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Voltage or current operated devices?

Discussion in 'General Electronics Chat' started by beakie, Jan 31, 2011.

  1. Ratchit

    Ratchit Well-Known Member

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    misterT,

    The color of the LED is immaterial. Find the desired current from the specifications. Subtract the given voltage drop of the diode from 5 volts, or use 0.7 to 1 volts if that information is not available. Calculate the resistance from the resistance formula R = V/I. Hook everthing up in series and voila.

    Ratch
     
  2. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    Yes, physics surely does take energy into account. And energy does move charge carriers, doesn't it? But voltage and energy are inextricable. As I said before, voltage is the energy density of the charge (joules/coulomb). In other words it, is the energy concentration wherever charge is present. So although voltage is not energy, you cannot have electrostatic voltage without both energy and charge.

    The only controller. The diode current is controlled by the concentration of electrical energy across the diode.

    You mean that electrostatic voltage cannot be defined without charge. The concentration of electrical energy has to exceed a minimum to move an electron through the band gap.

    You cannot have voltage without energy. Yes, you will create a higher energy concentration with photons, the same as if you applied the voltage directly with wires.

    Yes, voltage is controlling the current and not the other way around. Voltage and energy go together. Energy may do the work, but if the energy concentration is not high enough, then the work does not get done. I am not ignoring energy.

    Do you mean putting the devices in parallel? How would I be ignoring the current by driving many devices in parallel?

    What is the point of this fantasy circuit?

    Ratch
     
    Last edited: Feb 15, 2013
  3. misterT

    misterT Well-Known Member Most Helpful Member

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    See.. No physics needed. To make things even easier, you could just slap a ~1k..~330 ohm resistor in series with the damn thing and be done. The color is immaterial.. sure. I'll give you that.

    EDIT: How do you find the desired current from the specs?
     
    Last edited: Feb 15, 2013
  4. dave

    Dave New Member

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  5. Ratchit

    Ratchit Well-Known Member

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    misterT,

    You must have missed the other posts, where I wrote that a deep understanding of physics was not necessary for design procedures. On the other hand, design knowledge does not tell you how a device really works.

    Are you asking me how to read a spec sheet? Attach one and I will tell you. Or better yet, forget about it.

    Ratch
     
    Last edited: Feb 15, 2013
  6. misterT

    misterT Well-Known Member Most Helpful Member

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    Are you embloyed? I would hate to work with you. Or be working for you.. That would be the worst case scenario for me. I would quit so fast. You could run a nuthouse and you would be the nuttiest of them all.. and Sheldon Cooper would be your favorite patient. I can't explain why, but read my signature.
     
    Last edited: Feb 15, 2013
  7. Ratchit

    Ratchit Well-Known Member

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    misterT,

    You won't have to worry about that.

    Ratch
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,


    Devices in parallel means the current splits. If the current demand for all devices is the same, the current splits as I=I_total/N where N is the number of devices, or I_total=I1*N.

    With an infinite number of devices, the total current is:
    I_total=I1*infinity

    We can actual control an infinite number of theoretical devices this way because we can *state* that I1=0. But in real life, I1 (the current of one device) can never be zero. Even in the static case there's always at least some leakage current. So looking at this hypothetical circuit tells us that there isnt enough energy in the universe to power an infinite number of circuits of any kind, so a voltage source can never power an infinite number of devices.

    Yes we need the charge to move, and that requires *new* energy. And energy can never be extracted from a voltage without at least some current so accompany it. I cant say this any clearer than that.

    But i feel we may be getting off the point anyway. That's because the OP was asking about a practical device, not a PN junction. The PN junction is just part of the device not the whole thing, and we are not using a PN junction alone we're using a whole device.
     
  9. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    Did I ever say that any practical voltage source you can come up with and sustain is going to be infinite? The voltage souce will always be finite. But so what? What if current is involved? Voltage controlled devices rely on the energy concentration per unit of charge (voltage=joules/coulomb) to control the current, not the other way around.

    Yes, and the point is? Anyway energy = voltage x charge. That still means that energy concentration per charge (voltage) does control charge or current in semiconductors.

    The OP was asking about the "difference between voltage and current operated devices". We got on the subject of PN junctions because it is a voltage controlled device that is often mistakenly thought of as current controlled.

    Ratch
     
  10. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Ratchet,

    "Did I ever say that any practical voltage source you can come up with and sustain is going to be infinite? "
    YOU didnt have to say a voltage source is infinite, the current from it would HAVE to be infinite in order to drive an infinite number of devices that you are calling "voltage controlled". If they were truly voltage controlled, they would be ALL controllable from the SAME real voltage source. Obviously that's impossible.

    "What if current is involved? Voltage controlled devices rely on the energy concentration per unit of charge (voltage=joules/coulomb) to control the current,"
    But voltage itself isnt doing anything by itself. It's the energy. That's the whole point. Granted the current is low, and that's where the phrase "voltage controlled" comes from. But the point is that it's not just the voltage itself, that's just the way it is *viewed* for simplicity.

    I think that all that is needed here however is to clarify when we are talking about PN junction theory and when we are talking about total device theory, and what actually occurs in real life practice.
     
  11. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    Why are we talking about an infinite number of devices? I am only referring to one device. If there are several devices, then there would have to be enough energy concentration per unit charge (voltage) available to control those devices from a single voltage source. Just because a device is voltage controlled does not mean that current is not involved. As I said before, charge and voltage and inextricable.

    Yes, the voltage is doing something. It represents the concentration of energy per unit of charge. I am trying to get across the point that it is the concentration of electrical energy that controls the current in a semiconductor. The concentration of elecrical energy per unit charge (voltage) takes into consideration of both energy and charge.

    I think I did that in my previous posts.

    Ratch
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,

    Read your own words, first paragraph. If something was PURELY voltage controlled, then you would not have had to say that. By saying what you have in this thread, you are implying that everything is voltage controlled because it doesnt matter if it also needs current it's still voltage controlled. But the mere measurement of a parameter doesnt make it the controlling parameter.

    What i am stating on the other hand is that everything needs both voltage and current, and so more concisely there is a V I trajectory for every device (see attachment). To fully explain the device we need to know this information.
     

    Attached Files:

  13. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    I think you are making your remarks to me. I always try to name the person to whom I am talking.

    I see no dichotomy in what I said with regard to voltage control causing current (notice I said just "current", not "current flow"). You have to distinguish between control causation (voltage) and consequence (current). Just because current is a consequence of voltage does not mean that current is controlling something.

    And so what if there is both voltage and current in a device that can be plotted. I said before that you cannot determine what is controlling a device by graphs, equations, or measurments. You have to know the physics of the device to find out that information. For semiconductors, the physics point to the energy concentration per unit charge (voltage=joules/coulomb) that controls the device.

    Ratch
     
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    <still editing for format>

    Hello again,


    "I see no dichotomy in what I said with regard to voltage control causing current (notice I said just "current", not "current flow").
    You have to distinguish between control causation (voltage) and consequence (current). Just because current is a consequence of voltage
    does not mean that current is controlling something."
    That's an interesting statement. You are stating the current is a consequence of voltage, but that's only true in a separate circuit
    apart from the 'voltage'. Since that's not always the case, and when we want to know how a device is 'operated', we want to talk about
    the voltage and current in the SAME circuit. Let me try to make this a little more clear.
    When we energize say a gate, we are trying to control the transistor. Once we establish a voltage, we see drain source current flow.
    But the whole idea behind control is what are the signals that we are applying TO THE CONTROL terminal to get that control, not just
    some random signals that exist in other places in the device.
    So the whole point is what is going on with the control signal itself. And in order to drive the device we have to apply both current
    and voltage, because here is no way to apply a voltage from out of nowhere to a device.

    "And so what if there is both voltage and current in a device that can be plotted. I said before that you cannot determine what is
    controlling a device by graphs, equations, or measurments. You have to know the physics of the device to find out that information. For
    semiconductors, the physics point to the energy concentration per unit charge (voltage=joules/coulomb) that controls the device."
    I did not create the graph and then looked at the physics, i looked at the physics first and then came up with the graph. That graph
    shows how EVERY device behaves with respect to current and voltage, even if you could somehow prove that a device could be totally
    voltage controlled (pure voltage control is shown along the x axis). Trying to void that graph is the same as voiding your own
    statement that something can be purely voltage controlled because pure voltage control appears along the x axis. You're also trying to
    void many devices that exists because that graph represents all devices that are voltage or current controlled (if they could exist
    alone) and anything in between (except negative polarity signals were left out for simpliciy).
    You're also now stating that it is in fact the energy that makes the difference, but still claiming that voltage is the controller and
    even typed out a little formula for the voltage to emphasize this :)

    The circuit you seem to be working with is a totally fictious circuit in every sense of the word. You've somehow managed to get a
    voltage to appear somewhere you wanted it to be without moving any charge. I'd like to see how you did this :)

    This is why the transistor is sometimes also quoted as being "Charge Controlled". In this scenario the movement of charges controls
    the device. But we all know you cant move a charge without energy, and energy cant come from voltage or current alone but requires
    both.

    So back to your statement, "current is a consequence of voltage". Since the movement of charge is the only way to establish a voltage
    in the same circuit loop, any voltage that we apply has to come through wires and so there has to be movement of charge just to get
    that voltage there, which means charge has to move though the wires as the voltage is climbing.

    So when we try to get to the "underlying physics", we would like to know the most basic physical principles at work, not some super
    shelf where it looks like something else is happening.
     
  15. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    I never said voltage and current occur in a "separate" circuit. Of course, voltage and current are in the same circuit. The parameters that control a device are not determined by what signals one decides to apply that control. It is determined by what signals actually cause the device to be controlled. It does not matter if current exists from a control voltage. It is the voltage that is controlling the device.

    I do not disregard the graphs if you are talking about design. I do consider them irrelevant for determining control. A good example is the Ib-Ic graph for a BJT. That graph is good for design, but it does not tell you that a BJT is voltage controlled.

    You are misquoting me. I said that the energy concentration (voltage) is what controls a semiconductor.

    I never designated any circuit or schematic. The voltage will appear wherever the input signal is applied. I said several times now that current may accompany the voltage, but it does not control the semiconductor.

    And the "sometimes" pertains to nonlinear switching circuits where turn-on and turn-off transients come into play. This is not applicable to the active region region which is voltage controlled.

    Charge movement is going to occur from the region of higher energy concentration (voltage) to the region of lower energy concentration. The signal voltage is going to establish the voltage to do this. Charge is not going to move by itself. But all this is irrelevant with respect to what is controlling the semiconductor.

    And that is what I have been trying to expound on with such concepts as diffusion and lowering of barrier voltage.

    Ratch
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    But you're still talking about two circuits, one the controlling circuit and one the controlled circuit. That is apparent because you have a voltage source coming into being without stating how that voltage got there. If it is the "control" voltage then someone somewhere at some point had to do something to get that voltage there. Maybe they carried a battery to the device :) or they set up a circuit to do this. In other words, to explain the device correctly we have to not "have" anything from out of nowhere. How it got there is of great interest. And once it begins to appear there, there's no way to tell (looking near the zero on that graph) if the voltage came first or the current started immediately with the voltage. In fact, since the voltage is basically an accumulation of charge both had to occur at the same time. Hence my argument that there is nothing that is purely voltage controlled.
     
  17. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    Let's keep it simple. How many circuits are there in a diode in series with a voltage source and a resistor?

    I will leave it to your imagination as to how a voltage gets applied to the diode.

    It doesn't matter. First or second have no meaning in this discussion. Whether the voltage is caused by some charge gathered together, or the same voltage level is caused by less charge gathered closer together, the control for the semiconductor device is going to be determined by the voltage, and therby the energy concentration, according to the physics of the semiconductor. The charge is present but its amount does not control the semiconductor.

    Ratch
     
  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    Well since this discussion is never going to end, lets just say that your view is an internal view after the fact, and my view is external before anything anywhere by anyone or anything has done anything. So we could sequence your view and my view by placing my view just ahead of yours in the time line. After my view starts and initiates the control, sometime after that your view starts to take effect. So your view starts with prearranged conditions while mine assumes nothing except the very basic physics of particles.

    So physically, your view is internal and starts at a time period sometime after operating conditions have been established.
    Physically my view starts when the semiconductor is first made in the manufacturing plant, and so the time line starts at the birth of the device, but can be extrapolated to the time when the device is first used in a circuit (or used several times after that) but before any external energy can affect the device in any way, including heat or light energy, so that means it is held in complete and utter darkness and at near absolute zero until the first use. But again we can extrapolate to the time when some heat is already present and some light is already present, which of course changes the static operating conditions and the amount of energy needed for control.

    So in short, we're taking two different physical starting conditions and two different time lines.

    In short, my goal is to show that all devices need both current and voltage to operate, which constitutes energy, so that any control of any kind has to be able to supply energy.
     
  19. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    I don't see the relationship between viewpoints and starting points of time lines with respect to whether a device is voltage or current controlled.

    The need for energy never was in question. Voltage alone implies the availability of energy and charge, for otherwise, voltage cannot be defined. For a semiconductor, just energy is not enough. The energy has to be available in a sufficient concentration. That is what voltage indicates.

    Ratch
     
  20. fernando_g

    fernando_g New Member

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    When I first started following this thread, it immediately dawned on me that it would become one of those (post >100) threads...we are getting there.

    I only hope that the original poster is not scared of asking another question again!
     
  21. schmitt trigger

    schmitt trigger Active Member

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    [MODNOTE]Off Topic. Deleted[/MODNOTE]
     
    Last edited by a moderator: Feb 19, 2013

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