Circuits 101

Discussion in 'General Electronics Chat' started by rushton, Oct 15, 2009.

  1. rushton

    Thread Starter New Member

    Oct 13, 2009
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    Hi all,

    As you can see, this is my first post and as such I seem to be at the beginning of a very long road. I am a newbie, still trying to grasp these concepts.

    So here is my quandary; having looked at countless diagrams and schematics, I can't understand exactly how the current flows. I understand that the voltage pushes the current through resistance but does it always flow one way? I see arrows on schematics that sometimes don't make sense. Logic tells me it flows both ways, otherwise why have diodes, right? If so, how does that work?

    Also, I've read about electron flow and conventional flow and how the former is correct but the latter has been around so long it can't be forgotten. But does this mean that a circuit layout needs to be reversed if the flow 'style' is changed?

    A shiny new red capacitor (which I don't know what to do with) to the best answer.

    Thanks

    Scott
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
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    Welcome to AAC (acronym for All About Circuits), Scott!

    Have you read some of the AAC e-book yet? I'm sure it would have some of the answers you're looking for.
     
  3. rushton

    Thread Starter New Member

    Oct 13, 2009
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    I've read all sorts of stuff. On this site I've read quite a bit from Volume I - DC and I'm also more than halfway through Mims' 'Getting Started in Electronics'. I understand most if not all of what I see but at no point has any of my readings directly answered some of my questions. I guess I'm looking for the right way to understand this.
     
  4. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
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    Very well Scott,

    Imagine a simple water pipe, and also a pump. The more water pressure will increase the output potential, or in this case voltage. In addition, the width of the pipe can be described as current; the wider the pipe we have, more current can flow. Obviously, the current is flowing in only one direction and we can think of it as positive to negative. Suppose we add a resistor what would happen? Evidently it would decrease the current flow. We could widen the pipe letting more current go through to compensate for the loss current, but for now we want it there.

    Another way to think of it: Ever see those big electrostatic generators (Van de Graff generators)? Well when a big electrostatic potential builds up (+), it wants to go to ground. When you get ground near it, there'll be a big arc.
     
  5. bundick

    Active Member

    Dec 19, 2007
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    I remember a Tech class, where the instructor was trying to make a circuit with a Zener and a Crowbar work, demonstrating it using the Electron theory.
    Starting at the Ground points, he was ankle deep in Chalk dust before he finally gave up and used the conventional flow.
    He was done with the demonstration in five minutes using the Conventional flow.

    You have holes going one way and Ions going the other way. It's just easier to use the conventional flow.
     
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
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    when you start to really get into components and how they work in a circuit, you'll find that both conventional and electron current flow is used depending on the circuits being analyzed.

    Example:

    When dealing with voltage drops across resistors electron flow is good to use because you get a mental picture of electrons piling up on one side of the resistor making it more negative than it originally was before current began to flow.

    But when your dealing with positive going signals coming from transistor amplifiing stages, it is easier to follow conventional flow from the output of the stage into the inputs where the signal is flowing.

    Sometimes you'll find yourself using both methods in the same circuit, it depends on how you view the circuit, and what works best for you.
     
  7. Ratch

    New Member

    Mar 20, 2007
    1,068
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    rushton,

    It shouldn't be too hard if you know a little physics, understand the correct terminology, and don't rely too much on analog models that don't really explain what really happens.

    The first thing you should do is read the first post of the link below.

    http://forum.allaboutcircuits.com/showthread.php?t=11579&highlight=treatise+voltage

    Finished reading? OK. We will start with a metal wire. All conducting metals have a huge excess of loosely bound electrons. When a voltage is applied to a wire, the electrons at the negative end have a higher energy density than the electrons at the positive end of the wire. Because like charges repel one another, the electrons will move from the battery and through the wire from the excessive (negative) side to the sparse (positive) side and back into the battery. In other words, they will move from the higher energy density (voltage) to the lower energy density. This explains the direction of the charge carriers (electrons in this case). Charge flow in this case is from negative, through the circuit, to positive.

    There is a lot of misinformation about the two conventions. A lot of people blame Ben Franklin for the confusion. They say something about him calling a electron negative when it should have been named positive or some such silly thing. Here is what you should know. Although it is counterintuitive, you should not attempt to calculate currents based on the actual polarity of the charge carriers. Many times I have seen a circuit analysis done by assigning a current direction according to the way an electron physically moves in a circuit. If beginners follow that method, they will get wrapped around the axle when they try to analyze two different charge carriers like holes and electrons concurrently present in a semiconductor, or positive and negative ions concurrently present in electrochemistry. So the way to keep your sanity is to always assume charge flow is from the positive to the negative of a voltage source, and that charge flow from a current source always comes from the positive end. You can justify that assumption by reasoning that a negative charge carrier going in one direction is the same as a positive charge carrier going the opposite way. After making the calculations, then if necessary, the true direction of the current can be ascertained.

    If it is polarized, make sure the + side goes to a positive voltage.

    Ratch
     
  8. rushton

    Thread Starter New Member

    Oct 13, 2009
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    Thanks everyone! It's still a bit fuzzy, but I know I won't get it all in one day.

    One of my problems, i think, was that up until now I've only encountered one flow, that of electrons. The flow of 'holes' is one that I'm only now encountering. So there is always a contra-flow, with electrons (-) flowing one way and holes (+) flowing the other way. Anyway, I'll keep reading...more basic questions to come I'm sure.

    Thanks Ratch, you capacitor is in the mail...
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    The concept -
    is only valid in semiconductor PN junctions. Wires and conductors only have electrons carrying charge, so the only things moving are negatively charged electrons. There are no holes in conductors.
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,414
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    rushton,

    There is always a great danger when people try to change things in order to be politically correct (or in this case, scientifically correct).

    Describing electronics using electrons and holes may be appropriate for solid-state physics but not for electronics, despite what your textbook will tell you.

    Using the current flow model works 99.99% of the time in electronics.
    Just think of current flowing like water flowing from the top of a hill to the bottom. Current flows from a higher potential to a lower potential, i.e. from a place that is more positive in potential to one that is less positive.

    It's as simple as that. Works every time for me.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    With the little caveat that electrons are the only charge carriers, and that the charge they carry is negative.
     
  12. rushton

    Thread Starter New Member

    Oct 13, 2009
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    Ok, so the current always flows from + to - (following conventional flow). So what is the point of a diode? Why have an implied one way when the flow always favours one direction anyway?
     
  13. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Not sure I understand your question, but,

    It is the symbol used to show that this component is a PN junction, device.

    I guess that was your question.
     
  14. rushton

    Thread Starter New Member

    Oct 13, 2009
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    I'm not sure I understand my question either to be fair. I'll keep reading for now. Thanks for everyone's help.
     
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    1,512

    I'm a newbie too, but I think I understand what your saying here. A diode is used in this way to prevent a higher potential from back feeding into a noter part of the circuit. Someone will correct me if I'm wrong.

    Cary
     
  16. Ratch

    New Member

    Mar 20, 2007
    1,068
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    rushton,

    So as to prevent current from existing when the voltage polarity changes.

    In the conventional current model, charge is assumed to flow from the positive to the negative of a voltage source. The actual physical flow of charge depends on the polarity of the charge carrier. In metals, the charge carriers are negative electrons. So the actual physical flow is in the opposite direction of conventional charge flow. In an electrolyte, the positive ion flow will agree with the conventional current model, but the negative ions will have an opposite direction. Same goes for electrons and holes in a semiconductor. Conventional current is a mathematical artifice that engineers and scientists use so as not to get hung up on voltage and charge polarities. If you really need to determine the actual physical direction of the charge carrier, then you can do so easily after the calculations are made. Don't try to determine unknown current directions before computing.

    Electrical equipment and parts manufacturers follow the conventional current method. That means that charge will flow in a diode if you connect a positive voltage to the arrow and a negative voltage to the bar. The same is true of your ammeter or any other device that measures current.

    Another thing. Remember that when charge flows, then current exists. The term "current flow" is redundant and ridiculous. It means "charge flow flow".

    Ratch
     
    Last edited: Oct 17, 2009
  17. zgozvrm

    Member

    Oct 24, 2009
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    Are you sure about this? The electrons have to have some place to move to, don't they? If they run out of a piece of wire at one end, they must also run into the other end, effectively replacing was has been lost, right? If they're not called "holes" there must be some equivalent concept/name for where the electrons go.
     
  18. beenthere

    Retired Moderator

    Apr 20, 2004
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    Current is defined as a number of electrons flowing past a measuring point per unit of time. One ampere is 6.24 X 10^18 electrons per second, just to give some magnitude to the effect.

    Imagine a generator, wherin a moving magnetic field creates an electric field by passing past a coil of wire, providing a propelling voltage. Connect the external ends of this field coil with a 1 ohm resistor, so a conducting path is created and the resulting current is controlled. With each passage of the magnet, the induced voltage will cause current flow. The only elements in the circuit are copper wire and the resistance element - we'll say carbon for this example.

    No holes need be found. The induced voltage pressure compels the electrons in the wire and resistance to move. With one polarity applied, they all drift left. Reverse the polarity, and they move right. Since they all get displaced, no special area to receive them need exist. Imagine a bicycle chain similarly transmitting endless and continuous power as long as the input force is applied.
     
    Last edited: Oct 24, 2009
  19. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Also to go along with what "beenthere" said,

    electron and hole recombination only happen in semiconductive material.

    Hole flow is mainly at the potential barrier, until the bvarrier is broken, that's what makes the material semi conductive, due to the potential barrier that exists with holes and electrons, at the junction.

    But once the material has its potential barrier overcome, then electron hole recombo, no longer is prevalent where most of the electrons are drifting through without recombining.

    Something like that I read in my course books. Long time ago.
     
  20. Ratch

    New Member

    Mar 20, 2007
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    zgozvrm,

    [QUOTEAre you sure about this? The ][/QUOTE]

    You can believe it when someone tells you that holes only exist in semiconductors, and not in a conductive wire. A hole would not stand a chance of existing in a conductive metal. That is because there are so many loosely bound electrons that the hole would get filled faster than you can comprehend it. Only in semiconductors, where the hole and electron concentration of the N and P types is sparse, and tightly controlled by the dopants, can holes exist.

    Are you talking about the drift motion, or the random thermal motion. They don't have to have drift motion.

    I am not a big fan of analogies, but sometimes they are good to illustrate a narrow point. The old one of a hose filled with marbles is good to illustrate electron flow. Put a marble in one end and another different one pops out the other end. Where are the holes? Block the exits end of the hose and nothing moves. Do you wonder where water in a circulatory pump goes? You know it returns back to the pump, even if it is not immediately the same water inserted at the discharge of the pump.

    Ratch
     
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