Newbie Question about Current Flow Direction and Resistor Placement

Discussion in 'General Electronics Chat' started by sso311, Jul 20, 2015.

  1. sso311

    Thread Starter New Member

    Jul 16, 2015
    3
    0
    Hello, this is my first post here. I just started learning the basics of electronics and circuitry a few weeks ago, which is one of the reasons I joined this site. I have already read through a reasonable portion of text covering basic concepts in electricity and circuits but there is one thing that continuously confuses me due to some apparent inconsistency. As the subject says, I am confused about current flow direction and, by consequence, the proper location of resistors in a basic circuit.

    Case in point - scrolling down to the middle of this sparkfun tutorial "What is a Circuit" they show a diagram (in the subsection entitled "The Simplest Circuit" with a circuit composed of a battery, resistor and one LED:

    https://learn.sparkfun.com/tutorials/what-is-a-circuit

    The diagram also indicates the direction of the current flow with a clockwise circular arrow from the positive to the negative battery terminal. This in itself is a little confusing to me because I learned from previous tutorials and other sources and that "charge" is actually moving in the opposite direction, from negative to positive, due to the flow of electrons. I understand that current can actually flow in both directions and that positive to negative direction was established as a standard for consistent polarity starting with Ben Franklin. What confuses me is that many of the same sources also stated that electrons, and thus "negative" particles, are the only things that move in copper wires. Logically, this implies to me that there should be serious practical considerations for positioning components on in a circuit if the actual movement is occurring from negative to positive. I understand just from reading that this is not actually the case, but my mind cannot detach itself from this simple logic.

    Which brings me to my next point - the location of the resistor in this "simplest circuit." In the same diagram from sparkfun, the resistor is located "before" the LED in terms of the flow direction, accepting that the flow is from positive to negative, which is must be due to the polarity of the resistor and LED. Logically, this makes sense to me, since the resistor is needed to reduce current going into the LED. However, just below the diagram they post to a link to an animation in which the resistor is placed AFTER the led with respect to the current direction.

    I have since read other tutorials and texts where I have witnessed the same "inconsistency" - sometimes the resistor is placed before the LED, sometimes after.

    Can anyone please help me understand this? Thank you very much in advance.
     
  2. Brevor

    Active Member

    Apr 9, 2011
    279
    54
    It makes no difference the resistor will limit the current the same anywhere in the series circuit.
     
    #12 likes this.
  3. dl324

    Distinguished Member

    Mar 30, 2015
    3,235
    619
    Conventional current flows from the positive terminal of the battery or power source to the negative terminal. The location of a resistor in a "loop" is usually a matter of preference because the same current will flow through all elements in series. But sometimes, element placement will be done for safety. I'll post an example shortly...
     
    #12 likes this.
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    You have run into something everyone in electronics has to deal with, the difference between conventional and electron flow:
    http://www.allaboutcircuits.com/textbook/direct-current/chpt-1/conventional-versus-electron-flow/

    As for the location of the resistor relative to the "source" of the energy - doesn't matter. One of the rules of electronics is that in a series circuit, all of the current goes through all of the devices all of the time. Using the water analogy, if a pump is pushing 5 gallons per minute through a loop made up of pipes of several different diameters, all of the water goes through all of the pipes. If you rearrange those pipes in a different order, the same pump makes the same 5 gallons per minute through all of the pipes. Depending on what other circuit components are out there, other aspects of the circuit might change when you re-order things, but for a simple series circuit it doesn't matter. And by itself, a resistor does not have a polarity.

    ak
     
    Last edited: Jul 21, 2015
    #12 likes this.
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    But the LED does. It lights up only when current (there is only one definition of "current") flows from the anode to the cathode (implied by the arrow in the symbol). Read my sig line, so you know how I come down on direction that current flows...
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    The only time you actually need to consider electrons as the charge carriers is when studying solid-state device theory and the operation of vacuum tubes (valves for the English).
    Otherwise it's generally easier to use current as flowing from positive to negative.
    Thus, for example, current flows from top to bottom in common schematic layouts which has the positive voltage supply at the top and the ground at the bottom.
    Also current flows in the direction of the arrow in schematic symbols for diodes and bipolar junction transistors.
     
    djsfantasi, #12 and nsaspook like this.
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Hello sso311, welcome to the forums. You’ve asked a bunch of excellent questions.
    Now that is actually very difficult to explain, especially since there is a LED in the circuit. For now please accept that while the resistor is limiting the current into the LED that the LED is limiting the voltage across the resistor. They play so well together it matters not where the battery is in the simple series circuit.

    The math to solve for this may make that clear if you would like to see it please ask.

    As far as current goes, there is actually one true definition: that where a positive current goes from the positive voltage to the negative voltage. This is called conventional current and is consistent with many other units used in science and engineering. The current called “electron flow” current is inconsistent with those same units, do try to avoid it whenever possible.

    Just remember a negative charge going this way -> is the same current as a positive charge going <- that way and there need be no confusion.
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,163
    #12 likes this.
  9. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    When I was first learning, the process of figuring out a circuit was a matter of noting on the schematic what I knew and figuring out the parts I didn't know. Sometimes I would have so many doubts that I couldn't figure out who was in charge of current in this path or voltage in that path. That's called, "fail".

    I tried using flow from positive to negative. I tried using flow from negative to positive. Well, that was in the days of vacuum tubes. Guess which one worked most often?

    Then transistors became popular. When transistors rule the schematic it doesn't matter which flow direction you use, as long as you're consistent for the duration of that job.

    Eventually, there was a Convention and they created a convention that says electricity only flows from positive to negative. Apparently it is now a Law. I don't care. Electricity doesn't care. Use whichever direction you need to understand how it is going to operate in a circuit. By the time you get to the level of sophistication where it can only flow one way, you will be able to think in either direction and you will probably know Calculus and Latin.

    We already don't expect to see vacuum tube CRTs for sale. When we finally have x-ray LEDs, we will all have to think in positive to negative because it will work for any circuit.
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    While rarely is there an electrical reason, there can be a physical one. For a typical indicator like a 12V source on a pc board, a 1 K resistor to limit the current to 10 mA, and an LED in a remote location off board, resistor placement in the circuit matters. If you bring the 12 V out on the wire harness to the LED, then the return leg of the LED goes back to the board where it goes through the 1 K resistor to ground, you have exposed the system +12V supply to a possible external dead short to some other ground like a sharp edge of a piece of the chassis. On the other hand, if the circuit has the +12V, then the resistor on the board, then the external connection to the led, an accidental dead short does not compromise full system power, or overheat wires, or melt connector pins, or other bad things. Some high-reliability projects have rules about things like this.

    ak
     
    Last edited: Jul 21, 2015
    #12 likes this.
  11. sso311

    Thread Starter New Member

    Jul 16, 2015
    3
    0
    Wow, thanks for all the fantastic replies. Really amazed at the level of activity and helpfulness on this forum thus far. So now I understand why I kept seeing this "inconsistency" - the position of the resistor relative to the LED does not matter.

    Although I knew even before creating this thread that current flows in both directions and the standard convention for the direction of current flow is positive to negative, I am the kind of person that does not easily accept something without fully, completely understanding it. Maybe my problem is that I keep thinking of "current" as something that is *actually* flowing, as valence electrons or ions in solution might do. It seems to me that, since my understanding is that nothing is actually flowing from positive to negative in a wire, the best way to think about a positive to negative current direction may be a "realignment" of charged particles in relation to the electric field?

    I realize another one of my problems is that I keep thinking of electricity using water as an analogy. Although it can be helpful for understanding in some cases, there are distinct and strong differences. For one, water does not flow in 2 directions simultaneously (generally speaking. I'm sure someone can find an exception if they search hard enough).
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    Steam flows in both directions in some steam engine cylinders. Also, with two lumberjacks taking down a tree with one of those long two-handled blade saws, the net movement of the saw is zero yet work is accomplished - a fair analogy of AC in a light bulb.

    ak
     
  13. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    There is no "realignment" of charged particles when thinking of current flow. It's just a convention that assumes positive charge flowing from positive to negative.
    But if you feel more comfortable using electron flow from negative to positive, then go for it.
    Just be consistent in using it.
    And note that many references, when discussing circuits, use current flow not electron flow, so you will have to adjust for that.
     
  14. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,163
    That's a very good attitude to move from a physical analogy while building a new (and correct) mental electrical model.
     
  15. sso311

    Thread Starter New Member

    Jul 16, 2015
    3
    0
    OK, I guess my mind can't get over the idea of "flow" being associated with actual movement and to resolve this internal conflict I'm trying to understand what, if anything, is actually moving (on a subatomic level) from positive to negative in a standard circuit. It seems like there may be no answer and I will just have to get over it.
     
  16. dl324

    Distinguished Member

    Mar 30, 2015
    3,235
    619
    The "real" current is composed of electrons flowing from the negative terminal to the positive.
     
  17. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    I'm afraid you will. ;)
    The (positive) current flow perhaps can be viewed as a virtual flow.
    You can do all your calculations using that virtual flow and all the results for circuit operation will be correct.
     
  18. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    It's just a simple matter of sign. A negative charge –Q moving at rate R in a negative direction is an equivalent current to positive charge +Q moving at rate R in a positive direction, since (-Q * -R) = (Q * R)
     
  19. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,907
    2,163
    There is a definitive answer but it's not what you need to understand to see how circuits operate. You need to understand detailed signal/power 'flows' in circuits so the circuit theory abstraction of 'current' and voltage in conventional directions is what you need to worry about to understand the how of electricity first. Getting stuck on 'electron' movements will only confuse you until you have a much deeper understanding of the physics of electricity as to why things move on a subatomic level.
     
    Last edited: Jul 23, 2015
Loading...