Where's the ground? - Page 1
Forum section: electronics / beginners

author: ABCD
author_rank: Contributor
timestamp: September 08, 2011, 01:59:40 am
content: In a simple circuit consisting of a battery and a resistor, where is the "ground"? Is it the negative terminal? If so, what does it mean to tie this circuit to ground?

author: Computeruser
author_rank: Regular Contributor
timestamp: September 08, 2011, 02:04:30 am
content: It can be called "common" but it is not ground unless you ground it.So in a printed circuit you sometimes see (I sometimes see) a band of conductor surrounding the board (or most of it). The circuitry is inside this band. Then the battery negative might be connected to that band and that would be the common terminal.From there, you can (usually) ground the common terminal if you wish. ... C

author: IanB
author_rank: Super Contributor
timestamp: September 08, 2011, 02:14:36 am
content: There is no such thing as "ground". The whole thing is a common fallacy and very misleading.When you take an elementary physics class you learn that electricity flows in a circuit. It flows (say) from one battery terminal, through various circuit elements, and back to the other terminal. This was Truth. Later on, clever people corrupted the truth with imperfect lies.What happened was that electronic engineers decided that there were too many lines cluttering up their circuit diagrams. So they decided to arbitrarily assign one point in the system a potential of zero and call this "ground". This allowed them magically to erase many of those annoying lines. Any point in the circuit connected to this point of zero potential could be replaced with the ground symbol.Personally I find the whole thing very annoying, and it totally plays up with the obsessive compulsive tendencies to neatness within me. I want to see the current path, not a load of silly ground symbols!There are many extra subtleties about this that would take too long to explain, such as for instance that not all "grounds" are the same ground. This is because a plain circuit diagram cannot convey every piece of information about how a circuit should be built, especially at higher frequencies.But anyway, to start with, just assume all points with a ground symbol are joined together with wires keeping them at the same potential. And if you don't want to draw ground symbols on your circuits at all, you will get a great cheer from me!

author: Unknown
author_rank: Unknown
timestamp: September 08, 2011, 02:21:31 am
content: Quote from: IanB on September 08, 2011, 02:14:36 amThere is! And I'm standing on it.Ground potential there is an entirely different argument. In a floating circuit ground is not ground but the 0V reference in the OPs simple circuit any reference to ground actually means 0 Volts. IE: the supply negative terminal.

author: IanB
author_rank: Super Contributor
timestamp: September 08, 2011, 02:23:04 am
content: Quote from: ABCD on September 08, 2011, 01:59:40 amI realize I didn't answer your question. The answer is that "ground" is any point in the circuit you wish (as long as you are consistent about it). For instance if we look at car electrical systems in Britain, sometimes ground was the positive battery terminal and sometimes it was the negative battery terminal. (And sparks would fly if you forgot which was which!)To tie a circuit to ground means to choose some arbitrary point that you will call ground potential, and then to ensure that all points in all circuits that are supposed to be at this mystical ground potential are electrically tied together.

author: IanB
author_rank: Super Contributor
timestamp: September 08, 2011, 02:27:56 am
content: Quote from: Uncle Vernon on September 08, 2011, 02:21:31 amAh, but in the same way there is no such thing as "ground", there is also no such thing as "0 volts". Voltage measures the difference in potential between two points. There is no absolute voltage. So with a given battery you could say that one terminal is at 0 V and the other at +9 V, or you could say one terminal is at -9 V and the other is at 0 V. Or you could say one terminal is at +4.5 V and the other terminal is at -4.5 V. All statements are equivalent.

author: ThePranksta
author_rank: Contributor
timestamp: September 08, 2011, 06:42:09 am
content: As IanB puts it, Ground is only a reference point in simple circuit diagrams. Keeping with industry standard (at least in my opinion) usually the ground reference is set at the power supply anode (in the case of a alkaline cell the negative terminal).

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 08, 2011, 08:30:42 am
content: it is a concept to make engineering simpler. as a kid, we liked to play with positive number only. influenced by that experience, we as adult will try to make the lowest possible potential as a "ground" hence in calculation, we only deal with positive potential (volt). as others said, you can set any point as a ground (even +ve battery terminal, if you like negative number, which i also usually got confused).

author: ABCD
author_rank: Contributor
timestamp: September 08, 2011, 11:08:43 pm
content: Just to beat this horse a little more, if I understand in the case of the battery circuit the negative terminal serves as the common reference. What I don't get is what tying it to ground would do electrically. I think I understand that it would serve as a common reference, but the negative terminal is already doing that, right? The thing that got me all confused was the circuit on the right in the attachment. How would the right-hand circuit differ electrically from that on the left, if at all (e.g. if I measured the voltage drop across the resistor, would there be a difference between the two circuits)?For some reason "ground" has been a hard concept to grasp, only because there seem to be so many. I understand that to get current flowing, you need a potential difference, and that potential difference has to be referenced to some level so that you can determine how much of a difference you're dealing with. But when looking at circuits and in discussions, I see ground being used in many different ways.

author: amspire
author_rank: Super Contributor
timestamp: September 09, 2011, 12:06:43 am
content: The ground symbol doesn't do anything electrically.It has a few purposes.First it is very convenient to pick one node of a circuit as a reference node for the rest of the circuit. Otherwise, you can't even speak about supply voltages in a simple way.Say your battery was a 9v battery. Is the supply voltage +9 V or -9 V? You cannot say unless you have picked one node to be the 0V ground point. You could pick any point of the circuit, but it makes sense to pick the most obvious "common" part of the circuit to be the ground.It gets even more confusing if you have several power supplies. Say a circuit has a 15V supply, a -15v supply, a 5V supply and a 3.3V supply. The only way you can label the supply rails is if you have picked one node in the circuit to be the 0V ground. If you don't, then how are you going to label the supply voltages? The +15 rail is +30V if you measure it against the -15V rail, and it is also +10 V compared to the +5V supply, and +11.7v compared to the 3.3V supply. Very confusing, if you haven't defined one point in the circuit to be 0V.The second use of the ground is when you have multiple schematic diagrams that all have to work together, you have to know how the two circuit connect. When you see the grounds symbol on each diagram, you instantly know that the grounds on each circuit are connected together.The ground symbol usually means that that part of the circuit is connected to mains ground, but you will see lots of circuits using the ground symbol that have no connection to mains ground - eg battery powered devices. One reason people like using it is you only ever have one ground in a circuit - you will never have two different grounds in your circuit at two different potential. When you choose to use the ground symbol, it does definitely imply that that point of the circuit can be safely connected to mains ground without affecting the circuit in any way, and this is definitely useful to know. If the circuit cannot be connected to mains ground for some reason, you would choose a different circuit symbol to indicate the circuit's 0v reference point.Now you will have circuits that have isolated supplies - like some lab power supply circuits, bench top multimeter circuits, etc. Each part of the circuit will have a "0V" node allocated using different symbols - sometimes the "Chassis" symbol and sometimes an inverted triangle with a number in it. But you will only pick one point of the circuit to be called "Ground".So forget any magical properties to Ground. It is a very convenient reference point for the whole circuit and it is very convenient to understand how different schematics and devices connect.When you look at a new schematic, probably the first thing you will look for is the 0V reference node of the circuit, and usually is will be indicated with the ground symbol.Richard

author: IanB
author_rank: Super Contributor
timestamp: September 09, 2011, 06:33:00 am
content: Quote from: amspire on September 09, 2011, 12:06:43 amI just want to mention for completeness that while this is the ideal case, sometimes the ideal is not met. For instance where there is a power ground and a signal ground, or where there is enough current flowing between two "ground" nodes that they are actually at different potentials. This can be an issue sometimes with voltage regulation circuits in power supplies. There are times when the circuit diagram does not tell the whole story and you have to pay attention to the layout of traces on a circuit board and the distance between connection points.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 09, 2011, 07:56:09 am
content: Quote from: ABCD on September 08, 2011, 11:08:43 pmNo,both circuits are the same.You can call some point in your circuit "Fred" if you wish,but the circuit is still the same!As previous posters have said,it is necessary to have a reference point for multiple power rails,as descriptions of the supplies need in practice to mean the same thing.A -5volt supply & a +5 volt supply are both referred to the same reference point,although a meter connected between them will read 10 volts,so you could call the two in series a "10volt"supply.(But it would confuse people!)The terms "ground" & "earth"have a historical background,from when telegraph systems were single conductor,"earth return".One leg of the supply (battery) was connected to an earth stake,the other to the single wire,via a Morse key.At the other end,a very sensitive "sounder" was connected between the single conductor & an earth stake.In theory,if we can make a really good connection to the earth,even though dirt & rocks are not good conductors,the cross-sectional area of this grotty conductor is huge,so it will be a low resistance path between any two points.The problem is in getting this good connection!It made sense for old time Techs & Engineers to read voltages with respect to the "earth connection",& eventually the terms "ground" & "earth" became synonymous with the return conductor, even if there was norealearth connection.When 2-wire balanced systems came into use,the connection of "return conductor" & "ground" on the telegraph lines became lost,but it remained in use on the terminal equipment."Ground","common","reference","earth",are all terms that have been used to mean the same thing,although "earth" is more recently,usually reserved for actual physical connections to the Earth.You can declare any point in your circuit to be "ground",but it most commonly refers to the negative or positive supply rail in electronic circuits.The standard in many older British circuit diagrams was to have a "Supply rail" drawn along the top of the drawing,& a"ground" or "common" rail at the bottom,with the circuit drawn between them.American,Australian,& Japanese practice was to place "ground" symbols where necessary to indicate a return connection to the "common"rail.VK6ZGO

author: ejeffrey
author_rank: Super Contributor
timestamp: September 09, 2011, 08:02:27 am
content: QuoteConnecting to physical earth has essentially no effect on your circuit. It is mostly a safety thing. Connecting your circuit signal 'ground' (i.e., reference voltage) to the household wiring ground conductor does two things: it connects you to the physical earth, and it connects you to every other device in the house. This makes sure that your circuits signal reference is the same (or at least close) to that of other devices in the house.

author: Jimmy
author_rank: Regular Contributor
timestamp: September 09, 2011, 08:28:23 am
content: Grounding is done to Earth to keep everything referenced from earth. This is to stop height floating voltages from ground.

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2011, 04:32:42 pm
content: Yes, earth ground is irrelevant in a portable battery powered circuit.Earthing is only important in a fixed installation where lightning strike, accidental connection to the mains or an earth fault could cause the circuit to float at a much higher voltage relative to earth than intended. An example is a three phase generator with a neutral: suppose the nominal phase to neutral voltage is 230V, the phase to phase voltage will be 400V, if the neutral isn't earthed and a phase is shorted to earth the other phases will float at 400V, and the neutral at 230V. Even worse, if someone connects a 1000V step up auto transformer to the generator and the phase of the secondary is shorted to earth, the other phases will float at 1400V which could damage the insulation resulting in electrocution and fire. Earthing the neutral would ensure it stays near 0V and the other phases 230V.