Author Topic: Very Elementary Question  (Read 9469 times)

0 Members and 1 Guest are viewing this topic.

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Very Elementary Question
« on: January 07, 2014, 06:27:26 pm »
First please excuse my stupidity for asking this question, but...I could not really find an answer.
What does "floating circuit" mean?  :-//

 

Lurch

  • Guest
Re: Very Elementary Question
« Reply #1 on: January 07, 2014, 06:28:36 pm »
In what context exactly? Although the answer is basically same I think.
 

Offline c4757p

  • Super Contributor
  • ***
  • Posts: 7805
  • Country: us
  • adieu
Re: Very Elementary Question
« Reply #2 on: January 07, 2014, 06:37:24 pm »
Is this by any chance related to the floating circuit I recently posted?

Not referenced to an external ground. The usual version of that has one terminal, with the other terminal of the simulated inductor grounded. If you want that end connected somewhere else - tough luck. This version has two terminals like a real inductor.

For example, the LM317 voltage regulator is a floating regulator. It doesn't see ground. You can use it to regulate a 500V rail if you want, as long as you respect the maximum difference across it, because it can't see what you call 0V.

Or an oscilloscope has signal ground connected to earth ground. A floating oscilloscope does not.
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline A Hellene

  • Frequent Contributor
  • **
  • Posts: 601
  • Country: gr
Re: Very Elementary Question
« Reply #3 on: January 07, 2014, 06:38:29 pm »
It probably means that the circuit's ground line is not connected to the system ground (i.e. the circuit is not grounded), being probably powered by batteries.


-George
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #4 on: January 07, 2014, 07:35:01 pm »
Is this by any chance related to the floating circuit I recently posted?

Not referenced to an external ground. The usual version of that has one terminal, with the other terminal of the simulated inductor grounded. If you want that end connected somewhere else - tough luck. This version has two terminals like a real inductor.

For example, the LM317 voltage regulator is a floating regulator. It doesn't see ground. You can use it to regulate a 500V rail if you want, as long as you respect the maximum difference across it, because it can't see what you call 0V.

Or an oscilloscope has signal ground connected to earth ground. A floating oscilloscope does not.

Yes your post reminded me of "floating" circuits.

As I understand grounding means designating one node to be the reference node relative to which other voltages are measured. In this sense, it's a theoretical means by which we can analyse and tweak the circuit operation. So "floating" means...not referenced to any node or cannot be referenced? Impossible. I looked through many circuit analysis texts but could not find anything about this term.
 

Offline A Hellene

  • Frequent Contributor
  • **
  • Posts: 601
  • Country: gr
Re: Very Elementary Question
« Reply #5 on: January 07, 2014, 07:48:05 pm »
Quote
So "floating" means...not referenced to any node or cannot be referenced? Impossible.
Well, yes, and no!

For example, a car's ground is its own chassis, where the battery's negative terminal is also connected to, and we refer to it as the 0V point for the whole car; this is the car's ground line or, simply, the car's ground.

Now, if the car chassis is not connected to another external reference point we call 'earth' for example (which is the mains ground lead besides the 'live' (or 'hot') and the 'neutral' (or 'return') lines), then the car is floating in reference to the mains power grid. Yet, the car itself is perfectly functioning.


-George
« Last Edit: January 07, 2014, 07:56:04 pm by A Hellene »
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #6 on: January 07, 2014, 07:56:40 pm »
Regardless of mains and "earth" ground, since battery operated electronics are not earthed, what's the accurate definition of floating circuit?
 

Offline A Hellene

  • Frequent Contributor
  • **
  • Posts: 601
  • Country: gr
Re: Very Elementary Question
« Reply #7 on: January 07, 2014, 08:13:16 pm »
Would it help to firstly define what a floating line is?

A floating electrical line is a line that is left externally unconnected; a line that is not tied to an output line or to a power or ground line.

For example, an unused microcontroller's input line should never be left floating; it should be tied up to Vdd or down to Vss (or to 'GND' since Vss usually has the ground potential and we also call it GND) via a resistor (which is highly recommended) or directly (which is not always recommended).

In the same way, we have a floating circuit when its ground line is not connected to the ground line of a another system.


-George
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #8 on: January 07, 2014, 08:22:40 pm »
Any academic reference please?

I don't get it.  |O
 

Offline NANDBlog

  • Super Contributor
  • ***
  • Posts: 4636
  • Country: nl
  • Current job: ATEX certified product design
Re: Very Elementary Question
« Reply #9 on: January 07, 2014, 08:42:45 pm »
Any academic reference please?

I don't get it.  |O
Then ask a teacher. We are engineers.
The problem is that you use too loosely the world "ground". A better definition would look like: A floating circuit is a circuit, which ground only has a high impedance path tho the earth.
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #10 on: January 07, 2014, 09:01:45 pm »
Any academic reference please?

I don't get it.  |O
Then ask a teacher. We are engineers.
The problem is that you use too loosely the world "ground". A better definition would look like: A floating circuit is a circuit, which ground only has a high impedance path tho the earth.

So engineers don't know?
 

Offline AG6QR

  • Frequent Contributor
  • **
  • Posts: 826
  • Country: us
    • AG6QR Blog
Re: Very Elementary Question
« Reply #11 on: January 07, 2014, 09:03:47 pm »
Regardless of mains and "earth" ground, since battery operated electronics are not earthed, what's the accurate definition of floating circuit?

It has to be taken in context.  Usually, the context is the "system ground", and the "system" will vary depending on context.

Consider: most electronic devices could be operated on an airplane, perhaps even connected to the airplane's chassis and power system, if you have the right kind of inverter or generator to supply the right flavor of power.  Nothing on an airplane is referenced to an earth ground while the plane is flying.  The plane's body IS normally grounded while the plane is being refueled, however (reduces problems with static discharge around fuel vapors).  That doesn't mean we'd always apply the term "floating circuit" to everything on the airplane while it's flying, and call it ground referenced while the plane is being refueled.

The designers of a laptop computer probably used a ground symbol on their schematic.  A particular connector, say an audio line out, could be floating (through an audio transformer, maybe), or could have one side tied to the computer system's ground.  The fact that the laptop was brought aboard an airplane so that the computer system's ground can't be connected to earth ground is irrelevant.
 

Offline NANDBlog

  • Super Contributor
  • ***
  • Posts: 4636
  • Country: nl
  • Current job: ATEX certified product design
Re: Very Elementary Question
« Reply #12 on: January 07, 2014, 09:04:40 pm »
Any academic reference please?

I don't get it.  |O
Then ask a teacher. We are engineers.
The problem is that you use too loosely the world "ground". A better definition would look like: A floating circuit is a circuit, which ground only has a high impedance path tho the earth.

So engineers don't know?
Yes, we are total morons.
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #13 on: January 07, 2014, 09:21:42 pm »
AG6QR, you are talking about grounding concept in general vs. what I asked to define, which is a "floating" circuit.
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #14 on: January 07, 2014, 09:23:20 pm »
Any academic reference please?

I don't get it.  |O
Then ask a teacher. We are engineers.
The problem is that you use too loosely the world "ground". A better definition would look like: A floating circuit is a circuit, which ground only has a high impedance path tho the earth.

So engineers don't know?
Yes, we are total morons.

I don't believe you.  O0
 

Offline c4757p

  • Super Contributor
  • ***
  • Posts: 7805
  • Country: us
  • adieu
Re: Very Elementary Question
« Reply #15 on: January 07, 2014, 09:37:59 pm »
It's not an incredibly well defined concept. Anything can float or not float as you see fit (though that may require throwing a few rude gestures toward the datasheet). You can even float a 7805 on a pair of resistors like an LM317, though the current flowing out of the Ground pin may be higher and less predictable, causing a voltage drop in the resistors. Caveat engineer.

Think of it this way: I tell you Ann is five feet tall, and Bob is six feet tall. If Bob and Ann are both standing on the earth, then I know that the top of Bob's head is one foot higher than the top of Ann's. If Bob is floating in a hot air balloon, I know nothing about their relative heights (and could tether him to something and determine the height however I wanted).

Floating can refer to an entire circuit, like in the case of Bob or the LM317, or it can refer to one input, like in the case of a string tied to Bob or an unused input on a microcontroller. I can walk over and move Bob's string up and down with very little effort, though I may encounter difficulty attempting to lift it past (Bob.height + String.length) - in the same way that you cannot move a floating MCU input past roughly (VDD + 0.3).

The circuit I posted wasn't actually floating. It was a "simulated floating inductor" - within the limits of the simulation, it pretended to be a floating inductor, but it is of course still just an op amp.
« Last Edit: January 07, 2014, 09:42:56 pm by c4757p »
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #16 on: January 07, 2014, 09:42:05 pm »
It's not an incredibly well defined concept. Anything can float or not float as you see fit (though that may require throwing a few rude gestures toward the datasheet). You can even float a 7805 on a pair of resistors like an LM317, though the current flowing out of the Ground pin may be higher and less predictable, causing a voltage drop in the resistors. Caveat engineer.

Think of it this way: I tell you Ann is five feet tall, and Bob is six feet tall. If Bob and Ann are both standing on the earth, then I know that the top of Bob's head is one foot higher than the top of Ann's. If Bob is floating in a hot air balloon, I know nothing about their relative heights (and could tether him to something and determine the height however I wanted).

Now it makes more sense.  :-+
 

Offline Maxlor

  • Frequent Contributor
  • **
  • Posts: 564
  • Country: ch
Re: Very Elementary Question
« Reply #17 on: January 07, 2014, 09:47:21 pm »
I don't know about the precise academic definition, but for me this is useful:

  • A floating line has an undefined state, and can be at any random voltage level at any given time. In practice this means that the voltage level on the line tends to wander up and down, and might cause unexpected behaviour on anything connected to it. For example, a MCU input my randomly toggle between high and low if it is left floating. The wandering would be caused by effects outside of the circuit, i.e. changes in magnetic fields in the room, changes in temperature, humidity, which might affect parasitic capacities in the circuit and the like.
  • A floating circuit has a ground level that might be offset by an arbitrary amount compared to the ground level of any other circuit. This doesn't matter while they're isolated, but if two circuits like that get connected, there might be unexpected effects. I think ESD can be viewed as an example of contact between a floating and a second circuit (I think, but I defer to the physics/definition geeks here.)
 

Offline A Hellene

  • Frequent Contributor
  • **
  • Posts: 601
  • Country: gr
Re: Very Elementary Question
« Reply #18 on: January 07, 2014, 09:47:38 pm »
Quote from: IntegratedValve
[...]
Just try to understand that the term 'ground' in any closed system is a node that is arbitrarily chosen and considered to be the electrical reference point (of 0.0V) for this specific system.

Just as clocks do not measure the actual time, but only the time measured by other clocks (meaning that, since clocks are only measuring themselves because the objective referent of a clock is another clock), the ground node of a(ny) system is just a point of reference regarding the specific system's electrical potential reference (or voltage reference) of 0V most commonly. We set it to be the circuit's reference!

For example, older cars used to have their battery positive terminal grounded, and as a result we were measuring negative voltages and currents. Now that we have a slightly better grasp of what electricity really is, we decided to ground the power supply negative terminal and to be measuring positive voltages.


-George
« Last Edit: January 07, 2014, 09:49:25 pm by A Hellene »
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)
 

Offline AlfBaz

  • Super Contributor
  • ***
  • Posts: 2029
  • Country: au
Re: Very Elementary Question
« Reply #19 on: January 07, 2014, 10:11:46 pm »
Couldn't you just class a floating circuit or potential as one that is electrically isolated from your reference, in so far as something is floating depending on your context. You could say earth mass ground is floating with respect to your battery
 

Offline Maxlor

  • Frequent Contributor
  • **
  • Posts: 564
  • Country: ch
Re: Very Elementary Question
« Reply #20 on: January 07, 2014, 10:23:20 pm »
Couldn't you just class a floating circuit or potential as one that is electrically isolated from your reference, in so far as something is floating depending on your context. You could say earth mass ground is floating with respect to your battery
Yes! But bigger systems (especially the earth and everything connected to it) seem to be given reference status by everyone! This is totally unjustified of course, since I have the one true ground sitting here on my desk, and once you start believing, you'll realize that it is indeed the earth that it floating, and there is only the one true but narrow path leading to eternal grounding!  :-DD
 

Offline AlfBaz

  • Super Contributor
  • ***
  • Posts: 2029
  • Country: au
Re: Very Elementary Question
« Reply #21 on: January 07, 2014, 10:28:08 pm »
Yes! But bigger systems (especially the earth and everything connected to it) seem to be given reference status by everyone! This is totally unjustified of course, since I have the one true ground sitting here on my desk, and once you start believing, you'll realize that it is indeed the earth that it floating, and there is only the one true but narrow path leading to eternal grounding!  :-DD
Martians will be reading your post and thinking you are quite self centered  ;D
 

Offline Maxlor

  • Frequent Contributor
  • **
  • Posts: 564
  • Country: ch
Re: Very Elementary Question
« Reply #22 on: January 07, 2014, 10:32:41 pm »
Martians will be reading your post and thinking you are quite self centered  ;D

No no no, I'm grounded, not self centered. It's different... it's much more holistic and virtuous!
 

Offline A Hellene

  • Frequent Contributor
  • **
  • Posts: 601
  • Country: gr
Re: Very Elementary Question
« Reply #23 on: January 07, 2014, 10:41:29 pm »
Yes! But bigger systems (especially the earth and everything connected to it) seem to be given reference status by everyone! This is totally unjustified of course, since I have the one true ground sitting here on my desk, and once you start believing, you'll realize that it is indeed the earth that it floating, and there is only the one true but narrow path leading to eternal grounding!  :-DD

Exactly!

This is not a mater of magnitude (of the grounding mass, for example); if it was so, then we would only have the general term 'earth potential' and not the term 'system ground' of any little floating circuit or device we might be working on or with.

It is just a matter of our specific needs, in order to help us have the job done.

So, the term 'floating' is relative to any reference we might consider to be what WE set as a reference point at any given time.

For example, my circuit might be earthed (meaning, grounded to the earth potential) but also be floating in respect to my (ungrounded to the earth potential) multimeter/oscilloscope/bench/soldering iron/etc.


-George
Hi! This is George; and I am three and a half years old!
(This was one of my latest realisations, now in my early fifties!...)
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #24 on: January 08, 2014, 03:22:42 pm »
Quote
So, the term 'floating' is relative to any reference we might consider to be what WE set as a reference point at any given time.

Then it's not physical property until the circuit is implemented with the designated reference ground as the actual ground on PCB for example.
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9681
  • Country: us
Re: Very Elementary Question
« Reply #25 on: January 08, 2014, 03:46:50 pm »
As I understand grounding means designating one node to be the reference node relative to which other voltages are measured.

No, this is not what grounding means. Grounding means making a physical connection from some part of a circuit to an external point of known and stable potential (commonly the earth, but not always), such that all voltages in the circuit are predictable relative to that known potential.

This can be done for safety, circuit protection, or to eliminate unwanted noise signals in the system.

To give a contrived example:

You could have a battery powered signal source like a microphone with preamp, feeding a battery powered amplifier. Suppose now you attach the microphone to your nylon shirt that has a static charge of 10,000 V on it. The microphone is now sitting at a potential of 10,000 V induced by the nearby static charge. It doesn't care of course, because it is isolated and floating and does not know what the outside world is doing.

Now suppose we plug the microphone into the amplifier, which is sitting on a desk and not charged up to 10,000 V. As soon as we plug in the microphone there will be a voltage difference of 10,000 V between the microphone and the amplifier. This might be bad for the amplifier, since its input circuits are only expecting to see a few mV. The amplifier could get fried.

One solution to this is to establish a common voltage reference between the mic and the amp, so they are both sitting at the same potential. This is usually done with the shielding braid on the microphone cable. Both mic and amp  physically connect their circuit grounds to the common braid and now both of them share the same ground potential.
I'm not an EE--what am I doing here?
 

Offline smoothtalker

  • Regular Contributor
  • *
  • Posts: 76
  • Country: us
Re: Very Elementary Question
« Reply #26 on: January 08, 2014, 04:35:23 pm »
May i ask a Very Elementary Question too? i'm confused with mains earth reference. i'm a newbie too.

Why does grounding a circuit makes it reference to that grounded voltage? For.eg, the 230V AC line. The neutral is grounded. i know it doesn't affect the circuit connected to it. we will see 230v rms between hot and neutral when it's open, but why do we still see a 230 rms between hot and neutral when the circuit is loaded? isn't the AC sine wave also running in the neutral?

i don't understand how grounding affects a circuit.

thanks
« Last Edit: January 08, 2014, 04:43:05 pm by smoothtalker »
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #27 on: January 08, 2014, 05:41:07 pm »
As I understand grounding means designating one node to be the reference node relative to which other voltages are measured.

No, this is not what grounding means. Grounding means making a physical connection from some part of a circuit to an external point of known and stable potential (commonly the earth, but not always), such that all voltages in the circuit are predictable relative to that known potential.

This can be done for safety, circuit protection, or to eliminate unwanted noise signals in the system.

To give a contrived example:

You could have a battery powered signal source like a microphone with preamp, feeding a battery powered amplifier. Suppose now you attach the microphone to your nylon shirt that has a static charge of 10,000 V on it. The microphone is now sitting at a potential of 10,000 V induced by the nearby static charge. It doesn't care of course, because it is isolated and floating and does not know what the outside world is doing.

Now suppose we plug the microphone into the amplifier, which is sitting on a desk and not charged up to 10,000 V. As soon as we plug in the microphone there will be a voltage difference of 10,000 V between the microphone and the amplifier. This might be bad for the amplifier, since its input circuits are only expecting to see a few mV. The amplifier could get fried.

One solution to this is to establish a common voltage reference between the mic and the amp, so they are both sitting at the same potential. This is usually done with the shielding braid on the microphone cable. Both mic and amp  physically connect their circuit grounds to the common braid and now both of them share the same ground potential.

You are referring to "earth" ground. What we call reference node (ground) is relative and can be any node. However I'm wondering if there's one and only one node in the circuit that all currents sink into it or lets say "best" "ground" candidate.
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9681
  • Country: us
Re: Very Elementary Question
« Reply #28 on: January 08, 2014, 07:58:32 pm »
As I understand grounding means designating one node to be the reference node relative to which other voltages are measured.

You are referring to "earth" ground. What we call reference node (ground) is relative and can be any node. However I'm wondering if there's one and only one node in the circuit that all currents sink into it or lets say "best" "ground" candidate.

No, I am not referring to earth ground particularly, I am referring to grounding in general. I am trying to correct your misunderstanding about grounding.

Grounding has nothing to do with reference nodes in circuit analysis. Picking a "ground" reference on a circuit diagram is purely algebra, nothing more. It works like this: all circuit voltages are differences in potential between two points. Let's say we have three points on a circuit diagram, call them A, B and G. Let's reference the voltages to point G. So the voltage at point A referenced to G is VA - VG. And the voltage at B is VB - VG. Therefore, the difference in voltage between points A and B is:

  (VA - VG) - (VB - VG) = VA - VB

That's all it is. A pure numerical convenience to avoid unnecessary arithmetic and keep the degrees of freedom straight. There is no node where all currents "sink into" (since currents don't source or sink anywhere, they just flow round and round in circles). There is no "best" candidate other than the one that is most convenient for humans looking at the circuit.

Just keep in mind that grounding a circuit happens physically, in the real world. It can be protective earth grounding, or signal grounding, or combinations, but it is always a practical matter with a functional purpose. Do not confuse it with reference nodes in circuit simulations which have no functional purpose except computational convenience.
I'm not an EE--what am I doing here?
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #29 on: January 08, 2014, 08:15:46 pm »
Ok then how does a circuit know I used a specific reference node to get it working if it's not connected to some earth point? What about battery powered devices like cellphones?
 

Offline Len

  • Frequent Contributor
  • **
  • Posts: 515
  • Country: ca
Re: Very Elementary Question
« Reply #30 on: January 08, 2014, 08:30:45 pm »
Ok then how does a circuit know I used a specific reference node to get it working if it's not connected to some earth point? What about battery powered devices like cellphones?
It doesn't know! The circuit doesn't care which node you choose to put your black DMM probe.

It's like measuring distance, the world doesn't know or care where you choose to put the end of your ruler. There is no "centre of the universe" that everyone starts measuring from, and there is no "universal zero voltage" point in your cellphone. Any voltage measurement is measuring the "distance" between two points chosen by you. (That's why voltmeters have two probes, not one.)
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9681
  • Country: us
Re: Very Elementary Question
« Reply #31 on: January 08, 2014, 08:41:46 pm »
Ok then how does a circuit know I used a specific reference node to get it working if it's not connected to some earth point? What about battery powered devices like cellphones?

Again, you are mixing up two different concepts. Reference nodes in in circuit simulations have nothing to do with connections to earth points.

Reference nodes in simulations are to make the simulator work. They have nothing to do with whether or how the circuit works.

Earthing points in the real world have nothing to do with circuit simulations.

Therefore:

The circuit doesn't know or care what reference node you used to simulate it. The circuit will do whatever it does regardless. The circuit will work even if you don't simulate it at all and just go ahead and build it.

Battery powered devices like cellphones work because earth points are not needed for circuits to work. Currents flow in loops. If there is a closed loop current can flow and the circuit can work.

Consider a circle. Which side of the circle is the bottom?

Consider a battery circuit. Which part of the circuit is the ground?

See? The question doesn't really make sense.

Reference nodes in simulators are there to make the computer simulation have a numerical solution. They have no bearing on the function of the circuit being simulated.

Try this question:

Point A is 4 m higher than point B.
Point B is 2 m lower than Point C.

What is the elevation of point C?


I'm not an EE--what am I doing here?
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #32 on: January 09, 2014, 08:04:01 am »
This is one battery cell used to dual-supply Op Amp. Anything wrong with this design?

 

Offline NANDBlog

  • Super Contributor
  • ***
  • Posts: 4636
  • Country: nl
  • Current job: ATEX certified product design
Re: Very Elementary Question
« Reply #33 on: January 09, 2014, 01:41:04 pm »
This is one battery cell used to dual-supply Op Amp. Anything wrong with this design?
Yes. You did not decouple the virtual ground. The power supply voltages depend on the load current of the virtual ground.
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9681
  • Country: us
Re: Very Elementary Question
« Reply #34 on: January 09, 2014, 02:19:24 pm »
This is one battery cell used to dual-supply Op Amp. Anything wrong with this design?

With the partial circuit as drawn you can remove both resistors and the virtual ground and it will be exactly the same circuit. At present the resistors serve only to drain the battery.

In case you are puzzled by this, observe that the two resistors are not connected in any way to the op amp unless you complete the circuit by making some connections to the op amp inputs. Once you have wired up the inputs in this way and made all necessary connections, you will discover that the ground symbol you have drawn is not important to the operation of the amplifier.

Did you reach a conclusion on the question about the elevation of point C?
I'm not an EE--what am I doing here?
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #35 on: January 09, 2014, 03:42:12 pm »
Quote
Did you reach a conclusion on the question about the elevation of point C?

Relevant to what point? It's lower than A by 2 m and higher than B by 2 m.

This is an inverting amplifier configuration, I tried calculations using nodal analysis and I still get same results if were calculated without considering power supply. Where did I screw up?

 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9681
  • Country: us
Re: Very Elementary Question
« Reply #36 on: January 09, 2014, 04:04:53 pm »
Quote
Did you reach a conclusion on the question about the elevation of point C?

Relevant to what point? It's lower than A by 2 m and higher than B by 2 m.

It's relevant to voltages at nodes in circuit simulators and why you need to add a ground symbol somewhere when simulating a circuit.

Consider:

There are two equations in three unknown variables:

A = B + 4
B = C - 2

Since there are three variables and two equations there is one degree of freedom. It follows that we can assign any elevation to C that we wish. If, for example, we assign an elevation of 100 m to C, then B is at 98 m and A is at 102 m.

The same situation happens with voltages at nodes in circuit analysis. There is always one degree of freedom that must be arbitrarily satisfied. It is a convention to pick a node, any node, and arbitrarily assign it a voltage of zero. This is done by attaching the ground symbol to that node in the circuit. The circuit in real life has no physical device corresponding to that ground symbol.

Quote
This is an inverting amplifier configuration, I tried calculations using nodal analysis and I still get same results if were calculated without considering power supply. Where did I screw up?

When you do your nodal analysis you are considering the amplifier to be a "black box" and not including what is inside it. As a black box, the amplifier is assumed to be self powered and to have an infinite range of possible voltages. As such, the actual power supply (and any limitations it may impose) is not part of the analysis.

Therefore, if you erase from your circuit diagram the battery and the two resistors and the connections to +Vcc and -Vcc, you will still have the same ideal circuit. Furthermore, if you now connect each of the ground symbols with a line and erase all the ground symbols, you will still have the same circuit. The ground symbol is just a shorthand for "all these points are connected together".
I'm not an EE--what am I doing here?
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #37 on: January 09, 2014, 05:49:46 pm »
Correct me if I'm wrong, are you saying the ground between the two resistors of the voltage divider is not necessary? Lets consider the typical two battery supply connection presented all textbooks, it shows the node between the two batteries is "grounded", is this necessary? What type of ground they are referring to? Is it connected to OpAmp external circuit "ground"?
 

Offline IanB

  • Super Contributor
  • ***
  • Posts: 9681
  • Country: us
Re: Very Elementary Question
« Reply #38 on: January 09, 2014, 06:27:07 pm »
Correct me if I'm wrong, are you saying the ground between the two resistors of the voltage divider is not necessary? Lets consider the typical two battery supply connection presented all textbooks, it shows the node between the two batteries is "grounded", is this necessary? What type of ground they are referring to? Is it connected to OpAmp external circuit "ground"?

Yes, I am saying that, in a way. For the ideal black box op amp schematic the virtual ground is not needed. For the real op amp in a physical circuit it is needed if you want a reference voltage mid-way between the power rails. When it is needed, it is needed because the op amp output can source or sink currents and these currents need a return point to get back to the power supply.

But I see you are still getting stuck on this idea of "grounding". It would probably help you to start drawing circuit diagrams without any ground symbols, since the ground symbols seem to be causing confusion. Instead of using ground symbols, just draw a line on your schematic between points that share a common connection. Do that for a while and see if things become clearer.
I'm not an EE--what am I doing here?
 

Offline Galaxyrise

  • Frequent Contributor
  • **
  • Posts: 527
  • Country: us
Re: Very Elementary Question
« Reply #39 on: January 09, 2014, 06:31:27 pm »
Correct me if I'm wrong, are you saying the ground between the two resistors of the voltage divider is not necessary?
It is not necessary that you called it ground, or that you designated the connection with a net symbol instead of solid lines.

Quote
Lets consider the typical two battery supply connection presented all textbooks, it shows the node between the two batteries is "grounded", is this necessary? What type of ground they are referring to?
Just the "I hereby declare this to be 0V" ground in those simple examples.  You could just as easily let -VCC be 0V; it's not necessary to declare 0V to be between the batteries.  The circuit will function just fine whether you call one of its voltages "ground" or not.

And in your circuit, the voltage from VCC to "ground" will vary based on the output of the opamp, which is probably not the intent of the design. This is what NANDBlog was referring to. It is a common beginner mistake (I'm certainly guilty of it!) to design a part of the circuit in isolation (in this case, the resistor divider) and then use it in a way that makes it work very differently (by attaching the output to it.) And that's why the textbook example is two batteries instead of a battery and a resistor divider :)
I am but an egg
 

Offline IntegratedValve

  • Regular Contributor
  • *
  • !
  • Posts: 187
Re: Very Elementary Question
« Reply #40 on: January 09, 2014, 07:55:39 pm »
I think I got it now.

Thanks you all for your patience and informative help.

Conclusion:

1- The point between the two batteries has to be connected to the OpAmp external circuit so that the internal circuitry swings the output voltage between the two rails.

2- In case of using one battery of double voltage (2 x Vcc), although the voltage difference between +Vcc and -Vee OpAmp connections is the same, but connecting one end to the OpAmp external circuitry will cause the swing between 0 and 2xVcc?

3- the voltage divider method does not work because when it's loaded the voltages across each dividing resistor will change based on load current, which in turn will affect the normal operation.
« Last Edit: January 10, 2014, 04:13:54 pm by IntegratedValve »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf