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Offline Mr DTopic starter

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Talk me through this simple circuit?
« on: May 30, 2019, 08:24:54 pm »
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!



« Last Edit: May 30, 2019, 08:29:00 pm by Mr D »
 

Offline rstofer

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Re: Talk me through this simple circuit?
« Reply #1 on: May 30, 2019, 08:35:25 pm »
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?


It 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...

Quote

Also, the LED will get more or less bright depending on the load downstream? How does one think about this or calculate it?


Well, 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.
« Last Edit: May 30, 2019, 08:47:50 pm by rstofer »
 

Offline rstofer

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Re: Talk me through this simple circuit?
« Reply #2 on: May 30, 2019, 08:37:36 pm »
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.
« Last Edit: May 30, 2019, 08:53:01 pm by rstofer »
 

Offline Mr DTopic starter

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Re: Talk me through this simple circuit?
« Reply #3 on: May 30, 2019, 10:20:57 pm »
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 it otherwise 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?


 
« Last Edit: May 30, 2019, 10:43:33 pm by Mr D »
 

Offline virtualparticles

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Re: Talk me through this simple circuit?
« Reply #4 on: May 30, 2019, 11:04:03 pm »
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.
 

Offline virtualparticles

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Re: Talk me through this simple circuit?
« Reply #5 on: May 30, 2019, 11:07:46 pm »
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.
 

Offline SparkyFX

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Re: Talk me through this simple circuit?
« Reply #6 on: May 30, 2019, 11:22:21 pm »
But first this concept of ground, I'm still really struggling with.
Voltage 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.

Quote
just to be sure, all grounds in all 3 pix are attached to the same node, right?
Unless 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.

Quote
But how does one decide: "ok, we need to connect to ground here, here, here and here"?
Ground 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.

Quote
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?!
The 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.

Quote
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?
There´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.
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Offline Mr DTopic starter

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Re: Talk me through this simple circuit?
« Reply #7 on: May 31, 2019, 07:51:33 am »
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)
« Last Edit: May 31, 2019, 08:13:45 am by Mr D »
 

Offline timelessbeing

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Re: Talk me through this simple circuit?
« Reply #8 on: May 31, 2019, 08:15:41 am »
you have current in the 5k resistor because it's connected to the negative of the voltage source.

no connection, no current
 

Offline SparkyFX

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Re: Talk me through this simple circuit?
« Reply #9 on: May 31, 2019, 10:03:48 am »
Could someone please give me an ultra-simple example (maybe an EveryCircuit circuit) of such "referencing" taking place? (With an explanation)
The reference being talked about is where the other probe is attached, from which point to which other point a voltage potential difference is 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.
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Offline LateLesley

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Re: Talk me through this simple circuit?
« Reply #10 on: May 31, 2019, 11:03:11 am »
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.

 

Offline Mr DTopic starter

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Re: Talk me through this simple circuit?
« Reply #11 on: May 31, 2019, 09:01:01 pm »
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.
« Last Edit: May 31, 2019, 09:06:00 pm by Mr D »
 

Offline timelessbeing

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Re: Talk me through this simple circuit?
« Reply #12 on: May 31, 2019, 09:20:09 pm »
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 in reference to 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.
 

Offline Mr DTopic starter

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Re: Talk me through this simple circuit?
« Reply #13 on: May 31, 2019, 09:42:28 pm »
You're not understanding where my remaining difficulties lie, please re-read my previous post carefully.
 

Offline timelessbeing

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Re: Talk me through this simple circuit?
« Reply #14 on: May 31, 2019, 09:54:23 pm »
I read your post.

Can you please rephrase the question?
 

Offline SparkyFX

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Re: Talk me through this simple circuit?
« Reply #15 on: May 31, 2019, 09:54:43 pm »
It is not "referring to", it is "referencing of".
Your reference is whatever you choose to be the base level to reference from.
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Offline Yansi

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Re: Talk me through this simple circuit?
« Reply #16 on: May 31, 2019, 10:00:49 pm »
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?


It 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...

Quote

Also, the LED will get more or less bright depending on the load downstream? How does one think about this or calculate it?


Well, 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.

As 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...)
 

Offline Mr DTopic starter

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Re: Talk me through this simple circuit?
« Reply #17 on: May 31, 2019, 10:33:40 pm »
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?



« Last Edit: May 31, 2019, 10:46:55 pm by Mr D »
 

Offline LateLesley

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Re: Talk me through this simple circuit?
« Reply #18 on: May 31, 2019, 11:20:50 pm »
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.
« Last Edit: May 31, 2019, 11:22:35 pm by LateLesley »
 

Offline Nerull

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Re: Talk me through this simple circuit?
« Reply #19 on: May 31, 2019, 11:59:17 pm »
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.
« Last Edit: June 01, 2019, 12:08:50 am by Nerull »
 
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Offline SparkyFX

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Re: Talk me through this simple circuit?
« Reply #20 on: June 01, 2019, 12:59:13 am »
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?
Well, 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 called protective earth, namely RCDs. 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 or EMI requirements 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.
« Last Edit: June 01, 2019, 01:01:08 am by SparkyFX »
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Offline timelessbeing

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Re: Talk me through this simple circuit?
« Reply #21 on: June 01, 2019, 07:33:14 am »
if one takes the observer out of the equation, what is intrinsic to an electronic circuit that it would need a ground?
It doesn't

Anything 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 - 9V
or
0 +/-4.5V

Doesn'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.
« Last Edit: June 01, 2019, 07:36:00 am by timelessbeing »
 

Offline Mr DTopic starter

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Re: Talk me through this simple circuit?
« Reply #22 on: June 01, 2019, 08:05:03 am »
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 circuit on it's own in 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?



 

Offline timelessbeing

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Re: Talk me through this simple circuit?
« Reply #23 on: June 01, 2019, 08:46:37 am »
My question is simply this: if we cut all those connections to ground, will the circuit still work in exactly the same way?
The 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.
 

Offline vk6zgo

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Re: Talk me through this simple circuit?
« Reply #24 on: June 01, 2019, 02:39:51 pm »
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)

I 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.

 


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