| Electronics > Beginners |
| Beginners idiot question |
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| timelessbeing:
--- Quote from: Mr D on May 26, 2019, 07:56:38 am --- Ok, so am i right in thinking that in these circuits / discussions, the word "ground" could be replaced with "the negative terminal of the battery"? --- End quote --- Nope. You can attach the positive terminal to ground and it won't make a difference because it's arbitrary. |
| Cnoob:
The ground terminal is the simulator's reference point, and treats it at zero volts potential. Which is why in your top right hand circuit you have plus 833mV on the positive terminal and -166mV on the negative terminal. |
| magic:
A battery is a source of voltage, not potential. Voltage is a difference between potentials. A battery creates a difference of 1V (or whatever) between its two terminals. Ground (the rocky stuff underneath you) is a source of absolute potential: it is huge and electrically neutral, any small object connected to ground looses its static charge and becomes neutral too. You can connect the positive terminal to ground and then the negative is at -1V. You can connect a resistor divider to ground and then each terminal is at ±0.5V. Not all circuits are connected to ground. For those that aren't, it's usual to choose some point which is called "ground" and measure all voltages in the circuit with respect to that "ground". The absolute potential of that "ground" with respect to the physical ground can be anything and it doesn't influence the way the circuit works. |
| mikerj:
--- Quote from: vk6zgo on May 26, 2019, 05:46:28 am ---It's just one of the idiot things simulators do! ;D Another one is that as a default, LTspice shows an ac source as a DC one, then has a note alongside saying what it really is. --- End quote --- LTSpice shows a voltage source as a voltage source. You then define what kind of voltage waveform you want it to provide. --- Quote from: vk6zgo on May 26, 2019, 05:46:28 am ---I tried to like LTspice---- I really did!, but when it I couldn't simulate a basic CR network, I gave up. --- End quote --- This is a PEBCAK problem, not LTSpice. --- Quote from: vk6zgo on May 26, 2019, 05:46:28 am ---It's a bit like trying to remove cylinder head bolts with a Stilson Wrench------it might work, but it will take 10 times as long, & a lot of scraped knuckles along the way! --- End quote --- It's like trying to use a tool you haven't yet learned to use. |
| Brumby:
--- Quote from: Mr D on May 26, 2019, 07:56:38 am ---Thanks folks, I'm still having a hard time wrapping my head around it. Ok, so am i right in thinking that in these circuits / discussions, the word "ground" could be replaced with "the negative terminal of the battery"? But if that's correct, then the circuit in the top right of my image doesn't make sense to me. Because before the current enters the 200ohm resistor, it has the option to go to ground (to the battery neg. pole), which is less resistance........ so why would any current flow through the 200ohmR? (which according to EC it does). --- Quote from: Brumby on May 26, 2019, 01:24:21 am ---As for moving the "ground" connection around - this (generally) will make absolutely no difference to the operation of a circuit - as long as there is only ONE such point specified. Moving it about is like moving where you put the black probe of your multimeter ... and that's about it. --- End quote --- I suspect the above quote is the key to the answer, but i'm not getting it yet! --- End quote --- Yes - you will see the answer in there, once you "get it". (When you read the following, take it slowly and be comfortable you know what is being said before moving on. If you have a question - ASK! Either I or other members will help you as best we can.) Here we must take a minute to wrap our heads around some basic terminology... We have: 1. Ground 2. Earth 3. Chassis 4. Common 5. 0V 6. Reference point (doubt you will see this) In many cases, these mean EXACTLY THE SAME THING - particularly as far as the circuit under study is concerned. What adds to the confusion is that the two terms "Ground" and "Earth" DO NOT ALWAYS refer to a connection to the soil on which your building is situated (usually through the Earth pin on your mains power outlet). Sometimes they will, but not always. Quite often, these terms will be used when the term "chassis" would be more descriptive. But even then, you should check to see that there is an explicit connection to a metal framework and/or case before you take that literally. (And even if there is, it may not mean anything!) Fun, eh? The next point that you need to understand is that having ONE point connected to a Ground/Earth/Chassis will have no fundamental effect on the circuit operation. Any such connections will need a closed loop before you need to take them into consideration. A single connection cannot make a loop. As for the "Ground" or "Earth" allowing current to flow, even that requires a loop. In the case of a mains supply, that loop is made up of two parts. The first is where the pole transformer has wires carrying the normal active(s) and neutral running to your property and to the power points around the building. When your equipment is working properly, all the current flows through this system. The second part is made up of another wire from the pole transformer (connected to the neutral) going straight down to a rod buried in the soil below it - and - all the "Earth" pins of building's mains outlets connected back to a common point which then has a similar wire running down to another rod driven into the ground alongside the building. In normal operation, all the current coming "in" on the active will go "out" on the neutral (let's not get caught up on the nature of AC and let's leave 3 phase out of this) - but in a fault condition where the active comes into contact with an earthed part (that is a part that is actually connected to the building's earthing rod), In this case, the circuit loop goes from the pole transformer - through the active to the building - through the wiring to the power point - to the piece of equipment where it then takes a (fault) path to the earthed part. From there, the current goes back through the earth wire of the power cord - back through the earth wiring of the building - down to the earthing rod driven into the ground - and through the soil back to the earthing rod underneath the power pole, back up to the transformer neutral. NOTE: This only works when there is direct connection all through. When you have a circuit which has galvanic isolation - such as power from your typical mains transformer secondary - then there is no loop possible through the building earth system ... unless you deliberately connect to the mains side (which is not normally done). Where the terms: "Ground", "Earth", "Chassis", "Common", "0V" and "Reference point" are used, it is very often the case that they are talking about a point/track/conductor/metal part which provides a 0V reference point (a place to put the black lead of your multimeter) - and that's all! No actual connections to the soil need be involved. Now, sometimes there will be a connection to a real "Earth" - but if your circuit is isolated on the secondary side of a transformer, a single connection to such an "Earth" cannot be part of a loop and thus will not and can not have any influence on circuit operation. THUS - in the circuits originally posted, having ONE Earth point noted in two of them, there is no loop path for current to flow, so that Earth point means nothing (as far as circuit operation is involved). This is why the third circuit will operate in exactly the same way! SO - Why does this circuit simulator require an "Earth" point? That's easy - albeit a tad confusing. When we humans look at a circuit, we can pick out a likely spot to use for our 0V reference point - but for a simulator program, that's not necessarily a simple process. It could make a guess - but with a circuit having dozens of connection points, odds are the voltages it would show will be confusing. Certainly, it has enough information without such a point being defined to be able to described circuit operation. It could take a random point and use that - but when it wants to show you voltages that will mean something to you, it needs to know "where to place the black lead" (for want of a better phrase). The easy way to do this is to have you place a 0V reference point on the circuit - and the often misappropriated "Earth" point is the natural choice. I hope that has helped ... and not helped you into further confusion. |
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