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Constant current source?

  1. Jul 13, 2014 #1
    How are constant current sources different than any other source?
    Also, would they really have a constant current in any situation? Even when back-emf is induced?

    Can anyone explain the use of a constant current source, and how different they are from other sources?
  2. jcsd
  3. Jul 13, 2014 #2
  4. Jul 13, 2014 #3
    True "constant current sources" don't exist. Imagine removing such a device from a circuit; it would arc, and continue to arc until it either destroyed itself or was replaced.

    But, it can be useful to consider something a constant current source. If you have a constant voltage source V in series with resistance R, it will behave exactly the same (in terms of the circuit) as a theoretical constant current source I=V/R in parallel with resistance R.
  5. Jul 13, 2014 #4


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    This is precisely what can happen to a current transformer, used for measuring the current flowing in HV cables. They are always kept with their terminals shorted together until they are actually connected to the measurement circuit.

    It is strange that the 'Voltage source', as a concept gives us very little intuitive trouble. But perhaps not so strange, I suppose, since the invention of the Car battery, which maintains a pretty good PD, even when you roast and melt a thick piece of wire, held across the contacts. There, you have a source of 'as much current as is needed' to give you 12V. A very familiar piece of kit.

    If you still have a CRT based TV, the beam of electrons has a current which is set by the electron gun and the grid and the same current will fall on the screen for a massive range of screen / cathode voltages. There, you have 'as much PD as you need' to ensure the required current; a 'current source'.
  6. Jul 13, 2014 #5
    But what is a "constant current source"? I don't understand what it is, and what differs it from the other sources?
  7. Jul 13, 2014 #6
    constant current source is to current like a battery is to voltage.

    Think of the I-V curve. An ideal constant current source is a straight horizontal line, while an ideal voltage source (like an ideal battery) is a straight vertical line.
  8. Jul 13, 2014 #7


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    A Constant Current Source is a Fiction. It is not a real piece of equipment or component. It is an Ideal which is used in circuit analysis.* Its characteristic is that it delivers the required current into any load, whatever its value. Constant current supplies are less familiar, which is why they give problems for students. It is valid to assume a constant current in many real circuits, even when the source consists of real components.

    There is a parallel with the way that a constant voltage source is used. For example, an ideal power supply would normally maintain its nominal volts, whatever load you give it (in practice there would be limits). A good audio amp would behave very much like a constant voltage source, providing the loudspeaker with the same voltage signal for a wide range of load impedances. We often assume a constant voltage in these cases.

    A constant current supply will behave in a similar way but will deliver the required current whatever the load impedance. You can achieve 'near enough' constant current by using a high supply voltage in series with a very high resistance. The current that comes out will be (more or less) the same over a large range of (low) load resistances. A high enough supply voltage and high enough source (series) resistance will behave a a constant current source. Work out the current from a supply of 1000V in series with a source resistance of 1MΩ, when you connect it to 100Ω load or a 10Ω load. Near enough the same value for practical purposes.

    *Just the same way that a Voltmeter or Ammeter is included in a circuit; we usually ignore the fact that meters actually consume Power. Engineering is full of such dodges.
  9. Jul 13, 2014 #8

    jim hardy

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    Have you studied Limits yet in math ?

    A constant current source would be an infinite voltage in series with an infinite resistance. We all know that can't happen, how would you ever pick it up without getting electrocuted?.

    But how would a circuit behave if you had numerically equal voltage and ohms in series ?

    Let's start with one volt and one ohm, then see how it behaves as we increase them.

    One volt and one ohm would deliver one amp into a short circuit
    and one half amp into a one ohm load
    and one tenth amp into a nine ohm load,
    and so on. That's a long way shy of constant current....

    Now take 10 volts in series with 10 ohms.
    That'd deliver one amp into a short circuit
    and 10/11ths of an amp into a one ohm load
    and one half amp into a ten ohm load...
    better, but not close..

    Now take 100 volts and 100 ohms
    you get one amp into a short circuit
    0.990099.. amp into a one ohm load
    and 0.90909... amp into a ten ohm load

    Try the same arithmetic exercise with 10kv and 10 k ohms............

    As you see, the higher the internal voltage and internal resistance, the more nearly it resembles a constant current source.

    So: A constant current source behaves mathematically like the limit as n approaches infinity of n volts in series with n ohms.
    In reality no such device exists, but there do exist pretty good approximations of it.

    A real word example is a "Capacitive Coupled Potential Device"
    where perhaps 500 kilovolts is the driving voltage.

    Or Sophie's current transformer.

    An op-amp can be arranged to behave like one within the limits of its power supply. They're used a lot for providing constant current to electronic gizmos.

    So form yourself a mental image and work it in your head until it agrees with the math - it's a really useful concept, if a hard one to believe in .
  10. Jul 13, 2014 #9


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    You normally consider constant voltage sources, called batteries or regulated power supplies. The circuits are designed to draw the current they need from the constant voltage sources. Oscilloscopes and voltmeters are used to observe the signal and supply voltages. You only know the current if you also know the circuit's series resistance, and can compute reciprocals or do arithmetic division.

    In the other half of the universe, where I am, we use constant current supplies. I have linear constant current generators and switching current regulators that work really well. My circuits are designed to drop the voltage they require when powered by a constant current. I can still measure the voltage across the components, but I know the current is constant, so I can compute the circuit resistance as being directly proportional to voltage. I have no problem with circuit inductance.

    In both our worlds, the power distribution grid systems are based on constant voltage levels so any customer can independently draw, and pay for, the current they require from the supply. If a constant current was passed around the neighbourhood, then we would all need to drop only the voltage we required and there would be all sorts of problems with earthing and insulation breakdown, on very thick wires.

    Sometimes a constant current circuit is needed by an engineer in your world, where a circuit from my world must be used. But your education system grew up in the time of “batteries” of electrochemical cells, with all those crudely fixed voltage sources, so it has forgotten to teach you about the other half of the universe.
  11. Jul 13, 2014 #10
    I'm trying to grasp the idea... but Jim thanks to your relation to limits that accelerated it.
  12. Jul 13, 2014 #11
    Thank you for the help everyone.
  13. Jul 13, 2014 #12

    jim hardy

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    nowadays you'll see them in lots of configurations , used for LED drivers .



    you can think of it as a variable voltage source that adjusts its voltage to maintain the desired current. That's how you make an op-amp do the job.
    see the 'getting started' links here
  14. Jul 13, 2014 #13


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    For a constant current source:
    In just the same way as you think of a regulated voltage supply, as a device that adjusts it's current to maintain the desired voltage. You can also do that with an op-amp and series pass transistor.
  15. Jul 13, 2014 #14
    A good use for real current sources is lighting at airports. If you used a voltage source you would need to run 2 wires in parallel across every single light, one hot and one neutral. a lighting circuit at an airport can be miles long so by the end the voltage would be weak because of the resistance in the wire. Now if you use a current source you can put all the lights in series and you only need one wire going into and out of each light. Each light receives the same power because it has the same current. This also saves a fortune in copper because a good wire layout will have a single wire making a big loop around a runway or taxiway as it goes to each light.

    In case anyone was wondering, each light in an airfield is connected to an isolation transformer. If the light burns out, as they do, current can still flow through the primary winding of the transformer to all the other lights. One bad light will not take out the entire circuit.

    The current sources they use come in large sizes like 5kW to 25kW and more. A typical current is around 5 amps for a light circuit, depending on what kind of light intensity you want. That means that the current sources max out at 1kV to 5kV and higher because power = current X voltage.

    The circuits do fail sometimes and the danger of this setup comes from electricians failing to understand how it works as they troubleshoot it. Electricians spend most of their time working on voltage sources like you have in your house or business. Like others said, if a current source sees a high resistance it will pump out a high voltage to maintain a steady current. If there is a an open circuit fault on the airfield and an electrician tries to measure voltage on that circuit then the electrician may experience a short peak voltage of 10,000 Volts or more before the source shuts itself down.

    I worked in the airfield lighting industry for three years and I've seen some close calls.
  16. Jul 14, 2014 #15


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    That's a revelation! No wonder you have had "close calls". Your average EE would be totally flummoxed if he hadn't read the handbook!
  17. Jul 14, 2014 #16


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    Very interesting Okefenokee, thank you.

    I can't help wondering how history might have been different if Edison had chosen this scheme for the Edison Electric Illuminating company. Edison designed the whole electric utility business, (including generation, transmission, loads, and even billing models) for the express purpose of providing illumination. Contrary to public opinion, Edison was quite aware of AC and it's advantages. He may well have considered and rejected a design similar to what Okefenokee described. Instead, he made his own brilliant invention, the feeder, which is still vital today in power distribution.

    Why did he choose parallel circuits? Perhaps the complexity of manufacturing an isolation transformer with each light bulb. Perhaps the designs of isolation transformers back then were not good enough. But I suspect the real reason was that Edison planned on making big money in copper. He invested in the predecessor to Anaconda Copper. If thick copper cables became the norm, he would have profited greatly.
  18. Jul 14, 2014 #17
    Any battery or generator can be designed for constant current. The utility company spins their turbines at constant speed, resulting in constant frequency and constant open circuit voltage. Once load current is drawn, voltage drops due to synchronous reactance due to stator inductance (self). Adjusting field current in rotor regulates voltage to constant value.

    The power company could just as well spin their turbines at constant torque and adjust field current for constant current instead. Doing so is not done because it would mean that full current is always being distributed. Voltage varies with loading. Conductors lose more power than insulators, I2R vs. V2G. So constant voltage is used. Also, with constant torque and current, the speed varies, as does the frequency. Synchronous motor speeds would not be fixed, and paralleling generators with differing frequencies is not done.

    At dc, however, constant current would make sense for LED lamps. Connecting the lamps in series, and providing constant current is the best way to run LED lamps, as they are naturally amenable to current drive. A switching power converter can be set up for constant current to drive LED lamps. Did I help?

  19. Jul 14, 2014 #18


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    A series system wouldn't be too convenient round the house though. Some of the appliances would have to be operating at many kV Potential above Earth. Then there would be the problem of different powered devices and switching them in and out. It could just be my innate conservatism but I can see many problems which, although they have their constant voltage feed equivalent, would seem to be harder to solve. For instance, which is more likely in an electrical installation, a short circuit or an open circuit? All the sockets in the house would need to short out when an appliance was unplugged.
    I think History got it right for normal living but we may have missed out in some circs which are like the Airport lights.
  20. Jul 14, 2014 #19


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    So to sum up, a controlled current source basically a controlled voltage source to make up for voltage drop or voltage increase due to distance in lines and loads starting or turning off?
  21. Jul 14, 2014 #20


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    History did get it right and Edison got it wrong, but not because of the technical details. Edison's vision was that the geographical extent of a utility company would be no more than a few blocks, and no applications for the power other than light bulbs. There was no idea of a general purpose socket in houses. True, he did accommodate limited use of motors and other stuff, but that was secondary. Remote generation, intercity transmission, and diverse applications of electric power were beyond Edison's vision. He was a brilliant engineer, but a mediocre businessman. The name of his company said Illumination not Electric and that was his focus.

    The fascinating alternate history would have come if Edison had found a much superior way to supply illumination. If that had happened, we might have evolved two electric grids, one for illumination, and another for everything else. It would be the realm of SF to project how that might have changed the 20th century, and what artifacts of that history might linger today. Come to think of it, that might make a fun science fiction story. It would be a challenge to make readers understand how technology/business/society interact.
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