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Series lc circuit and clamp-on ammeter

  1. Jan 22, 2015 #1
    In a series LC resonant circuit the capacitor acts to cancel out the inductance of the the circuit. With no inductance in the circuit the magnetic field will collapse. So my question is, with this collapse of the magnetic field will it be be harder to measure the current in the circuit with a clamp on ammeter that measures the current from the magnetic field it develops around the conductor?
     
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  3. Jan 22, 2015 #2

    sophiecentaur

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    There is always some inductance in a circuit - even if you just consider the wires connecting the ends of the Capacitor. The resonant frequency of such a circuit will be higher than if you include a (perfect) inductor in series but that's the only difference.
    As you mention a Clamp-on Ammeter, you are clearly considering pretty low frequencies then the practicality of your experiment would mean that the frequency response of your Ammeter would be too soggy to observe the rapid pulse of current at switch on without a significant series L..
     
  4. Jan 22, 2015 #3

    berkeman

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    That's true for the transformer-based clamp-on probes for use with AC Mains. But I use a LeCroy Hall effect clamp-on meter in our Lab, and it works from DC-50MHz.

    No, the magnetic field from the current flowing in the wire will be the same, given the same current level. The magnetic field that a clamp-on probe measures comes from the current flowing in the wire it is clamped onto.
     
  5. Jan 22, 2015 #4

    sophiecentaur

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    50MHz could be thought of as a pretty low frequency for a self resonating Capacitor. But my point remains that you can never have a circuit with no inductance.
     
  6. Jan 22, 2015 #5
    When calculating the capacitance for the circuit I did include the inductance of the wire. The resonate frequency is less than a kilohertz. I measured the circuit inductance with and inductance meter and the value dropped below the inductance of a single wire. I just wanted to be sure because that the clamp on meter would still be accurate.
     
  7. Jan 23, 2015 #6
    Some AC clamp on ammeters are good enough only up to 400-500 Hz. Readings at 1 kHz can be quite off with them. Do you measure circuit at steady state AC power or want to measure its' transient response? What exactly is your circuit and how do you know resonant freq is below 1 kHz?
     
  8. Jan 23, 2015 #7

    sophiecentaur

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    What sort of Capacitance are you using in your calculations? Resonant frequency is 1/(2π√(LC))
    C in Farads and L in Henries; you have to include the 'micros' and 'picos' in this.
    What Inductance Meter did you use to measure the inductance of a short loop of wire, btw?
     
  9. Jan 23, 2015 #8
    The capacitance in the circuit is 500uF, the inductance of the circuit including the wires is 810.56uH and the resonant frequency is 250Hz. I measured the inductance of the wire to be 13.5uH with a MTP MS5300 inductance meter. The clamp meter I am using is good for frequency from 50Hz to 500Hz. I wanted to make sure that the clamp on ammeter was reading properly because without the capacitor the current output was higher than the supply voltage of the circuit, and with the capacitor the current was much lower than the supply voltage. But the impedance with the capacitor was much lower.
     
  10. Jan 23, 2015 #9
    Probably you have situation where adding the cap decreases or increases overall impedance/reactance of the circuit. That depends on paralell or series connection of two reactive components with respect to the source. That's a normal thing, your ammeter is OK.
     
  11. Jan 23, 2015 #10

    Svein

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    No, that is not quite true. What happens is that a energy flows from current in the inductance to voltage across the capacitor and back again. Since the voltage and current is 90° out of phase, no power is involved. The amplitudes can be quite high, though.
     
  12. Jan 23, 2015 #11

    sophiecentaur

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    But it's true that the Reactance of one will cancel out the reactance of the other at resonance.
     
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