earth ground, chassis ground, power supply return and shield are all different. This video explains why.
“Earth ground has a very precise meaning and a very precise name, and it’s earth ground. My professor, James T. McLaughlin, at Kettering University previously General Motors Institute, pointed out [that] earth ground is a ten foot copper-clad steel rod,” says Rako.
“And you hammer it into the dirt. And you make sure there’s moisture so it has conductivity. The minute you hook a wire to it, well now you got some inductance. And 12-gauge wire all the way to get to where earth ground has to get, which is this third pin on your wall socket, well now it’s got a little resistance, as well.”
Rako also points out that a car isn’t grounded.
“What you want to use is this symbol, which is chassis common. And chassis common, it’s not just cars, but television, radios, PCs with metal things. Anywhere there’s a metal case or a metal mounting point, that’s chassis common. In America, if a human being can touch that metal, you have to connect earth ground at chassis common,” he notes.
“Underwriter’s Laboratory requires a ring terminal so it doesn’t get pulled off. And that way, if there’s a short of high voltage on to the chassis — a wire or something falls down — then it can seek a ground through this earth ground and trip a circuit breaker instead of electrocuting your customers.”
As Rako emphasizes, semiconductor companies who make chips should be using this symbol, the triangle.
“That is power supply return. You may connect your circuit board in the corners, it may connect to chassis, and maybe you want that,” he adds.
“Maybe you want it to connect at 100 places to get a really good RF connection between the circuit board and the metal chassis. But this symbol would be improper on a circuit board, and certainly earth ground is wrong.”
You can watch the video above for more information about schematics. Readers may also want to check out Rako’s previous Debug 101 episode here.
Important discussion Paul. Definitely a pet peeve of mine. So easy to forget that these symbols and terms actually mean something. Like VCC & VDD as well. I’ve had dozens of schematics that use these symbols / terms incorrectly and it’s only an issue, when it becomes an issue…But when it DOES become a problem? It’s usually not pretty.