Resonance-help me!!

[niga]

Hi there,

Resonance happens in RLC(parallel & Series)
LC(parallel and series) when XL=XC.

1.Is this resonance a transient behaviour in ac circuits?

2.For how long is it?
The transient period of dc ckts RC,RL,RLC series is 5time constants.
What abt ac RLC -parallel and series? :?

3.What is the time constant of LC across ac and dc?

3.This stuff is related to damping i guess....

Can someone help me out PLEASE.............???

niga

 

[niga]

someone please answer......

 

[Oznog]

I don't understand what you're asking:

1. Resonance is an AC process. Like L and C elements independently, they have a transient response. Resonance most often refers to a state sustained by an external driving source.
3. LC has a freq, not a time constant. If we had a known R element it will decay and that can be calculated. Time constants do not apply to fixed DC sources, a change in voltage/current is required. With a constant DC source the C appears open, the L is a short, and the R is the only thing that affects the circuit.

 

[Russlk]

Resonance occurs because energy is stored in the capacitor and inductor. Just as a pendulum oscillates with the energy alternating from kenitic to static, the L -C circuit oscillates with the energy alternating between the inductor and capacitor. Since the circuit has resistance, there is a time constant, the amplitude will exponentially decrease unless energy is constantly added.

 

[niga]

Thankyou Oz and Russ for responding.
Yes,i understand that resonance is an energy...
what i am quite stuck abt is..........

1. When RL and RC is conntd in series to DC there is Timeconstant(TC)- L/R and RC but when in parallel the resistance is extremely small so TC is less.Okay.Now
LC in parallel across dc-
here there is energy transfer btn L and C,am i right?
Fine, but since its in parallel for how long is this transfer?how long does it take to reach the steady state?


2.When RLC is conntd in Series or parallel across AC supply,what is their transient behaviour?Resonance happens only when XL=XC.
I haven come across time constants in this case only the resonant frequency.

Plaese do help.i am realllllllllly confused. :?

 

[Russlk]

If you connect a parallel L-C to DC, a series resistor is needed to limit the current. You can combine that resistance with the series resistance of the inductor to find an equivalent series resistance. The transient response of the L-C circuit generally responds as exp(-t/RC). Steady state for the transient is zero amplitude, which takes an infinite time. It is more common to rate L-C circuit using Q = Xl/Rs.

2. The transient response to an AC source depends very much on the impedance of the source. If the source is low impedance, the time constant will be correspondingly short.

 

[ljcox]

A parallel reasonant circuit needs to be driven by a current source and a series one by a voltage source.

If you apply a current pulse to a parallel LCR circuit, it will (assuming it is under damped) show a damped oscillation on a storage oscilloscope.

Depending upon the value of the R, the circuit will be either, over damped, critically damped or under damped.

If you want to calculate the reponse to an impulse, you need to solve a second order differential equation and in the process, determine whether it is over damped, critically damped or under damped.


Alternatively, use circuit simulation software to see the result.

Len

 

[niga]

thanku russ and len for ur replies.u have indeed helped me out a lot.
If i may pease ask why

"A parallel reasonant circuit needs to be driven by a current source and a series one by a voltage source. "

Why shud it be like that?
i think i sound stupid but i really donno the answer and am trying to learn thr basics.
thanku so much for having responded.

 

[ljcox]

Please don't feel stupid, we all had to learn and are still learning.

If you drive a Parallel reasonant circuit with a voltage source, the source has a low resistance and so the voltage across the L & C is fixed at the voltage of the source. So if you vary the frequency, there will be no reasonance.

But if you use a current source, it has a very high source resistance so the voltage across the L & C will rise to a peak (ie. reasonate) as the frequency is brought to the reasonance point.

For a series reasonant circuit, the same applies in reverse. If driven by a current source, the current is fixed at that of the source. But if driven by a voltage source, the current will peak at reasonance.

I hope this helps, if not, don't be afraid to ask for more advice.

Len

 

[Russlk]

Adding to Len's answer, the capacitive and inductive reactance (Xc and Xl) cancel in the parallel circuit to produce an infinite impedance at resonance (with ideal components). Consider the formula for parallel resistors: Rp =R1*R2/(R1+R2) But since Xc is negative, the denominator is zero which gives an infinite result.

In the series circuit, the Xl and Xc cancel to produce zero impedance at resonance.

 

[niga]

thankyou soooooooo much len and russ for ur replies.u have really helped me and i sincerely appreciate it.

thanx!!!

niga

 

[ljcox]

You're welcome

Len

 

[vk5ajl]

Whether simple L charged (or discharged) with DC or simple C charged (or discharged) with DC, the charge rate (or fall) is determined by the law of natural decay. This is an inverse exponential function and thus the time it takes is infinite ie. the closer it approaches some limit, the slower it goes and therefore theorectically never makes it. A series or parallel RC curcuit charged with DC has similar effects in both components. It never nmakes it either but there are more than one resonance with AC. Driven resonance is where the circuit only shows a resistive component, natural resonance is the frequency it will continue to ocsillate at when the exictation is withdrawn. These two frequencies are not necessarily the same. Only natural resonance is where Xl = Xc. Driven resonance depends on those values AND the value of any resistance and where it is.

 

[blueroomelectronics]

I wonder if the OP was still waiting for an answer after 5 years.

 

[vk5ajl]

Probably (probably definately) not but I found it so put in my bit in case someone else does.