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B Direction of current? Pos to neg?

  1. Apr 26, 2016 #1
    Does current flow from the positive lead to the negative lead of a battery? Does this ever reverse for any normal reason? I'd like to make my own circuits and I figure if current only travels in one direction then I wouldn't need a diode to stop the current from going back.
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  3. Apr 26, 2016 #2


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    Yes. Like water in a hose flows from the high pressure side of the pump to the low pressure side at the spout. Without any diode.

    And if the battery is empty, the current stops flowing.
  4. Apr 26, 2016 #3
    In DC current do the electrons actually move like water through the wire? And AC current they only rock back and forth?
  5. Apr 26, 2016 #4


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    At the most basic level of understanding, yes and yes.

    Most people who ask these kinds of questions do so because of automobiles. Is this your reason, or is it a homework question?

    DC current flows from negative to positive in electron theory, the accepted theory in almost all of electronics and physics. The electrons "move" (so to speak) through the wire.

    Automobiles were designed with negative chassis "grounds" based on the Franklin theory, the idea that "holes" being vacated by electrons provided the electric power. IMO as a theory it should have died with Franklin.

    You may be unaware that some cars are designed with "positive earths" where the chassis is positive. British cars often are wired this way. You cannot use a negative ground stereo in a positive ground car.

    Most AC components (stereos, TVs and such) are ultimately internally converted to DC to run the electronics. The older ones may have a negative ground, but not necessarily. Some guitar amplifiers have a ground switch on the back to change ground polarity in case the local wiring isn't up to code. Apparently some electric guitarists have been shocked by their guitars and requested this remedy.

    Of course, when you factor in coils and capacitors, semiconductor behavior, and the different ways AC and DC affect them, you create all kinds of scenarios. The math isn't hard once you learn the formulas. I never took anything higher than Geometry in HS.
  6. Apr 26, 2016 #5
    I'm trying to build some robotics sensors but I'm trying to visualize what is actually happening in a circuit so I can have a better grasp on things.

    And why do we say current flows from positive to negative when electrons flow the other way?
  7. Apr 26, 2016 #6


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    No, this is not correct. Current flows from positive to negative through a resistive load.

    If the charge carriers are electrons then they flow in the opposite direction of the current, but regardless of the charge of the charge carriers the current always goes from positive to negative.
  8. Apr 26, 2016 #7
    If electrons are not the current than what is?
  9. Apr 26, 2016 #8
    It rarely makes any difference which direction current is conceptualized as flowing, as long as you're consistent.
    Conventional current flows from positive to negative. Just a convention upon which authorities have arbitrarily agreed.
  10. Apr 27, 2016 #9


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    Current is the motion of charge. Since electrons have negative charge, their motion is in the opposite direction of the current.

    Again, because electrons are negatively charged their motion is by definition in the opposite direction of the current.
  11. Apr 27, 2016 #10


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    Current can flow the other way around in a battery if it is forced to, as in a battery charger. By forced to, I mean the voltage between the two terminals is set higher than the electrochemical potential of the battery.
  12. Apr 27, 2016 #11


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    Yes, that is why I specified "through a resistive load" in post 6.

    Note, in a battery very often some of the charge carriers are positive, so current is in the same direction as the motion of those charge carriers. Current remains in the opposite direction of any negative charge carriers. Batteries are one clear example of why it is more useful to think in terms of current than in terms of the motion of specific charge carriers.
  13. Apr 27, 2016 #12
    Yes. Conventional current flows from positive to negative :smile:
  14. Apr 29, 2016 #13


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    I think he means "Holes;" "Holes" carry the current (direct quotes in a letter from Mr. Benjamin Franklin, Esq. to Sig. Allessandro Volta). This is why book learned engineers and boots on the ground technicians don't often play well together. You should see some of the field fixes some of us have had to make in response to their "applied science." If you are an engineer with limited field experience, be sure to consult an experienced tech concerning ANY complicated project. For instance, Hondas are great cars, but you have to take the engine out of my wife's Honda to change her $10 transmission filter. No one asked a mechanic on where to place it, I'm sure of that.

    You are working digitally if you are working in robotics; current really isn't the issue, potential is, so you are working with the voltage side of the equation. Most semiconductors have an IR drop of only 0.6V. You're dealing in about 5V DC for the controller, am I correct? And with a computer clock frequency? Your sensor, is it optical, a NO/NC micro switch, or a weight sensor/pressure pad, and I assume it is shielded to protect sensor output from EMI from the stepper motors nearby? The cable connecting it should be shielded too, at the least be a twisted pair.

    If you are dealing with AC then you can cut one leg off of a transistor that can handle the power, and voila, you have a diode, but the new DC frequency will mix with your clock frequency in the controller and could create any of multiple new frequencies. Most people don't know the different touch tones on a phone are made by the mixing of only two audio frequencies. One bad diode in an alternator will cause all sorts of havoc in a late model car for this reason.

    Of course, if this is a hobby robot then you probably won't be dealing with huge EMI.

    Either way, I would suggest making the sensor as simple as possible, and keeping all of the controlling aspects in the controller. If you have to somehow make the sensor fit the controller, you need to find a different model sensor. [EDIT: I was thinking "component" when I said "sensor." My intent was to say something like this, an extreme example I know: if you have to build a power supply and its own controller to mount on the end of the robot's arm just to use the component you chose, you're doing something wrong.

    I've heard of one company asking a new engineer to power an LED in an already laid out AC circuit just to have him come back with a beautifully designed bridge rectifier when a resistor would have sufficed.]
    Last edited: Apr 29, 2016
  15. Apr 29, 2016 #14


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    Sorry for the rant earlier, kolleamm. As to circuit flow regarding the sensor: start at the power supply side, and note all of the components that hook in parallel to it. I'd imagine most will be switched by transistors or ICs, but I don't have access to your schematics. Current is controlled (switched and directed) by transistors and other semiconductors. Knowing whether the chips have AND, NAND, NORs, etc. isn't as important as knowing that a certain input here gets a certain output there, but whether the transistors are NPNs or PNPs is because that matters to which component is powered or switched by them. Remember resistors lower voltage, not current; bigger resistors have to be made to handle larger amounts of power (in Watts or "E", which is a multiple of current, in Amps or "I"). Your sensor component sends a signal that has to be sent in a manner and within a range of variables that can be understood by the controller, however that should look on your O-scope or freq counter.

    You can look up your IC by manufacturer and number in any semiconductor catalog and find out pins and outputs, major logic patterns, etc. and see if it helps you any. You can look up transistors the same way, but the issue will be NPN vs PNP. You can find semiconductors and LEDs with lower voltage thresholds if that is an issue as well. IIRC germanium takes only 0.3V.
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