understanding ground

[calvingloster]

I'm slightly confused about ground on dc circuits. The way I understand ground is a place for electrons to flow to.

What confuses me is this, if I have an arduino switching an npn transistor, I would connect a digital pin to the base pin and my power circuit on the emitter and collector. Do I need to connect my arduino's ground to the emitter?  Why cant my arduino's digital pin use the power circuits ground?

[JimboZA]

I think you mean the digital pin to the base? (Edit: I see you fixed that already. It's good practice when you edit, to show what you changed... else others will think I'm imagining things....)

The npn requires the load on the high side, so from the top down, you have:

Power +ve
Load
Collector
Emitter
Ground (same as power -ve)

Then on the "side" so to speak you have:

Controller digital pin to base (through a resistor)
Controller ground to same ground as the transistor, which is the same as the Emitter.

But I don't really understand your question.....

The grounds need to be common so that all voltage measurements have the same reference. If the controller ground isn't the same as the equipment ground, then the transistor has no 0 for the controller io signal, and it's a bit like imagining one hand clapping.

Edit: I guess it's easiest to say that the Arduino ground, the power supply -ve, and the emitter are the same point.

[calvingloster]

I think you mean the digital pin to the base? (Edit: I see you fixed that already. It's good practice when you edit, to show what you changed... else others will think I'm imagining things....)

The npn requires the load on the high side, so from the top down, you have:

Power +ve
Load
Collector
Emitter
Ground (same as power -ve)

Then on the "side" so to speak you have:

Controller digital pin to base (through a resistor)
Controller ground to same ground as the transistor, which is the same as the Emitter.

But I don't really understand your question.....

The grounds need to be common so that all voltage measurements have the same reference. If the controller ground isn't the same as the equipment ground, then the transistor has no 0 for the controller io signal, and it's a bit like imagining one hand clapping.

Edit: I guess it's easiest to say that the Arduino ground, the power supply -ve, and the emitter are the same point.

Yes sorry I did edit it, you are not imagining things lol.

Basically what I'm asking is why can u just have 1 wire coming from the arduino into the base or the transistor? The current will flow from the base to the emitter , base being the "positive" side of the circuit and the emitter being the "ground".

If I can put it in another way, say u have 2 1.5v batteries, or two power supplies. You take one battery/power supply and connect a wire from the positive terminal to the load, then you take the second battery/power supply and connects the ground wire to the negative side of the load. You see the two batteries/power supplies
Are separate from one another but the load still see's a positive and a negative? Why can't this work?

I just wana know why I can't ONLY put 1 arduino digital pin on the base of the transistor and not connect the arduino ground. Why can't the arduino use the higher power circuits ground? Why can't the electrons flow from the arduino to the ground of say, a battery?

[JimboZA]

The current will flow from the base to the emitter , base being the "positive" side of the circuit and the emitter being the "ground".

Not quite: electrical ground isn't quite like a bucket into which the electrons flow. (Leaving aside for now the issue of the actual direction; "conventional" flow is + to - but we know it's really the other way)

If ground were a bucket, the source would soon "run out" so to speak, so there has to be a loop. Hence the Arduino + goes to ground thru the base and the emmiter, and the ground back to the Arduino provides the other side of the circle.

But also hear what I said about the common ground being the reference for everything to measure against. The Arduino "knows" the signal is 5v, since it measures against its own ground. The transistor only knows that as 5v if it measures from the same refernce.

[JimboZA]

If I can put it in another way, say u have 2 1.5v batteries, or two power supplies. You take one battery/power supply and connect a wire from the positive terminal to the load, then you take the second battery/power supply and connects the ground wire to the negative side of the load. You see the two batteries/power supplies
Are separate from one another but the load still see's a positive and a negative? Why can't this work?

Becasue the +ve of a cell or power supply isn't +ve in some absolute sense, it's only positive to its other end.

Take those 2 cells and put them on the table, apart from each other. Put one end og a volt meter on one cell's +ve and the other on the -ve of the other. You'll get no reading, or rather a reading of no magnitude.

[calvingloster]

Ok thanx, that kinda makes. Just a another question, why does the power circuit the arduino is driving not effect the arduino's ground? Like what prevents ,say we are switching a 100v 20A power circuit, the high current from not going into our arduino's ground?

does the current say "no wait that's not my vcc's ground hence I will not go there"

[JimboZA]

Hmmmmm, good question. I'll leave that to the electronics boffins!

[retrolefty]

Ok thanx, that kinda makes. Just a another question, why does the power circuit the arduino is driving not effect the arduino's ground? Like what prevents ,say we are switching a 100v 20A power circuit, the high current from not going into our arduino's ground?

does the current say "no wait that's not my vcc's ground hence I will not go there"

Electron flow is from the negative terminal of the voltage source (power supply) through various loads and then onto the positive terminal of the voltage source. As long as the negative terminals (ground in out case) are wired together the actually flow of current through each part of the system is only what it requires. The base arduino board consumes around 50 ma of current regardless of what other loads are drawing from an external DC supply.

That probably doesn't help all the much. Normally one picks up this knowledge as part of learning basic electronics fundamentals using a structured teaching plan starting with basic DC circuits using ohms law. Learning good electronics fundamental via just building arduino projects is sure to be a slow and frustrating experience, but perhaps still fun enough if really learning electronics was not a primary goal.

[MarkT]

The important point to realise is that a circuit is a circuit - electricity flows round a loop
(if it tried to pile up somewhere in any quantiy the voltage rises to thousands or millions
of volts - static electricity is exactly this, small amounts of charge stuck on the surface
of an insulator).

So in normal circuits everything just flows round so for any two parts of the circuit
to be connected they have to share at least two wires.  Or put another way connecting
two circuits together with a single wire is simply trying to use on as a radio aerial on
the other (and vice versa).

Voltage is always a difference, in fact the official term is "potential difference".

With the high current circuit and its control circuit the important point to realize is
that the transistor controls the high current circuit with the low current circuit - were
it some other device, or connected wrongly, yes the current could be reflected back
into the Arduino.  The properties of the amplifying device are all-important.

[Grumpy_Mike]

The way I understand ground is a place for electrons to flow to

That is where you are wrong. A ground is a bit of an abstract concept and any point in a circuit can be considered a ground.
But it has to be a circuit, see:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

[calvingloster]

The way I understand ground is a place for electrons to flow to

That is where you are wrong. A ground is a bit of an abstract concept and any point in a circuit can be considered a ground.
But it has to be a circuit, see:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

thanx man, what prevents the high load from going in to the arduino? in the picture i have attached what prevents the 20A flow of electrons from not going to the ground of the arduino?

[Grumpy_Mike]

What prevents it is that current can only flow between voltage differences. The ground of the arduino is at the same potential as the ground of your external supply so there can be no current flow into the arduino.

[retrolefty]

Maybe starting with a ground rod would help make things more clear? 

http://www.wlrc.net/wp-content/uploads/2013/04/Ground-Rod-Pic3.jpg

[l_dowell]

A ground is a bit of an abstract concept and any point in a circuit can be considered a ground.

I'm going to counter this slightly.    Ground is exactly that. Earth connection, like through a lightning rod or grounding connection on your fuse box. What most of us here are discussing is a circuit COMMON. The circuit common may or may not be grounded (earthed). Neutral is the 'common' of your house AC supply. It is tied to ground at the fuse box so it is ground referenced. The negative side of your car battery is probably chassis connected, but it is not grounded.  It still is the reference (common) of most automotive power systems.  US telephone systems are 48V DC based. The positive terminal of their batteries are tied to ground so in essence their systems are at -48V relative to ground. You could use ground as a reference and everything you measure is a negative value, or you could measure from the -48V battery terminal and everything is a positive value. Ultimately, the concept of a circuit COMMON is that it's the base point from which measurements are made so everyone who needs to, knows how to get the same measurements.  Make sense?

Lyle
{and I occasionally slip and say "ground" when I mean common    )

[crazypj]

It may be easier to just think of the path of least resistance?
Arduino and power supply are connected to 'ground' (common)  but 'power' just want's to go where it's easiest - direct path rather than into a bunch of electronic components
Now it's probably time for someone to point out electron flow so 'nothing' is flowing to ground