Talk:Electrical resonance

hum when two devices plugged in

Dear sirs, no matter how far away they are in the house from each other, when I plug in

• Electric blanket (with sine wave chopping current limiter) + uninterruptible power supply, or
• Electric frying pan (with sine wave chopping low power setting) + 12 V amateur radio power supply

I get this terrible mains hum out of the latter of each, due to some kind of electrical resonance. Is there some kind of electronic filter one can use? And is this phenomena documented in Wikipedia? Jidanni (talk) 19:08, 3 December 2008 (UTC)

OK, it is due to Silicon-controlled rectifiers. Jidanni (talk) 03:35, 1 January 2009 (UTC)

Yes, that seems like a correct assessment. I would not call that resonance - I would call that harmonic noise due to the fact that the AC supply is not sinusoidal. You might have an older inverter that creates a simple square wave. Many modern inverters have multi-step wave shaping so that the generated waveform has reduced harmonics, and you can further reduce them with an LC filter tuned for maximum Q at your line frequency. --Alex146 (talk) 15:32, 11 May 2011 (UTC)

tank circuit?

"Resonance of a circuit involving capacitors and inductors occurs because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, and then the discharging capacitor provides an electric current that builds the magnetic field in the inductor, and the process is repeated continually."

Isn't this describing a "Tank" circuit(LC Circuit)? So described because like a tank of water tilting back and forth, the energy oscillates between being stored in the capacitor and the inductor like the water shifting between the two sides of the tank. —Preceding unsigned comment added by Shoez (talk • contribs) 21:39, 5 March 2009 (UTC)

First Equation Error?

The first equation, w = 1/sqrt(LC), seems to be missing an f term. Where does the term go? Thank you.

Alex146 (talk) 06:15, 13 March 2010 (UTC)Alex146

Its not w, but ω, which is angular momentum. As it says under the equation, ω=2πƒ. The ƒ in this case is the resonant frequency. Wocrepus (talk) 12:25, 21 March 2010 (UTC)

Revealing the truth about electrical resonance phenomenon

Bless my soul! I know it looks strange and incredibly, and probably you will not believe me... but I have finally revealed the secret of ubiquitous electrical resonance phenomenon! It is interesting that the negative impedance phenomenon has helped me to find out credible intuitive explanations about the impedances of series and parallel LC circuits. I would like first to share my insights with you here; then to compress these lengthy explanations into a few sentences and to place them in the main article...

Realizing the LC arrangement

The fault of the classic formal approach when explaining the zero and infinite impedance of series and parallel AC-supplied LC circuits is that it implies two dual impedances (inductive and capacitive) that cancel each other thus giving total zero or infinite LC impedance. But this widespread assertion is misleading...

It is hard for people to imagine how two humble impedances can cancel each other as "impedance" gives an impression of something passive. Two passive "things" shouldn't cancel each other; one of them should be active (a source). So, we have to consider an LC circuit as a combination of two elements: a source (active element) driving a load (passive element). Depending on the situation, the either element (the inductor or capacitor) can act as a source; meantime, the other element will act as a load. Strictly speaking, both they are sources containing energy (magnetic or electric); but figuratively speaking, the load is a source that is "forced" to act as a load (like a charging accumulator). They can be distinguished by the signs of the current through and the voltage across them - in the source they are different while in the load they are equal.

Why the impedance of a series LC circuit is zero

We have an arrangement consisting of four elements connected in series: an AC input voltage source, an inductor, a capacitor and a load (a resistor). Or, we may combine the input voltage source and the resistor into a real voltage source (with internal resistance).

The main article says: "Inductive reactance magnitudeX_L increases as frequency increases while capacitive reactance magnitude X_C decreases with the increase in frequency. At a particular frequency these two reactances are equal in magnitude but opposite in sign; so X_L and X_C cancel each other out. The only opposition to a current is coil resistance. Hence in series resonance the current is maximum at resonant frequency". Let's now try to comprehend this magic...

According to the considerations about LC arrangement above, we can think of the series LC circuit as of an AC source and impedance connected in series. At the resonant frequency, this "source" has the same polarity as the input source; the two AC voltages are in phase with each other so they add together. Let's consider the voltage polarities travelling along the loop at both the half waves.

Positive input half wave: -V_IN+ (source), -V_L+ (source), +V_C- (impedance), +V_LOAD- (impedance). The charged inductor acts as a source that "helps" the input source. Note the voltage across the inductor (the source) is equal to the voltage drop across the capacitor (the impedance) so the total voltage across the series LC circuit is zero. Its total impedance is zero and it does not impede the current. Very interesting... as though the inductor acts as a negative capacitor that neutralizes the capacitor impedance! Well, there is still a subtle difference:) The true negative capacitor uses an additional external energy (a power supply) for this purpose while this "negative capacitor" draws energy from the input source.

Negative input half wave: +V_IN- (source), -V_L+ (impedance), +V_C- (source), -V_LOAD+ (impedance). Now the charged capacitor acts as a source "helping" the input source. The voltage across the capacitor (the source) is equal to the voltage drop across the inductor (the impedance) so the total voltage across the series LC circuit is zero again; its total impedance is zero and it does not impede the current again. Now as though the capacitor acts as a negative inductor that neutralizes the inductor impedance!

We can generalize the two cases by one conclusion. An AC supplied series LC circuit consists of two elements connected in series and having equal voltages across them; one of the elements acts as a voltage source while the other acts as impedance.

Why the impedance of a parallel LC circuit is infinite

Now we have a simpler arrangement consisting of two elements: an AC input voltage source driving an LC tank.

The main article says: "Let R be the internal resistance of the coil. When X_L equals X_C, the reactive branch currents are equal and opposite. Hence they cancel out each other to give minimum current in the main line. Since total current is minimum, in this state the total impedance is maximum."

To comprehend this assertion as above, we can now think of the parallel LC circuit as of an AC "helping" source and impedance connected in parallel. At the resonant frequency, the "source" provides all the current needed for charging the impedance to the input voltage; so there is no need the input source to do this donkey work:) The helping "source" as though acts as a load canceller (i.e., as a negative impedance again)! Actually, this arrangement is similar to the exotic bootstrapping technique: a voltage "source" (i.e., the LC tank) is connected in opposite direction to the input voltage source; as a result, the current is almost zero and the impedance is infinite. Of course, there is a subtle difference again:)

Circuit dreamer (talk, contribs, email) 22:12, 21 July 2011 (UTC)