Author Topic: Very Elementary Question  (Read 9474 times)

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Offline IntegratedValve

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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

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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

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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.
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Offline A Hellene

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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 »
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Offline IntegratedValve

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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

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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

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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!...)
 

Online AlfBaz

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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

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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
 

Online AlfBaz

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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

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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

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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

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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.
 


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