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Current source

Discussion in 'Electronic Basics' started by Hamidreza Shaiganfar, Nov 1, 2003.

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  1. Dear friends,

    I am trying to design an AC current source with power of 1kVA. Anyone
    who has an idea I will be appreciated to know it.
  2. mike

    mike Guest

    I have these little sockets in every wall that meet your specification,
    not to be confused with your requirements which were not disclosed.
    You probably have them too.

    Or maybe, for those of us who don't have the psychic hotline on speed
    dial, you could quantify some of those pesky little requirement
    parameters like frequency, voltage, current, minimum source impedance
    vs frequency, overload protection,
    input power source parameters, and, and, and.

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  3. scada

    scada Guest

    Generator, Inverter!
  4. Jim Large

    Jim Large Guest

    Strange, I have little sockets like that in my house too,
    but mine are voltage sources, not current sources.

    -- Jim L.
  5. Peter Meyer

    Peter Meyer Guest

    No use being flippant...
    Have you tried a short over those little "sockets"? V=0 and I=destructive!
  6. Haven't you heard? The evil French Léon Charles Thévenin* has been
    replaced with Mr. Norton.

    *Althought some claim Helmholtz did it first anyhow.

    Best regards,
    Spehro Pefhany
  7. Thanks to all for that nice discussion.

    I still remember those primary classes, where I studied that voltage
    difference is necessary to make the current flow in the circuit.

    Current source consists only current so how they are able to make the
    current flow?

    Or the voltage drop first occurs across parallel Thevenin Resistance
    (in Norton Equivalent) which facilitates the current flow.

    All seems to be silly & confusing!

    Animesh Maurya
  8. default

    default Guest

    I take it you mean current limited AC source? Several ways to do it.

    Fuse the circuit. Fuse the circuit with a resetable circuit breaker.

    Connect a variable transformer (variac) in series with the load and
    adjust the current manually. Or design a motor drive if speed of
    response isn't a consideration, and it needs to be automatic.

    Build an inverter, sense current, limit drive when current is

    Wind a transformer with a magnetic shunt. Core is rectangular with
    the primary on one side secondary on the other, in between is a gapped
    magnetic shunt (laminations). Secondary tries to draw too much
    current and the magnetic path finds it easier to get through the
    shunt. Works well with a sharp "knee," good efficiency, high
    reliability. Downside is big heavy expensive - unless you do it

    Use a DC current limiter and put it between the plus and minus of a
    bridge rectifier. Wire the bridge in series with the load.

    Design a phase control circuit and limit the firing angle as current

    You would do better to post what you hope to accomplish as the end
    goal in this endeavor. There may be a way to do it that doesn't
    involve limiting AC current.
  9. Reg Edwards

    Reg Edwards Guest

    Insofar as low load resistances are concerned, a current source is a high
    voltage in series with high resistance.

    With an accuracy of 0.1 percent or better, a 1 megohm resistance in series
    with 1000 volts behaves as a 1 milliamp constant current source to variable
    loads less than about 1000 ohms.

    It's such a simple idea some people can't believe it. But you are not one
    of them are you?

    Forget about Norton and Thevenin. Neither of them are of much use except to
    make speakers sound clever to an ill-educated audience. Ohm is good enough
    for most things. He allows you to invent your own theorems to fit your own
    immediate applications.
  10. I read in that Animesh Maurya
    A current source can be modelled as a generator in series with a very
    high resistance. With no load on the output, the generator voltage
    appears there.
  11. Ban

    Ban Guest

    Dear friends,

    somehow I have the feeling the OP doesn't even know what a current source
    is, (he talks about 1kVA) sounds more like an Inverter 24V->120V~ or
    something to me. At his level of knowledge it seems highly unprobable he
    manages more than typing above question.
    Or maybe it is a software guy?

    ciao Ban
  12. Current (and voltage) sources are theoretical devices.

    A voltage source will produce its 'rated' voltage across either an open
    circuit, which isn't a big deal. Or it will produce its rated voltage
    across a zero ohm conductor, which is impossible in the practical world,
    since that results in an infinite current.

    Likewise, a current source will produce a current through an impedance,
    including an open circuit. This will result in an infinite voltage.

    In real life, each of these sources is incorporated into a circuit model
    with the appropriate series (for a voltage source) or shunt (for a
    current source) impedance, which gives the model a behavior that
    approximates a real world circuit.

    On the other hand, there are no real world circuits that behave exactly
    as a pure current or voltage source.
    The purpose of the Thevenin and Norton equivalent circuits is only to
    analyze their behavior at the circuit terminals. You can calculate what
    might be going on inside each circuit, but that isn't necessary and
    doesn't correspond to anything that would occur in real hardware.
    Just wait until you whip out the old Simpson multimeter and try to
    measure an imaginary voltage. ;-)
  13. Jim Large

    Jim Large Guest

    Tried? No I've never tried. I've DONE it plenty of times,
    just never on purpose. Sadly, I've never had a meter handy
    to measure the voltage. My gut feeling though, is that the
    AC mains are a lot more like a voltage source than a current
    source. The voltage is around 125 volts RMS when nothing's
    plugged in, and it's around 125 volts RMS when I've got air
    conditioners running and hair dryers running and etc. Heck,
    I've even lived in places where we heated the apartment with
    electricity -- fifty or sixty amps -- and the voltage was
    *STILL* about the same.

    -- Jim L.
  14. Jim Large wrote...
    Probably dropped about say 5V at 50A, implying a current of
    over 1200A into a full short on the electric-heater lines.
    For a few milliseconds that is.

    - Win

  15. Ken Taylor

    Ken Taylor Guest

    Yep, them generators don't slow down much no matter how many people park
    'emselves across the socket!

  16. I read in that Jim Large <>
    The source impedance at a US mains socket is around 0.3 ohms.
  17. Norm Dresner

    Norm Dresner Guest

    Not to be too argumentative, but where does this number come from?
    There have to be so many variables entering into the impedance like the kind
    of service, the wiring, and even concurrent loads, that I can't believe that
    this is anything but a rough estimate.

  18. And DO NOT try to measure it with an ohm meter!

    Hello John ;-)
  19. Ben Bradley

    Ben Bradley Guest

    Oh yeah, well the ones in MY house are sources of both AC voltage
    AND AC current!
    The Thevenin equivalent is about 120VAC with less than an ohm
    resistance. The Norton equivalent is, well, a huge waste of electrical
  20. Glen Walpert

    Glen Walpert Guest

    The actual impedance at any "mains socket" varies widely, and could be
    calculated fairly easily if you know the transformer rating and how
    much of what size wire is between the transformer and receptacle; this
    is routinely done in large industrial distribution systems but
    essentially never for residential distribution because the available
    fault current is limited to less than 10,000 amps, and you cannot buy
    listed distribution components rated for a lower fault current. The
    10 kA limit on 240 VAC implies a lower limit of .024 ohms at the
    service entry, add to that the resistance of the wire between the
    panel and the receptacle for the worst case impedance at a given

    The nameplates on some "pole-pig" distribution transformers can be
    read with binoculars from the ground, and the maximum fault current
    from any distribution transformer with the worst case "infinite bus"
    assumption (input voltage does not drop during fault) is the nameplate
    rated current divided by the impedance factor, typically 0.05 or a bit
    less (often expressed as percentage eg 5%).
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