I've read that "ground is not ground". I still don't totally understand that but that's never stopped me....lol

So, in my limited understanding....I have a power brick (wall-wart) that is either a switching PSU or a linear one (transformer). Both take AC and step it down to convert to DC. Most of the ones I have are not grounded that I see (two prongs).

Then you have the normal ground rail/plane in your circuit that is essentially isolated from the "real earth" ground. Meaning, it's not connected to a cold water pipe, earthing rod, etc.

Then you have "case ground". Which, again as I understand it, ties the metal casing of the enclosure (computer case, project box, etc.) to the circuit ground above.

That's a long winded paragraph to simply ask....should I tie my circuit ground to the mounting holes in my board? So that when screwed down, the case (or enclosure box) becomes grounded along with the circuit? So, should it be left isolated?

Thanks!

PS, I plan on eventually installing a panel mount DC barrel jack on the case so that I don't have to go directly to the board. I also plan on installing serial/parallel jacks. So I assume they would also be grounded to the case and internal board.

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Cat; the other white meat.

I am sure there will be others on the forum that can supply a better definition.

Ground is truly one of the worst concepts ever conceived for electrical/electronics. Over the years, I've come to realize that ground is better thought of as the negative reference plane of an electrical circuit. Ground generally refers to the fact that in most buildings and similar electricity-based facilities, the local voltage of the earth, or ground, is used as the reference against which the other voltages are measured. In a localized environment, the local ground will appear to be 0V.

This is not strictly true, and is one reason for power transmission systems to rely on balanced AC circuits and transformers to isolate local ground potential from the power source. Many years ago, a two phase pump was installed at the cistern at the bottom of the hill. The outside two wires from the transformer at the house was run down from the house to the pump house where the two "hots" were connected to the pump. An awful racket was the result upon initial application of power to the motor.

A few measurements with a 90V neon AC tester indicated that one "hot" relative to the local ground connection at the pump was way low in voltage relative to the same measurement for the other "hot". The problem was that there was approximately 40 V of ground potential between the house at the top of the hill and the pump house at the bottom. The solution to this problem was to run the "ground" from the house down to the pump. A local protection ground, i.e. "earth", could be provided at the pump house for protection of the local inhabitants.

In your case, the neutral (wide plug) from the wall is supposed to be 0V, or ground, for your standard 115V ac outlet. That connector and the "earth"/"ground" connector are essentially wired to the same point at the entry to your house. Thus, if no AC current were to be "flowing" in the neutral, the neutral and the ground should be at the same potential, i.e. a 0V difference. Because there is AC current flowing in the neutral from the devices plugged into the "hot"/"neutral" AC wire pairs, and these wires have resistance, when there is an AC load there should be a small difference between the neutral and the ground of 1-4 VAC (under normal usage).

If your device is fully isolated, then I would wire the mounting holes together, but not in a full loop, and tie that trace back to the common reference of the power supply feeding your board. If those mounting holes connect to a metallic case, there could be a substantial difference between that reference and the AC neutral. Therefore, to dissipate the potential difference between your supply common and the neutral, I would connect a large value resistor (DC) and capacitor (AC) to "equalize" these two references.

That being said, the fact that your supply plugs into your AC outlet using only two prongs, means that the neutral can be at a fairly high potential above the local "ground" if a fault occurs. Fully isolated supplies are great in many situations, but the equipment must also be constructed in such a manner as to prevent contact with it when a fault occurs. I personal prefer to have a three prong AC-DC supply, and to connect my equipment case and ESD components' conduction path to the third connector, i.e. "earth/ground".

There should be a very small amount of current on that circuit except under fault conditions. Hopefully, the fault conditions are such that the circuit breakers interrupt it before any significant harm comes to anyone or anything.

Summary: a circuit ground planes are just another voltage reference. They are really the circuit return current path. They can be at thousands of volts relative to the local ground.

Hope this helps, and I apologize if my explanation is not on target or not clear enough.

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Michael A.

The reason for the 3rd plug (the wide one) is in its name - protective ground. It is a provison (and mandatory for metal cases) that if the "hot" wire gets off its terminal - accidentally - it connects to the protective-grounded case and trips a fuse or (in modern electric wiring) a "fault current detector" which cuts both ac rails immediately.

This protective-grounded case should not be directly (hard-wired) connected to your “GND”. It is a common practice to have a resistor (which has a working voltage greater than line voltage, e.g. 250V) of 10 MOhm between “GND” and case. Parallel to that resistor an Y-capacitor (>= 250 Vac) of 1 to 2.2 nF is placed. That is to keep both potentials nearly same in case of fast voltage transients (“ESD”). The construction of a Y-capacitor is considered to be fail-save against internal shorts even under over-voltage conditions.


Regards,
Arne

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The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

We have to differentiate between two "different grounds".

The one I talked about is sometimes called "protective" ground and is related to the local "earth" potential.

The one Garth talked about is usually referred to as "signal" ground. This should be the reference potential for some other signal. If one forgets to consider the resistance of a wire that carries some current, the "connection" to ground is actually a voltage source with an alien signal. In cases like Garth mentioned only a fraction of a millivolt may have a severe impact. One way to reduce problems like that could be working with differential signalling.

In "usual" electronic circuits the "ground" is isolated from "earth" due to transformers (even in SMPSs). It is similar to running the circuit with a battery. The transformer causes the insulation. It is not a perfect insulation, but that isn't necessary. Using a SMPS you will have a higher hum voltage than a classic transformer (50/60 Hz) would produce - that may result in an uncomfortable tingling sensation when touching any signal (including ground). (I hope that we can omit special considerations when using "autotransformers"...)

If you use a metal case and inside of it there are wires running at mains voltages - then protective grounding the case is mandatory. Otherwise you could simply forget "earthing" as long as that hum does not matter.

The frequent hints other members had already given that you should consider a 4 layer pcb (having a true "ground" plane) or at least using very fat traces for ground (and V+) are of more importance.


Regards,
Arne

Ahh, Ground vs. GND vs. Earth vs. 0V

Back when the first electrical items ran from one wet cell (or a number of cells wired in series), there were two battery connections to the circuit, positive and negative.

But someone at some stage decided they needed a ‘centre tap’ connection. Now, what to call this third connection? Can’t call it positive, can’t call it negative, as we have those already....

The second problem was that some people had the switching in the positive wiring. While others had the switching in the negative wiring.

Right, who said that? Was it you at the back that shouted “standard”? Don’t do such a silly thing again. Of course there were no standards, this was the “Wild West” where anything goes (or more often, didn’t go/work!).

Anyway, the end result is that with low voltage DC circuitry, whether based on the switching and control being in the positive or the negative circuit, the return path has become known as 0V or in some cases, as GND.

The ‘GND’ bit is misleading, as normally there is no connection to either mains ground/earth or to the soil/dirt outside and under your home or office.

But, in some devices/equipment, this 0V/‘GND’ is connected to either mains ground/earth or to the soil/dirt outside and under your home or office (using a ground rod/earth rod/stake).

If the device or equipment is mains powered and there is a risk of a single insulation failure resulting in the metalwork of the item becoming live at mains voltage, then as described above, the regulations in most countries requires that a safety electrical ‘earth’ or ‘ground’ connection is made to the metalwork of the case. This connects to the mains earth or ground connection.

I hope some of that helps.

Mark

Would it be possible to use a plastic enclosure?

No, you should not attach the enclosure to your circuit ground. Unless there's a specific reason otherwise, you'd be best off using a plastic enclosure to avoid this issue entirely. However, if the case is metal, and there is any danger of high voltage (> 50V) coming into contact with said metal enclosure (i.e. if a wire came loose inside), you must attach the enclosure directly to Mains Earth for reasons of electrical safety.

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Just to be clear. ANY exposed metalwork that may become live at a dangerous voltage should be earthed/grounded to mains earth/ground via a direct cable/wire/conductor. Regulations require the resistance between ANY part of ANY exposed metalwork to be a very low value to the mains plug earth/ground.

It is up to you if you decide to earth/ground the 0V/GND rail on your PCB/circuit board to the mains earth/ground. Either directly, or via a parallel resistor/capacitor. There are advantages and disadvantages. What else you connect (or want to connect) to it often determines the answer.

Mark