Talk me through this simple circuit? - Page 1
Forum section: electronics / beginners

author: Mr D
author_rank: Regular Contributor
timestamp: May 30, 2019, 08:24:54 pm
content: Hi folks,I'm really a beginner!I'm trying to get a foothold, and i found this simple looking circuit, and i was wondering if anyone would like to talk me through it.So let's start at the beginning with the power supply. The placement of the ground means that the voltage rails to the OSC section are +-4.5V.But does this matter? What would have been wrong with 0-9v?Could R2 & R4 have been replaced with one 20k resistor?Also, the LED will get more or less bright depending on the load downstream? How does one think about this or calculate it?Thanks in advance!

author: rstofer
author_rank: Super Contributor
timestamp: May 30, 2019, 08:35:25 pm
content: Quote from: Mr D on May 30, 2019, 08:24:54 pmIt matters if the intent is that some signals swing + and - around ground. An audio signal, for example. The signal could as well swing around 4.5V and use blocking capacitors to remove the DC offset. Either way...ETA: I didn't see the Op Amp on the first go-around. The op amp doesn't care whether you power it with +-4.5V or 0-9V but the datasheet for the 741 shows a minimum voltage of +-10V (20V total) on the power pins. I'm wondering...QuoteWell, sure, the brightness can change depending on load. The LED is across the battery and we assume, when first looking at the circuit, that it is a perfect voltage source. It puts out 9V no matter the load.But it isn't a perfect source, it has internal resistance and when the load increases, the internal resistance drop more voltage. Also, the battery voltage falls as it is discharged. The arithmetic for determining voltage versus load as impacted by source resistance (inside the battery) is a little more involved than just talking about it.The entire purpose of R2 and R4 is to create an artificial ground so that signals revolve around 0V.

author: rstofer
author_rank: Super Contributor
timestamp: May 30, 2019, 08:37:36 pm
content: If you were going to go with 0-9V, you wouldn't use R2 and R4 at all. You would just call the bottom rail ground and the top rail +9V. That may or may not be helpful.ETA: You would need to think through what happens if you don't create the +-4.5V/Gnd reference. I'm not convinced the circuit will work as drawn but I think the artificial ground is required. This kind of thing (rail splitter) is done all the time with single supply op amps and that is exactly what is happening here. The author is taking a bipolar op amp and running it on a single supply.

author: Mr D
author_rank: Regular Contributor
timestamp: May 30, 2019, 10:20:57 pm
content: Many thanks, there's a lot for me to mull over there.But first this concept of ground, I'm still really struggling with.So from my other thread from a few days ago i think i understand what ground conceptually is: basically a single wire (or node), and wherever a ground is indicated on the circuit, there's a connection to this wire or node. (just to be sure, all grounds in all 3 pix are attached to the same node, right?)But how does one decide: "ok, we need to connect to ground here, here, here and here"? And surely doesn't adding one more ground connection totally mess up the whole circuit? Because aren't you adding a new path down which the current can flow, having a knock on effect around your whole circuit?!And to zoom out even more: is there some sort of technique one can use to design or analyze a circuit, working backwards?I mean: you start with the end goal (in this case to output a square wave at variable audio rate frequency), and then work back from there, designing (or analyzing) what components go where and why?(i still want to come back to the specifics of your reply, but these are the things that are bugging me the most right now!)EDIT: i've been playing with EveryCircuit for a few minutes, and i think something just clicked: you use all those grounds in your circuit to persuade current to flow where itotherwise wouldn't want to flow? And then you use resistors in the circuit to balance the whole circuit out, speeding or slowing this flow back to ground?

author: virtualparticles
author_rank: Regular Contributor
timestamp: May 30, 2019, 11:04:03 pm
content: A ground is a common node in the circuit as you surmised. If your circuit is driven with a single +9V supply, the ground could attached to the negative terminal of the battery and we would call it 0V for reference purposes. We often then make the rash assumption that all connections to that ground and the implied connections between those grounds are all superconductors. In actual fact, there will always be some small voltages between any two ground nodes depending on the resistance and the amount of current flowing into each ground node. Frankly, it's just cleaner on a schematic to use a ground symbol rather than showing all the connections that go to a single wire which goes back to the negative side of the battery. But you can do that if you like and never use the ground symbol.As a practical matter, it is common to build a circuit on a PC board that has all copper on one side. If you use that as your ground, you can attach things to it and "skywire" a circuit together.You asked how to work backwards from a requirement to an implementation. This is what electronic design is all about and it takes a little experience and analyzing other people's circuits to learn the ropes. To generate a square wave, you would need a switch. That could be a biploar transistor or it could be a FET. Either one could do the job. I would suggest you load "LTSpice" from the Analog web site and experiment with transistor circuits to get the hang of it.

author: virtualparticles
author_rank: Regular Contributor
timestamp: May 30, 2019, 11:07:46 pm
content: Oh, one more thing. You asked about resistors. The first thing they teach in electronics is Ohm's law. Look that one up and then see what happens in LTSPice if you put two resistors in series and apply a voltage to one end and ground the other end. Measure the voltage in between the two.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: May 30, 2019, 11:22:21 pm
content: Quote from: Mr D on May 30, 2019, 10:20:57 pmVoltage is just a potential difference between two points. A voltage source forces it on these lines and polarity obviously defines which direction the current is gonna flow. These potentials are to be seen relative to each other, not necessarily absolute.Some circuits make use of a virtual ground using a voltage divider (R2 and R4 in your first schematic), which can be made to tap only a part of that voltage potential and reference signals from there. This works, as long as it is not in conflict with other reference potentials - meaning to design a short in the circuit, which usually exceeds the current limit of the source.QuoteUnless marked otherwise, yes. Same symbol usually indicates the same signal. It might get a bit tricky if circuits should be electrically isolated from each other, using a transformer or similar, and the same symbol is used on both sides.QuoteGround is more or less just a naming convention for a voltage level or signal, and it needs to be supplied wherever you need it to be referenced from. You could also reference from somewhere else, but all parts have limits to operate within and therefore you can ensure to stay within limits by using this known reference.QuoteThe schematics are just an abstraction of the real circuit and drawing a ground (or any other) symbol is more or less just a shortcut to declutter its graphic representation from lines that go across the diagram. Such labels can be anything from text to symbols, with some industry standard so others understands what it means.In other words these connections are meant to be connected to the same conductor. And yes, current flow does impact several things in the circuit, but it is the job of the designer to figure out (first by calculation, later by testing) how the result will stay according to the intention/specification - e.g. by sufficiently sized conductors (trace widths, wire size) or component selection to withstand heat dissipation.QuoteThere´s usually more than one way to do a certain task, for "typical" problems there are some circuits more popular than others and you can find them collected in electronics books, forums or magazines. The other question is if this is the only function and which other functions you might also need and therefore share some parts of the circuit with those.As hobbyist you usually try to stick to the parts you have at hand or can work with comfortably, which means you might prefer certain solutions over others, requiring different parts, have to do the calculations and end up with a working device.

author: Mr D
author_rank: Regular Contributor
timestamp: May 31, 2019, 07:51:33 am
content: Thanks............Here's what i'm still struggling with in terms of ground:Attached are two pix. In the first pic, with have a leg of the circuit going nowhere, so no current through the 5k resistor.In the second pic, i've grounded that leg, so current can flow.Hence my understanding that ground can be used to make current flow where it otherwise wouldn't want to flow.But by this logic one could have multiple different "ground" planes, not connected to each other. But this isn't something i've seen, therefore i surmise this isn't actually the purpose of ground.Ok, so ground is often (also in this thread) described as being used as a "reference". This is the part i don't understand. When i think of "reference" i think of an intelligent agent referencing some information from a user manual or wikipedia or something like that. But i have no clue as to the meaning of the word "reference" as it's being used in this context.Could someone please give me an ultra-simple example (maybe an EveryCircuit circuit) of such "referencing" taking place? (With an explanation)

author: timelessbeing
author_rank: Frequent Contributor
timestamp: May 31, 2019, 08:15:41 am
content: you have current in the 5k resistor because it's connected to the negative of the voltage source.no connection, no current

author: SparkyFX
author_rank: Frequent Contributor
timestamp: May 31, 2019, 10:03:48 am
content: Quote from: Mr D on May 31, 2019, 07:51:33 amThe reference being talked about is where the other probe is attached, from which point to which other point a voltage potentialdifferenceis being seen/measured. Without that, a voltage potential simply "floats", it would not have a base (hence "reference"), it would not be possible to measure/quantize any difference from a single point.A voltmeter measures a potential difference, e.g. if the probes are reversed the reading has opposite polarity. But it requires both to be connected somewhere, otherwise the air gap to the other terminal would be part of the measurement circuit, but it´s resistance is usually too high and the conductivity too low, so no current flows to make a measurement (actually electromagnetic stray fields nearby are what dominates a reading in such cases, making it impossible or at least very hard to differ it from the reading you actually wanted to take). This is where ideal voltmeters hit reality.The 500mV on the non connected leg of the 5k resistor are there, because the current required for an ideal voltmeter would be zero, the resistor therefore has no influence on the voltage ( U = R / I , for very little current ), almost as if it was not there, therefore having the voltage potential of the other leg.Even a real voltmeter draws very little current (it has very high input impedance/resistance, because it would influence it´s own reading otherwise), so you have a 5k resistor in series with a much higher resistance, which limits the influence of the 5k resistor on the measurements a lot.Remember that in the moment a voltmeter is connected, the leg is actually connected to something.

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 31, 2019, 11:03:11 am
content: As others have said, Voltage is just potential difference between two points. The concept of "ground" comes literally from the earth - IE one connection of the power supply is connected to a metal rod in the ground, and that becomes the reference point for voltages - so the ground you stand on is literally the reference point. Now, in small circuits that beginners mess about with, many are battery driven, and there is no connection or reference to ground. But We usually, if it's a single voltage supply take the negative pole as being ground. In multiple rail supplies (like your circuit which has +4.5V and -4.5V) it gets more complicated. You could look at your circuit as being +4.5 0 -4.5, or as 0 4.5 9 , depending on what we decide is "ground". The reason they choose the midpoint in this case, in an oscillator, is to get the idea of positive and negative voltages, and the main reference point of the circuit has the most connections, and Op Amps are conventionally run from a positive and negative supply. (That's not always the case, but common in older circuits and audio circuits). Also, the equipment you connect to, may be grounded (amplifier for example), and it may provide a path to ground via it's PSU and circuit reference point. I'm probably over complicating it now of a beginner, but in general, the negative in single rail supplies, or the midpoint in dual rail supplies are mostly (but not always) considered ground.The first pic of your circuit, is a wee trick to generate a dual rail supply (+-4.5V) from a single +9V supply, plus an LED with current limiting resistor for power indication. The other two resistors are creating the "Ground" of the dual rail supply for the oscillator circuit in the next pic. We would call that a "Virtual Ground" in this case. Compared to the negative of the 9V battery, it sits at 4.5V, but the rest of the circuit references off that line, and if properly grounded via a wire to actual ground or through other equipment connected to the output, its suddenly seen as 0V, and the battery negative is -.4.5V in respect to ground, and the positive is +4.5V.

author: Mr D
author_rank: Regular Contributor
timestamp: May 31, 2019, 09:01:01 pm
content: Hi folks,I believe i understand ground and virtual ground etc.The piece of the puzzle i'm still missing is this:I keep reading about the ground being needed to "reference" to.Could someone please give me a ultra simple example of why this is needed in a circuit and how (and what) components or parts of circuits do this referring?Like i said before, the word "reference" suggests to me a thinking agent that will "refer" to something, like we might refer to a user manual or a wikipedia article.But an electronic circuit doesn't contain thinking agents, so how can anything in the circuit "refer" to anything else?This is (i believe) the only part of this ground story that i'm still having trouble with.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: May 31, 2019, 09:20:09 pm
content: A bit of English misunderstanding there I think.Here we use the noun form of the word "reference", not verb."The use of a source of information in order to ascertain something."For example a dictionary.When you ask the question "what is the voltage?", the correct answer is "compare to what?" because it's always measured inreferenceto another voltage. The correct question should be "what is the potential difference between these two points?". That's why multimeters have two probes.You can think of voltage like your weight. When you tell someone you weigh 200lbs, they are able to get an idea of your mass. But how? Your weight is different at home from what it is in an airplane, or in orbit, on the moon etc. We can imagine 200lbs because we all live at the surface of the Earth where gravity is the same. The surface has become the "reference" point.In the same way, we can tie a segment of a circuit to Earth's electrical potential as a reference point, to which all other potentials can be compared to and measured.

author: Mr D
author_rank: Regular Contributor
timestamp: May 31, 2019, 09:42:28 pm
content: You're not understanding where my remaining difficulties lie, please re-read my previous post carefully.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: May 31, 2019, 09:54:23 pm
content: I read your post.Can you please rephrase the question?

author: SparkyFX
author_rank: Frequent Contributor
timestamp: May 31, 2019, 09:54:43 pm
content: It is not "referring to", it is "referencing of".Your reference is whatever you choose to be the base level to reference from.

author: Yansi
author_rank: Super Contributor
timestamp: May 31, 2019, 10:00:49 pm
content: Quote from: rstofer on May 30, 2019, 08:35:25 pmAs an interesting side note, UA741 seems recommended minimum 10V, while LM741 is 20V. I have never noticed this either! (have not used 741 opamps much anyway...)

author: Mr D
author_rank: Regular Contributor
timestamp: May 31, 2019, 10:33:40 pm
content: Ok, let's try it another way with a thought experiment:Imagine one believes in God.God creates an electronic device.Because He is all knowing and all powerful, He doesn't need to design or test the device.So, he never needs to analyze the design or probe a prototype with a multimeter.He simply creates it fully formed, perfect in every way. He never needs to think about where his ground might be, the circuit just works perfectly.Therefore this device doesn't need a ground, right?Well, wrong, i presume.So what i'm asking is: if one takes the observer out of the equation, what is intrinsic to an electronic circuit that it would need a ground?Could someone give me an example of a circuit component or element that would need a ground, how it would work, and why it wouldn't work without this ground reference?

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 31, 2019, 11:20:50 pm
content: Technically, any circuit doesn't need a ground. It only needs a potential difference of the right amount. A 5V circuit would work, if you connected the negative to 200V, and the positive to 205V. There's only 5V difference, and the circuit would work fine. The problem comes with us humans, we stand on the (sometimes wet) Ground. And to an electric circuit, we are just one big whopping resistor. And if you stick a fair few mA through that resistor, it kills it, it dies. When wet, the resistance goes down, and the current increases. So it's good to Reference the circuit to actual ground, to stop you getting zapped. 5V won't kill you. 205V may be a different story. The circuit only needs the potential across the two (or more ) power points to be within the right range, the problem is us humans can't help ourselves and keep poking at metal things, adding a big resistor into the circuit. And where there's potential difference, current will flow. So the whole idea of ground is to teach you not to let angry pixies (electrons) to flow through the 2 year old that poked a knife into your circuit.

author: Nerull
author_rank: Frequent Contributor
timestamp: May 31, 2019, 11:59:17 pm
content: You are probably surrounded by battery powered and double insulated circuits that have no earth ground connections at all.Referencing a circuit such that ground is 0V is just an arbitrary choice done to set the scale. There isn't any more meaning to it than that.You can think of it like a thermometer. Unless you're using kelvin, your thermometer references zero to some arbitrary point. There is no special magical property to what we chose to call zero degrees, it just is. We had to pick somewhere, and so we did. You can't say 'the temperature is 5 degrees' unless you define where zero is.Voltage works similarly, though it lacks any absolute reference at all. When assigning voltages in a circuit, you have to pick a point to make zero. It doesn't matter what part you pick. Nothing about this choice will change how the circuit functions.For ground referenced circuits, we have conventionally defined 0V at ground potential. Many circuits are not ground referenced, and we just choose a part of the circuit to call 0V. Conventionally its the primary return current path.A circuit only *needs* an Earth ground connection if it is using the ground as a conductor to pass current. Most circuits don't - their grounds exist purely to give any fault current somewhere other than the poor person touching the device a place to go.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: June 01, 2019, 12:59:13 am
content: Quote from: Mr D on May 31, 2019, 10:33:40 pmWell, the concept of ground as a symbol in a wiring diagram has been elaborated a few posts above, you should read it. The soil you are standing on has barely to do with it.There are some devices that are only supposed to work with respect to a signal calledprotective earth, namelyRCDs. They do not work as intended in isolated circuits, as they are supposed to break the connection of a circuit if too much current does not flow back to the source through the RCD, the only other path being through "protective earth". I suggest reading that wikipedia article about it, but it bears not much relevance for the circuit you initially posted.Otherwise circuits can perfectly work without contact to "the ground", except protection orEMIrequirements call for something to be connected to protective earth.Imagine flying a helicopter. The lights work without contact to "the ground", but after some hours of flying around the friction of the blades in air might have charged the whole chassis up to several thousand volts -with respect to the earth´s voltage potential. As long as the lights are only connected to "chassis ground" and the voltage source, the lamp will still work. But anything that touches the surface first will need to dissipate the charge of the chassis.I am sure you have seen planes or helicopters.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: June 01, 2019, 07:33:14 am
content: Quote from: Mr D on May 31, 2019, 10:33:40 pmIt doesn'tAnything that is battery powered isn't Earth grounded.And your amplifier circuit doesn't need to be grounded.So back to your original question.0 - 9Vor0 +/-4.5VDoesn't matter, it's the exact same thing.And this audio circuit only needs access to the signal and the zero AKA the DC reference, the common mode voltage. This "ground" (virtual in this case) is the potential around which the audio signal swings.

author: Mr D
author_rank: Regular Contributor
timestamp: June 01, 2019, 08:05:03 am
content: Let's take it back to the original circuit in the first post of this thread.Let's put aside rstofer's comments about the efficacy of this circuit, i presume it at least works to some degree.Let's place this circuiton it's ownin intergalactic deep space and turn it on.Now, this circuit contains 6 connections to ground.My question is simply this: if we cut all those connections to ground, will the circuit still work in exactly the same way?If not, why not? Could someone give me one or two example of why the components in the circuit won't do what they're expected to do without these ground connections?

author: timelessbeing
author_rank: Frequent Contributor
timestamp: June 01, 2019, 08:46:37 am
content: Quote from: Mr D on June 01, 2019, 08:05:03 amThe nodes symbolized as ground need to be connected to each other for the circuit to work. They do not need to be bonded to Earth ground, though it might help with shielding out interference.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 01, 2019, 02:39:51 pm
content: Quote from: Mr D on May 31, 2019, 07:51:33 amI used to work at a TV Station, & on occasion had to climb the tower at the Transmitting site.This tower was 145m high, so when I was replacing the topmost clearance light, I was that height above the base of the tower.(or "ground")OK, I was 145m high----- but wait! the site was about 300m above sea level, so was I (145+300) metres high?So the question is which?Well, if we are referring to the ground at the base of the tower, it is 145m , if we are referring to the ground at the very point where the beach meets the sea, then I am 445m above ground.Of course, the reference for height doesn't have to be the ground, either.The CH2 tower is a bit further up the hill, & is about the same size as our one was, so I may have been about minus 4m with reference to the top of that tower.You can see that the same vertical position in space can be different heights, depending upon where you measure them from.You will probably have seen the initials "w.r.t"----these mean, either "with reference to" or "with respect to" which are just different ways of saying the same thing.On top of the tower, I was 145m high w.r.t the tower base, but I was 445m high w.r.t. sea level, & around minus 4m "height" w.r.t. the top of CH2's tower.In all cases, a word which could describe the place my height was measured from is a "reference".As in height, so with voltage.In your original schematic, using the point you put a "ground" symbol on as your reference, the positive terminal of the battery is 4.5v w.r.t. your reference point.The negative terminal of your battery now becomes -4.5v w.r.t. your reference point.Nothing has happened to your battery, if you ignore the point with the ground symbol, & hang your meter across it, you will still read 9v.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 01, 2019, 03:29:37 pm
content: But wait!---- There's more!Back in the day, there were two schools of thought on grounding.One, which I might call "The British way", drew schematics with a lne at the bottom normally called the "earth rail" (Another term for what others called "ground") & one at the top called the "supply rail".The top one was usually the positive side of the supply, & the bottom one was usually the negative side of the supply.(not always, though, as with early PNP transistor circuits, the connections were reversed)The supply side of each circuit component ultimately was connected to the top "rail" & the ground side to the bottom "rail".This showed clearly where all the "ground" points were connected together, but was a bit unwieldy.The other school of thought was what I might call "The American way",With this method, the "ground" was indicated by a special symbol, which meant that all points which had this symbol were connectedJust to show that they weren't complete "party poopers" the Americans often drew simpler schematics in the British style.European schematics were usually "somewhere in between".Ultimately, the "American way" won out, as, with the advent of multiple supply rails, the other way became very messy to implement.Unfortunately for "noobs", it doesn't "lay it on the line" as to how ground connections work, like the old "British way" did.

author: Nerull
author_rank: Frequent Contributor
timestamp: June 01, 2019, 06:05:19 pm
content: Quote from: Mr D on June 01, 2019, 08:05:03 am"Ground" in a circuit without any actual earth-ground connections is just a shorthand. It represents an invisible wire connecting all the 'ground' points together, just as if you have multiple power nodes named 'Vcc' you would assume they are connected.This is the exact same circuit, drawn two different ways:We have chosen, by convention, to call the common return path in DC circuits "Ground", regardless of whether it has any real connection to earth ground. It really doesn't mean anything more. It seems like you are vastly overthinking this.

author: DimitriP
author_rank: Super Contributor
timestamp: June 01, 2019, 09:41:57 pm
content: QuoteBut...you can only simulate one of the two. The one on the left.

author: Mr D
author_rank: Regular Contributor
timestamp: June 01, 2019, 09:55:34 pm
content: I don't agree i'm overthinking it, i'm simply trying to get a clear picture of what is actually meant by the ground symbol in a circuit schematic!You mentioned the "return path", this is the first time anyone has called it that in this thread and that makes things a bit clearer.I think part of my confusion is that there are two things under the umbrella term of "ground", and they seem (to me) totally unrelated.Firstly you have the safety issues and the connection (for whatever reason) with the real ground, chassis or whatever.....Secondly you have this return path, whose purpose is sometimes to close the circuit (although not in my circuit example in this thread) and give you a short cut route for any component in the circuit to a common point of lowest potential, typically just before the negative terminal, or if you're using a rail splitter, to that point in between the two resistors.So in this usage, the word "ground" could be substituted with "return path".And if we were only talking about circuits in intergalactic space, the term "ground" would be a meaningless and confusing obfuscation.Once these circuits are returned in a spaceship to planet earth, we could happily continue using the "return path" nomenclature, adding, when necessary, "this point needs to be connected to the chassis or real ground or whatever".So could someone look at the circuit in the first post and choose one of those connections to ground (apart from the rail splitter connection) and explain why, at that point, the designer said to himself "right, here we need to connect to the return path"? (he probably thought of it as "ground")

author: timelessbeing
author_rank: Frequent Contributor
timestamp: June 01, 2019, 10:53:32 pm
content: In the amplifier circuit, the ground is connected to the chassis.Also note, "return path" implies negative polarity, which is not always the common or ground node. For example, on old cars, the chassis and engine block were connected to battery positive. In that case the chassis acts as source rather than return.Also, note that "return path" doesn't make sense in AC circuits, where current reverses direction in each cycle. In differential circuits, you have positive and negative signals which are referenced to neutral which is grounded.

author: DimitriP
author_rank: Super Contributor
timestamp: June 01, 2019, 11:42:23 pm
content: QuoteNot for earth people. (pardon the pun)All those terms, no matter what they are called mean what they mean in the context they are being used.A flashlight uses a ground connection. It won't make sense until it makes sense.Analyzing your post makes obvious two issues.The first issue is your use of the circuit simulator. Ditch it.The second issue is not understanding the concept of single vs dual rail power supplies.QuoteAs for that first circuit.....I'm not sure what you mean by "apart from the rail splitter connection".There are three connections available +4.5V , ground, -4.5V

author: LateLesley
author_rank: Frequent Contributor
timestamp: June 02, 2019, 05:56:58 am
content: Well, another name that gets used for ground is "common". It is the common point that all the other rails are "referenced" to. So all those ground symbols in the circuit are wired, or commoned, together. If you start disconnecting them all, the circuit won't work, as there is then no path for the electrons to flow. Remember circuits need to complete the loop to work. In most cases, the ground, or common is connected to the chassis (well at least in metal boxed equipment) and that got used as the reference point to measure the other voltages in the circuit. Some of them may have been positive in respect to the chassis ground, others may be negative.Anyway, the upshot is, yes those grounds all need connecting together, and if you just start disconnecting components of the circuit from it, then it won't work as designed, as you'll have taken that component out of circuit.

author: virtualparticles
author_rank: Regular Contributor
timestamp: June 04, 2019, 05:50:47 pm
content: Yet another way of thinking about a "ground" is to simply consider it to be a "net". It is common on a schematic to label a node with a "net" name, like "Vout-1". On the schematic it might be shown as a line that ends in a flag shape with the net name inside it. It might also be just be a small circle on the end of a line with the net name written next to it. Somewhere else on the page another circuit might use that output as an input and the "net" name will be repeated and a wire will be shown going from the net to the second circuit. This is done to reduce clutter on the schematic. Functional blocks are fully represented with labeled ins and outs, nets. "Ground" can be considered to be just another net.Some circuits on here show a direct connection to the battery. One could easily have the battery symbol sitting off by itself with a ground symbol on the negative and a "+V" net name attached to the positive side perhaps with an arrow symbol. Then whenever the positive voltage is needed in the schematic one simply draws a little arrow and labels it "V+" and you have an implied connection. The same is true of the ground but the net name isn't required because the symbol itself is unique.

author: Mr D
author_rank: Regular Contributor
timestamp: June 04, 2019, 08:33:06 pm
content: Hi folks,I feel like i'm still banging my head against a wall, i just don't think anyone is understanding what i mean!Let me try, at the risk of boring you all to tears, one more time, after which i'll shut up about it and move on.What i'm not getting is whether there is anything SPECIAL about ground in a battery powered circuit where there is no physical earth to contend with, and no observer to worry about a reference.Under these limited conditions, i can see no reason not to substitute the word "ground" with "piece of wire".So if you agree with that (i'm sure you won't), surely a circuit could have multiple "pieces of wire"?In the attached pix i have drawn the same (useless, do-nothing) circuit in two different ways.In the second pic i have used two (yes, count them, 2!) grounds, on the basis that "ground" can be substituted with "piece of wire".So to be clear, in the second pic i'm assuming that ground 1 & 2 are not attached to each other.(EDIT: i'm also not expecting the two versions of the circuit to perform the same way in EveryCircuit, i was just using it to create the images)So is it correct that if we were only dealing with this limited case, you could create as many seperate (unconnected to each other) "grounds" as you might like?Before you answer please remember i'm talking about a circuit in intergalactic space where no-one will ever observe or measure anything!

author: hamster_nz
author_rank: Super Contributor
timestamp: June 04, 2019, 10:00:07 pm
content: Completely unscientific here... by convention, for battery powered devices, even when a design doesn't have an external Ground/Earth connection, GND is now usually the negative terminal of the main battery.One of the first things I do when looking at a schematic is "how is the power supply connected to the GND symbol".It wasn't always the way, if you look at old schematics from the days before EDA & CAD tools if they do have a GND it is the physical external connection, and is conceptually different from the negative power rail, even if it is connected directly to it. Most schematics for battery-powered pocket radios usually doesn't have a GND at all.It is now not uncommon to have multiple GNDs in a design - for example, an analog GND and a digital GND, but you pretty much be sure that they are solidly connected together at some point, unlike your schematic where the two GNDs will be at different potentials.

author: LateLesley
author_rank: Frequent Contributor
timestamp: June 04, 2019, 11:16:27 pm
content: If any electrical or electronic engineer looked at the second circuit, they would assume that all the ground symbols were connected together. Multiple grounds get complicated, though some circuits do have them. The thing they have in common, USUALLY (but not always), is they are the 0V point in the circuit. It is usually the point where you would measure other voltages from, so is assumed to be 0V.So your second circuit would differ from the first, as the first circuit would have voltage on that line with 3 resistors, but in the second circuit, it would be read as being connected to the other grounding points in "Ground 1", or at least at the same potential IE 0V.But there is nothing "special" about it, it IS just a "piece of wire" as in your analogy. The only thing special, is you don't want to be touching it if you touch something else with say 100V on it. otherwise you may get a tingle.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: June 05, 2019, 03:46:09 am
content: Some pieces of wire CAN be at ground. But a piece of wire carrying an audio signal, for example, cannot be ground.The second circuit is wrong. You cannot have two grounds at different voltages in the same circuit. There is one ground (0V) and the other is something else.Usually, convention is that the negative would be ground.

author: virtualparticles
author_rank: Regular Contributor
timestamp: June 05, 2019, 10:03:53 am
content: Once again, the ground symbol is a net. The grounds you drew in the second circuit would ALL be connected together unless you used a different symbol for the "ground" on the right side. The symbols are important. They have meaning.

author: pianovt
author_rank: Newbie
timestamp: June 05, 2019, 05:31:22 pm
content: QuoteWe understand you better than you can imagine. Below is a detailed response to each step of your thinking:QuoteDo that, it's correct.QuoteThis, too, is correct.QuoteThis is where you joined the dark side. Work on this last step of your reasoning. You have made the following logical error:A. Ground is a piece of wire.B. I can arbitrarily define any piece of wire in a circuit to be called "ground".Therefore: I can call all the wires in the circuit "ground".The fallacy is that you can call "ground" only one node in a circuit. The word "node" is very important. A "node" consists of all the places in the circuit which are connected to each otheronly with wires and nothing else. The ground symbol represents a wire. You are free to pick which node you want to call ground, but you get to pick only one.QuoteYou have just violated the premise in the beginning of your post. In that premise, you said the following:QuoteYou said that the ground symbol represents a piece of wire. Then you connected the twonodesin the second circuit together using the "ground" symbol, yet you say that the two grounds are not connected to each other.If you decide that you want to call some other node in your circuit "ground", then you will have to rename your original ground and give it a different name. Specifically, in your second circuit, you have placed a short circuit across two of the (many) 1k resistors. You "shorted" two nodes.Make a habit of using symbols like R1, R2, ... Rn instead of just values. In your second circuit, I have no way of referring to a specific resistor unless I draw a circle around it and post that picture.

author: Mr D
author_rank: Regular Contributor
timestamp: June 05, 2019, 07:20:38 pm
content: OK!You almost understood what i meant, but not quite, sooooooooo........... :Here are two more pictures.Would you agree that, in intergalactic space, with no observers within a lightyear distance, the two circuits are equivalent and the choice to place the grounds where they are is completely arbitrary?........and in fact no ground need even be indicated in the diagram (as long as the ground symbols are replaced with a wire)?And just to be crystal clear: someone might ask: "if there no observers, why would you even need the diagram?". So let's say for the sake of this thought experiment that a robot needs to mass produce the circuits and will need the diagram for that purpose.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: June 05, 2019, 07:45:06 pm
content: Quote from: Mr D on June 05, 2019, 07:20:38 pmYesQuote from: Mr D on June 05, 2019, 07:20:38 pmInthis particular circuit, functionally it is arbitrary where you put ground. In AC and RF circuits, not so much. You can read about the concept of the "ground plane". As was explained, we do have conventions about where we put ground. Not to mention that thoughtful choice of ground location can make calculations easier.Quote from: Mr D on June 05, 2019, 07:20:38 pmIn this instance yes. In other situations connecting high voltage to chassis ground can be lethal.

author: Mr D
author_rank: Regular Contributor
timestamp: June 05, 2019, 08:02:08 pm
content: Thanks, we're now getting close to it being straight in my head.But still just one more thing:Quote from: timelessbeing on June 05, 2019, 07:45:06 pmAre you saying that in intergalactic space where a robot had to assemble a circuit from a diagram, some circuits (AA, RF?) wouldn't work in the same way if the ground symbols were replaced with a wires leading to a common wire, and some other bunch of nodes that lead to a common wire in the circuit was designated as ground? So, in the way i indicated in my two last pix?This is the nub of what i'm trying to get at: when there's no observer to reason about it, that ground as indicated on a diagram, is completely arbitrary in any closed, not actually physically grounded circuit!

author: timelessbeing
author_rank: Frequent Contributor
timestamp: June 05, 2019, 08:18:13 pm
content: Yes, that's what I said and I told you where you can read about it.https://en.wikipedia.org/wiki/Ground_plane

author: Mr D
author_rank: Regular Contributor
timestamp: June 05, 2019, 08:24:36 pm
content: OK, that's a special case where the ground is more than a wire, it has some specific physical property required for the circuit to work.So anyway, apart from that, i guess the answer to my last question was "no"!.Thanks all for your patience, i'm going to move on now, but i'll be back soon with some infuriating questions about op amps!

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 05, 2019, 08:27:06 pm
content: Quote from: Mr D on June 05, 2019, 08:02:08 pmOk, Ground point is not completely arbitrary in any real circuit. It is intended to be the point at zero volt potential. This is important when you are actually going to connect one circuit to another (think: one piece of equipment to another). Even in intergalactic space this must be done non-arbitrarily. Also, in any analog circuit using a dual power supply (e.g. +-15volts) the ground is almost always chosen as the zero volt point at equal potential to the positive and negative supply because this maximizes various specifications, e.g. for an operational amplifier CMRR, voltage swing, etc. etc.It will also become important in intergalactic space when one arbitrary circuit is grasped with one hand and the other grasps another arbitrary circuit at a potential difference of more than 40 volts. Perhaps.

author: Mr D
author_rank: Regular Contributor
timestamp: June 05, 2019, 09:08:33 pm
content: You've set the cat amongst the pigeons there, mate!But i don't think it really has a bearing on the basic fundamental that i'm trying to grasp, as any of the things you described could be considered as actually just being part of the same circuit!Sure, once you insert a reasoning agent into the thought experiment that needs to decide: "how do i connect this circuit to that circuit to get XYZ result", i guess it becomes an issue?!

author: larrybl
author_rank: Regular Contributor
timestamp: June 05, 2019, 09:40:32 pm
content: What i'm not getting is whether there is anything SPECIAL about ground in a battery powered circuit where there is no physical earth to contend with, and no observer to worry about a reference.Under these limited conditions, i can see no reason not to substitute the word "ground" with "piece of wire".Ground can also be a metal chassis as depicted in this lawn mower diagram.

author: larrybl
author_rank: Regular Contributor
timestamp: June 05, 2019, 09:56:06 pm
content: I meant to add in the above diagram that there is "not" a wire connecting all the connections on the right side. Most all ground connections are connected to the chassis.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: June 06, 2019, 02:05:26 am
content: Quote from: larrybl on June 05, 2019, 09:40:32 pmThat doesn't sound right to me. I think that is has nothing to do with the power source. Take for instance a portable radio. The antenna is a piece of wire, but it cannot be "ground". If you connect it to the chassis then surely the radio won't work right. The same goes for various other parts of the input circuit.

author: hamster_nz
author_rank: Super Contributor
timestamp: June 06, 2019, 02:50:38 am
content: Quote from: timelessbeing on June 06, 2019, 02:05:26 amAre you sure? my cell phone (and maybe even Walkmans) use the GND of the earphones as the FM antenna...If you want to see how it is most likely done...https://electronics.stackexchange.com/questions/273469/separating-an-fm-antenna-from-a-topology-which-uses-headphone-ground-as-antenna(spoiler: there is an inductor between the headphone ground and the design's local ground).