Series lc circuit and clamp-on ammeter

In summary, a series LC resonant circuit with a capacitor can have a resonant frequency of less than a kilohertz, and the use of a clamp-on ammeter may not be accurate due to the low frequency response of the ammeter. The resonant frequency can be calculated by including the inductance of the wire, and adding or removing the capacitor can affect the overall impedance of the circuit. The cancellation of reactance between the inductor and capacitor allows for high amplitude voltage and current, but no power is involved due to the 90° phase difference.
  • #1
Idea04
194
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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  • #2
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..
 
  • #3
sophiecentaur said:
As you mention a Clamp-on Ammeter, you are clearly considering pretty low frequencies

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.

Idea04 said:
With no inductance in the circuit the magnetic field will collapse.

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.
 
  • #4
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.
 
  • #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.
 
  • #6
Idea04 said:
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?
Idea04 said:
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.
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?
 
  • #7
Idea04 said:
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.
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?
 
  • #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.
 
  • #9
Idea04 said:
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.
Probably you have situation where adding the cap decreases or increases overall impedance/reactance of the circuit. That depends on parallel or series connection of two reactive components with respect to the source. That's a normal thing, your ammeter is OK.
 
  • #10
Idea04 said:
In a series LC resonant circuit the capacitor acts to cancel out the inductance of the the circuit.
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.
 
  • #11
Svein said:
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.
But it's true that the Reactance of one will cancel out the reactance of the other at resonance.
 

What is a series lc circuit and how does it work?

A series lc circuit is a type of electrical circuit that consists of a resistor (R) and a capacitor (C) connected in series. When an alternating current (AC) source is connected to the circuit, the capacitor stores energy and the resistor dissipates energy, resulting in a lag between the voltage and current. This lag is known as the phase angle and it can be used to calculate the impedance of the circuit.

What is the purpose of a clamp-on ammeter?

A clamp-on ammeter is a type of electrical measuring instrument that is used to measure the current flowing through a conductor without having to physically disconnect the conductor. It is commonly used in situations where it is difficult or unsafe to break the circuit to insert a traditional ammeter. The clamp-on ammeter works by using a magnetic field to measure the current passing through the conductor.

How do you measure current in a series lc circuit using a clamp-on ammeter?

To measure the current in a series lc circuit using a clamp-on ammeter, simply clamp the ammeter around one of the conductors in the circuit. The ammeter will measure the current flowing through the conductor and display the value on its screen. Make sure to position the clamp-on ammeter perpendicular to the conductor for accurate readings.

What are some factors that can affect the accuracy of a clamp-on ammeter?

The accuracy of a clamp-on ammeter can be affected by factors such as the position of the ammeter on the conductor, the orientation of the ammeter relative to the conductor, and the presence of other nearby electrical or magnetic fields. It is important to carefully follow the manufacturer's instructions and guidelines for accurate measurements.

Can a clamp-on ammeter be used to measure DC current?

Most clamp-on ammeters are designed to measure AC current only, but there are some models that can measure both AC and DC current. It is important to check the specifications of the ammeter to ensure that it is capable of measuring both types of current before use. Using a clamp-on ammeter to measure DC current is similar to measuring AC current, but the polarity of the leads must be correctly aligned with the polarity of the current for accurate readings.

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