Electronics > Beginners
Some noob questions
The Soulman:
--- Quote from: Mr D on August 17, 2018, 08:04:10 pm ---Thanks for trying but, urrrghhhh, it's not making any sense to me.
Just when i thought i had my head around Ohm's law (which i do), this ground thing looms it's ugly head.
Let me try to precisely express what i'm not understanding:
In the attached schematic, both the pos. & neg. terminals lead to this undefined "ground". Apparently according to James S this is not real earth, but, for example the metal case of my device.
So the current will travel from pos., down the circuit, into the metal case, then back into the neg. terminal, right? So, we have a complete circuit.
--- End quote ---
Yes! :-+
--- Quote from: Mr D on August 17, 2018, 08:04:10 pm ---But in such a device, the case is often earthed, right? So why would the current choose to travel back into the neg. terminal when there's an easier path (with virtually no resistance) to real earth?
--- End quote ---
No, The current path needs to lead back to the power source/battery, additional connection to earth makes no difference in this circuit.
--- Quote from: Mr D on August 17, 2018, 08:04:10 pm ---Or to turn the question round the other way: I take a long nail, wet my hand and jam into the positive hole of a wall socket. I'm likely to get killed, right?
--- End quote ---
Yes! :--
--- Quote from: Mr D on August 17, 2018, 08:04:10 pm ---But why? The current can flow into my body, but not back into the negative terminal, so there's no complete circuit so no current flow, right?
--- End quote ---
To be accurate, there is no positive or negative terminal in a wall outlet because its AC not DC, the propper term
would be phase and neutral.
The neutral is always connected to a ground connection "somewhere" (but never in your house or in any appliance).
Therefore current can flow from phase (thru you...) to ground.
--- Quote from: Mr D on August 17, 2018, 08:04:10 pm ---Until now i've been dealing with simple circuits: a battery, one or two resistors, and that's it. Can you guys not at least partly understand how this idea of a ground leading somewhere undefined (at least in my mind) might be somewhat confusing? Is it the case of the device? Or real earth. Or an earthed case ........ or.....?
Cheers! ......... D
--- End quote ---
meh, don't worry, if you read all of this back in a couple of months you will also have a good laugh. :)
The Soulman:
--- Quote from: Mr D on August 17, 2018, 08:14:44 pm ---But if i was to stop trying to intuitively understand it, is this a correct summary of what i need to know about ground:
Every time i see a ground symbol in a schematic, i just have to imagine a wire travelling from that symbol to a single bus, with that bus (that has zero resistance) leading back to the negative terminal?
--- End quote ---
Good enough, also in a schematic almost all voltages (unless noted differently) are referenced to ground (connect black probe of voltmeter/multimeter to ground).
So don't worry if you see negative voltages somewhere, it is a matter of perspective.
rstofer:
Of course it's not as simple as that! There are at least 3 ground symbols in common use.
https://www.rapidtables.com/electric/Ground_Symbols.html
The one we care most about is the COMMON ground - the triangle with the end pointing down. This is electrical COMMON whether it connects to EARTH ground by way of CHASSIS ground or not.
This is an arbitrary point and it is not always the (-) end of a battery. I could put a voltage divider around the outside of a 12V battery, ground the center point, call it common ground and the result is +6 and -6 volts relative to common ground. Think about a center tapped transformer. 6.3V either side of the center tap and 12.6V across the ends.
When you study circuit analysis, ground will usually be obvious and, for homework problems, is generally the lower left corner of the circuit. But it doesn't have to be! You can pick ANY point as 'ground'. Some choices will work better (easier) than others but they will all work.
When you get to op amps and look at single supply variants, you will see that arbitrary midpoint of the power supply created in every design (except for bizarre boundary designs). It is important to bias the op amp to mid range such that signals can vary above and below ground (AC signals). Again, this is where w2aew's op amp videos come in handy because he sometimes uses single supply op amps. For dual supply op amps, ground is at the midpoint between the supplies (usually).
Further in the op amp stuff, you will find out about a 'virtual ground'. An interesting situation where the op amp tries to create enough feedback on the (-) input to keep the difference between the inputs at 0V. If the (+) input is grounded (0V) then the (-) input is a virtual ground and also 0V. This is an IMPORTANT bit to understand. Kirchhoff's Current Law comes into play here.
The law is simple: The sum of the currents entering a node is exactly equal to the sum of the currents leaving the node. In short, current can't pile up nor can it spill on the floor. Or, what goes in comes out...
13 minutes into this video, you will see a voltage divider on the (+) input of an op amp. It is probably described much earlier. I just clicked an arbitrary time and the schematic showed what I wanted to talk about
That voltage divider puts the (+) input at half the supply voltage so the negative feedback will hold the (-) input at that same level.
Why don't you look at the Electrical Engineering path over at Khan Academy? I did the entire course in a day or so and it's quite good. True, it was all review but the explanations are pretty good. If you catch the math for capacitors and inductors, that's great. If not, there are other tutorials you can work through later. Don't worry too much about the math, most hobbyists don't need it to enjoy the hobby. But it is elegant!
What you WILL see is the range of applications of Ohm's Law and how it applies everywhere.
rstofer:
--- Quote from: Mr D on August 17, 2018, 08:04:10 pm ---Or to turn the question round the other way: I take a long nail, wet my hand and jam into the positive hole of a wall socket. I'm likely to get killed, right?
But why? The current can flow into my body, but not back into the negative terminal, so there's no complete circuit so no current flow, right?
--- End quote ---
You sort of understand the idea of a circuit. From 'circus' meaning, I imagine, 'around'. A circuit is complete when there is a path around. The path may be low resistance or high resistance. This determines how much current will flow when there is an applied voltage. But always a circuit - a complete path.
So, when you jam that nail into the wall outlet, assuming you hit the narrow blade, the phase, there is a voltage applied to your body but unless some part of your body has a low resistance path to neutral (ground) there won't be any current flow. That's the key point! There is no 'circuit'. Now, grab a water pipe with your other hand and you're dead meat.
This idea of a short circuit to ground is a lot more complex and I don't want to get into it.
--- Quote ---
Until now i've been dealing with simple circuits: a battery, one or two resistors, and that's it. Can you guys not at least partly understand how this idea of a ground leading somewhere undefined (at least in my mind) might be somewhat confusing? Is it the case of the device? Or real earth. Or an earthed case ........ or.....?
Cheers! ......... D
--- End quote ---
We have common or circuit grounds all the time without ever referencing them to chassis ground or earth ground. Battery circuits will almost never have an earth ground reference unless it comes in from the scope probe. Why would it? Circuits will be complete without needing an external conductor to earth.
OTOH, it is common to earth ground all exposed non-current-carrying metallic surfaces of electrical appliances (exemption for double insulated). That's why the BNC connectors on test equipment are tied to earth ground. Hook up you scope to your battery powered robot and now things are earth referenced.
Those animations you posted earlier showing a path through real earth are not helpful. Yes, they are correct but they are a distraction. We really can't talk about that path with out a lot more understanding of fields and such. Not important at this point.
Think 'circuit', 'circus', 'around'. There is a complete path from (+) to (-). If not, no current will flow. Voltage may be present at some points but without a circuit, no current flows around.
Mr D:
Many thanks so far guys, i'm starting to develop the smallest inkling of understanding.
But still some things not making sense.
Attached is a pic with two, independent circuits.
To my mind, the connection of the ground at different points in the circuits should make no difference whatsoever, as they're simply connections to a bus that itself isn't connected to anything, not even to the negative battery terminal.
However, what i observe is that the voltage potential is different in the circuits. How can the voltage potential be effected when the ground is going to a dead-end street (bus leading nowhere)?
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