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Voltage or current operated devices?

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

  1. beakie

    beakie New Member

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    How do I tell which a device is?

    I want to connect the following to a 12V source. Which need resistor consideration and which can be connected directly?

    12V Fan
    12V heater
    12V solenoid valve
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    None of the above
     
  3. beakie

    beakie New Member

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    I assume you mean they can be connected directly.

    Can you please name some components that require resistors? Just so I know...

    Cheers
     
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    All of those devices listed can be run from 12 volts.

    Many devices are voltage operated in that you apply a fixed voltage and they run fine. That's usually because at that voltage they draw a certain amount of current and if the voltage changes by a small amount the current changes by a small amount. If the device under consideration is run by a voltage like that but when the voltage is changed by a small amount the current changes by a large amount (looking at the ratio of voltage to current) then the device is usually considered current operated. In other words, we want good control over what is happening to that device so we use whatever method works best.

    This comes about because it is normal for the supply voltage to change by a small amount in many applications, and if that causes too much of a current change then something might burn out like the device itself, so rather than try to regulate the voltage to some extreme accuracy we drive the device with a current instead and let it seek its own comfortable voltage operating point (knowing the current will always be close to perfect).

    The good example of voltage operated is the light bulb. Applying the right voltage causes the right current to flow and everything works fine even if the voltage changes by a small amount.
    The good example of a current operated device is an LED, where the current must be set to some specific value and held relatively constant but there may be a range of voltages that different physical pieces of that part might operated at. In other words, when we buy an LED rated for 20ma at 3.5v, the most important spec there is the 20ma not the 3.5v because the 3.5v is just an approximation and may be as low as 3.2 or somewhat higher. This means we can not use a voltage supply of 3.5v because that may not cause 20ma to flow but rather either too little (like 5ma) or too high (like 50ma). If, on the other hand, we drive it with a current of 20ma then we dont have to worry about what the voltage is as it will be whatever that particular LED happens to be at that time and temperature. Note temperature changes the voltage characteristic too, so we can not even use a highly accurate voltage supply because it would only work at one temperature.
    For LED's, often a current limiting resistor is used in series with it to make sure the current doesnt go too high when it is run from a voltage source. That means that an LED can be run from a voltage source when it has a resistor in series with it of the right value, but should be run from a current source if it does not have a resistor.

    Note that it is the non linear devices that have to be looked at more carefully.
     
    Last edited: Jan 31, 2011
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  6. beakie

    beakie New Member

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    That makes absolute sense now! Thank you, Sir. A great response and a very much appreciated one!!!

    Cheers
     
  7. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Another example of a current operated device is a bipolar transistor. It's base-emitter junction looks like a diode and requires a resistor in series with the base to provide the desired base current when operated from a voltage source.

    A MOSFET, on the other hand, is a voltage operated device and you can apply a control voltage directly to the gate.
     
  8. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Voltage operated devices are characterized as "Ohmic Devices" (their current is more or less proportional to the applied voltage, I = E*R)

    Current operated devices are characterized as "Non-linear Devices or NON-Ohmic devices" (the current through them is NOT proportional to the applied voltage).

    {at least by me}

    Look at the following circuit and plots. It consists of two black boxes wired in parallel. A voltage source is connected, and is swept both negative and positive. The applied voltage is the independent variable, and is plotted on the X-axis. The dependent variables are the two currents, Ix(x1:A) and Ix(x2:C) through the black boxes, X1 and X2, and are plotted on the Y axis, respectively.

    By looking at the shape of the I/V curve, try to figure out what is inside each Black Box. Hint: each box contains only a single two-terminal electronic component.
     

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    Last edited: Jan 31, 2011
  9. Ratchit

    Ratchit Well-Known Member

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    crutschow,

    When you say "current operated", I hope you mean functionally speaking, and not operationally caused by current, or being current activated. Taking the causal point of view, the voltage applied to the base-emitter junction lowers the barrier voltage between the junctions, and allows the charge carriers to diffuse into the base region and on to the collector. The BJT needs the voltage across the junction to operate. The inevitable base current that occurs is a waste current that is not needed to operate a BJT. It is useful for design and calulation because it is somewhat linear with respect to collector current when the BJT is operated in the active region. So a BJT mimicks a current controlled device, but it is really a voltage controlled device. The only true current controlled device I know of is a magnetic amplifier.

    MikeMl,

    Are you defining active elements as "ohmic" or not? I thought only inactive elements such as loads had that designation. If a current source supplies current to a resistor submersed in a temperature controlled oil bath, would its output voltage not be proportional to the supplied current?

    Ratch
     
    Last edited: Jan 31, 2011
  10. BrownOut

    BrownOut Banned

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    There is no such thing as 'waste' current. The base current in a BJT is essential to the operation of the device. It doesn't mimic a current controlled device, it is a current controlled device. This has already been proven beyond the shadow of a doubt by several senior members on this site.
     
    Last edited: Jan 31, 2011
  11. Ratchit

    Ratchit Well-Known Member

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    BrownOut,

    Sure there is. The voltage across the base-emitter junction is absolutely essential for the BJT to work, for the reason previously given. No voltage-No operation. A true current amplifier would have low voltage or no voltage across its input. The base current is what is unavoidablly diverted from the emitter, and does nothing for the basic operation of the BJT. The higher the beta, the less waste current and power, the less relevant is the base resistor, the more the collector is controlled by the voltage across its emitter resistance, and the more the BJT acts as a better tranconductance amplifier--voltage in, current out.

    Not so. They presented arguments to counter my thesis, but they did not disprove it.

    Ratch
     
  12. BrownOut

    BrownOut Banned

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    No there isn't. The current is essential to the operation of the BJT. No such thing as waste current. No base current; no BJT operation. Beta relates collector current to the controlling base current, and does not make the base resistor less relevant -- it's just as relevant as if Beta was lower. Just try to bias a transistor without it and you'll quickly see how relevant it is. This has all been proven. You didn't have a thesis, just some made up stuff about waste current, of which no such things exists. Senior members have proven current control of BJT with terminal voltage and current equations, device physics, real charts, all of which were in perfect agreement. You can’t just make up stuff and pretend it’s real.
     
    Last edited: Jan 31, 2011
  13. crutschow

    crutschow Well-Known Member Most Helpful Member

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    You hope correctly. I was speaking functionally, as do most circuit design engineers. I don't give a rat's posterior about what the theoretical equations for the BJT may or may not imply about the internal behavior of the electrons and holes, if they are counter to how the device appears to behave and is normally used in the macro world. This subject has been thoroughly beat to death by you and others in another thread. You quixotically support an interesting theoretical view of how a BJT operates and you are welcome to it, but I imagine you will continue to espouse your view whenever anyone mentions your perceived blasphemy that the BJT is a current operated device.

    From an external point of view, the base voltage you mention can be just as easily (and more intuitively) be interpreted as the base emitter forward diode voltage necessary to generate a specific base current which is how it appears on a curve tracer, not the other way around.

    In practical applications (other than perhaps at RF) the BJT looks like a current operated device, acts like a current operated device, and is spec'd like a current operated device in the data sheets. To state otherwise just confuses the issue for the neophyte circuit designer. But you appear to enjoy doing that so I suppose we're sort of at a Mexican Standoff.

    If the base current is just waste current, then is it theoretically possible to build a BJT that takes no base current? And if it is, why can't it be done?

    P.S. I totally agree with your sign-off phrase.
     
    Last edited: Jan 31, 2011
  14. Ratchit

    Ratchit Well-Known Member

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    BrownOut,

    You are wrong, because it is not the current that lowers the barrier voltage caused by the junction diffusion. Without that counter voltage, the BJT ceases to provide diffusion current. You are confused by the current involvement. The base current exists, but it is the base voltage that is needed.

    The collector current is exponentially related to the base-emitter voltage. So is the base current. Dividing the collector current by the base current cancels out the expontial terms and give a more or less linear relationship, which is useful. That does not prove that Ib is essential for transistor operation. Only that if it exists, it can be useful.

    Actually, it would be easier. If the beta were so high as to make the base current irrelevant or essentially nonexistent, then the bias could be accomplished by a simple voltage divider.

    You said that before. So I will say again that my thesis has been argued against, but not disproven. If you say that again, I will say the same thing in return.

    Of course I had a thesis. You might not have agreed with it, but it was a thesis. The stuff I "made up" was given careful consideration and analysis. In fact, I am not the only one who thinks this way.

    No they did not. Many of the arguments were irrelevant, put downs, or misunderstandings. They did not by any means "prove" it. As I said, I did not just make up my thesis willy-nilly.

    Ratch
     
    Last edited: Jan 31, 2011
  15. BrownOut

    BrownOut Banned

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    You are wrong. The base-emitter junction is current controlled. No voltage can lower another voltage. As we've already proved, the junction voltage is lowered by charge which neutrailzes depleation layer ions. No other physical means exists to lower barrier voltage. No voltage can lower another voltage.


    It's not meant to prove Ib is essential, but Ib is essential. If it stops, all transistor action stops. Show me any example where base current stops and a BJT continues to operate. Unless you show a real example, then base current is proved to be essential.

    Beta can never be made so high that base current is irrelevant or nonexistant. Base current will always be essential to BJT operation. Using a simple voltage divider doesn't eliminate base current. In fact, transistors are already biased that way. The voltage divider resistors not only divide the voltage, but also limit the base current, just as a resistor in series with the base input resistor would do. You cannot bias a transistor without limiting current, and a series resistor is always required, no matter how high the beta is. Even a voltage divider has a resistor in series with the base bias voltage.

    Yeah, any 9 year old argues that way.


    Hardly. Nobody espouses the fantasy of waste current. You can't just make up stuff and pretend it's real. Let's keep the discussion about real physical quanties, and not make stuff up.


    Our senior members do not misunderstand. They have the experience and background to know what they are talking about. Those who have not the same experience would do well to learn from those who do.
     
    Last edited: Jan 31, 2011
  16. beakie

    beakie New Member

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    Thanks for your thoughts guys.
     
  17. BrownOut

    BrownOut Banned

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    Well, Rachit will insist on voltage control until kingdom come. Don't believe it, no matter how many cartoons he posts. Just stick with the definitions given before his posts, and you'll be fine.
     
  18. beakie

    beakie New Member

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    Thanks all :)
     
  19. Ratchit

    Ratchit Well-Known Member

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    crutschow,

    There are two points of view which complement, not oppose each other. The functional view is what you and just about everyone else uses because it works for design and calculation. The causal view is what the really makes the BJT work. You are right, most folks don't care about the inner workings of a transistor, only what and how it can be made to do things. But I take umbrage when they say it is a BJT by itself is current amlifier when in fact, it is a transconductance amplifier. It can be easily made to act as a current amplifier, but it should be made clear the difference between functionality and causality.

    You are sure correct about that.

    Thank you, yes.

    Translate "external" above to "functional" and we pretty much agree on the above.

    I don't mean to confuse anyone, and I do advocate using the useful relationship of beta for calculations and design. That superfluous base current does have to be taken into consideration, and the functional point of view is the best way to do it.

    It is because of diminishing returns. The voltage has to be applied to base-emitter, but there is no way to prevent some of the current from diverting from the emitter into the base circuit. BJT's are a mature technology, and the transistor makers have done just above everything they can do to prevent this from happening. When they succeed a little further, the beta increases.

    Why thank you. It is a warning and a statement of honesty. I was banned from another forum because I pointed out that the log of a negative number did exist. That was the last straw.

    Ratch
     
  20. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    When we classify a device as current operated or voltage operated (or both) we dont really look at the most basic physical theory for that device anyway. We look at how it operates under the influence of voltage and current and pick the one easiest to control. That's the way most devices are classified. That doesnt mean that for EVERY operating point we can claim complete accuracy for our chosen classification, but it does mean that for the majority of applications that classification will apply.
    For example, we can say that a zener diode is voltage operated before it starts to zener because for a small change in voltage we get a small change in current, but that is not a good overall description of how the zener diode works. As we get near the zener point, a small change in voltage causes a large change in current (unlike before) nd this is the way the zener is normally used so it would be best to classify it as current operated.
    For charging a battery we need a voltage higher than the terminal voltage, but it's not the voltage we have to pay attention to it's the current because the current is what adds charge to the battery.

    Here's an experiment we can perform that will help to classify a device as either voltage or current operated for most applications, but again part of it depends on the bias and just how far we want to go to run it as voltage operated when it is really current operated or vice versa.
    We start with a really good power supply that has two knobs on the front panel, one to adjust voltage from 0 to 50v and one to adjust current limit from 0 to 100 amps. We turn the voltage to zero and the current limit to 100 amps and connect the device to be tested up to the output terminals. We then turn the voltage knob up near the typical operating point and measure the current with a current meter, and note the resulting current flowing. We then turn the voltage up a little bit more and note the new current reading. We then turn the voltage up a little bit more taking the same increment as we did before and note the current again. This means that we supplied the device with three voltages v1,v2,v3 (where the difference between v2 and v1 is the same as the difference between v3 and v2), and obtained three current readings i1,i2,i3. Now if (i3-i2) is greater than (i2-i1) we have a current operated device, but if (i3-i2) is less than (i2-i1) then we have a voltage operated device, but if (i3-i2) is equal to (i2-i1) then it can be either current or voltage operated.
    That's a basic test for what type of operation the device would be classified as, although there would be some error limit to consider.

    Another way of stating this is:
    If we apply V volts and obtain I amps at an operating point P(v,i) and we choose a constant A such that:
    V/(A*I)=1
    and then for an increment dv in the voltage V we obtain the increment di in current then:

    if (V+dv)/(A*(I+di))>1 then the device is voltage operated, but
    if (V+dv)/(A*(I+di))<1 then the device is current operated, and
    if (V+dv)/(A*(I+di))=1 then it can be operated either way.

    That's the way we usually classify a device in the real world, but note it implies an operating point which could include temperature too: P(v,i,T) and possibly other things like pressure. This doesnt mean that we cant take extra steps to run a current operated device with a voltage or vice versa, but it does mean in the usual case we would not want to go through the trouble of making sure it worked well when doing it that way. It's much easier to run the device as it is classified, and that's also how we classify it in the first place. It doesnt matter if the device can be characterized by an equation like i=f(v) where f is some nonlinear function, it matters how the current changes when we run it with a voltage or vice versa.

    Note that for a current operated device when we turn up the voltage from the power supply we get some current level and then when we turn it up a little more we get some more current, but if we turn the voltage up some more we might get such a huge increase in current we burn it. Thus, if we are determined to run the current operated device with a voltage (like a base emitter junction) we better be darn careful that we dont overshoot or that transistor is history. This doesnt mean that we can never do this, it just means that the steps we would have to take to ensure that the device operated properly would be a lot more involved than what we do when we drive it the way it is classified.
    Note that when we operate the current operated device with a current we can increase the current a little at a time and as long as we stay within the limits of the device spec we dont have to worry about burning it out because the voltage changes very little with small step changes in current.

    One other interesting point here...

    If we try to run an LED (like a base emitter diode) with a voltage, we may find that we can not even do this in any practical case. Remember that a voltage source has very low impedance too, so any error in the voltage set point measns a huge current could flow or very little current at all. Thus, we can not state: "Operate your LED at 3.5 volts all the time" because it we tried to do that we would run into lots of problems with different physical parts, and even if we got lucky if the temperature changed it could still blow it out. On the other hand, it's very easy to state: "Operate your LED at 20ma all the time" because that way any LED we chose from that family will work just fine if the voltage is 3.1v, 3.2v, 3.5v, 3.6v, etc., and if the temperature changes the voltage changes a little but the LED still gets 20ma just as nice as can be :)
     
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  21. Ratchit

    Ratchit Well-Known Member

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    BrownOut,

    The charge cannot move until the barrier voltage is lowered. Although the individual voltage sources do not get lowered by another voltage source, the net voltage is lowered by Vbe.

    False logic. Base current is always present when forward Vbe exists. Ib can be reduced by higher beta, but not eliminated. The existence of Ib does not prove it to be essential.

    So why do you keep repeating yourself?

    False accusation. See BJT vs FET current Mirroring , post #4, second paragraph, first sentence. When folks worry about it, they may call it something else other than "waste". You acknowledge the loss everytime you realize that not all the emitter current winds up in the collector circuit.

    Most of the time they do, but they are human like everyone else here.

    What cartoons? I believe in the voltage causality of a BJT. Control depends on the circuit where the BJT resides.

    Ratch
     

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