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# to study the effect of frequency of basic RLC circuit

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#### PG1995

##### Full Member level 5
frequency and simple RLC circuit

Hi

https://img3.imageshack.us/img3/9393/img0003es.jpg

I think Vab is applied potential (peak to peak).

Frequency is the frequency of the generator being used.

I think Vab (rms) is wrong. Do you make any sense out of it like what it was supposed to be (perhaps it was supposed to be something else instead of Vab (rms)?

What is Irms? It has different values in the table.

XL is reactive inductance which increases with increasing frequency.

EDIT: Could some moderator please change the title of this thread to "frequency and simple RLC circuit"? Thanks.

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Apparently the table is a protocol of a lab experiment. I don't think that it makes any sense to guess about it's meaning without knowing either the circuit or the instructions.

If I should guess about it, I assume that the XL calculation is wrong.

Re: frequency and simple RLC circuit

Are you trying to study AC circuits on your own... this table is a rather big step ahead of the DC circuits we were working on.

Only if the voltage waveform is sinusoidal 10Vp-p => 10/2/SQRT(2) = 3.53 V rms

Apparently the table is a protocol of a lab experiment. I don't think that it makes any sense to guess about it's meaning without knowing either the circuit or the instructions.

If I should guess about it, I assume that the XL calculation is wrong.

Yes, it is a protocol of a lab experiment. The guy who made this table on the Word could be wrong and the lab instructor could equally be wrong when he didn't correct the table.

I have read that XL is proportional to the frequency and it is suggested by the calculations in the table. So, what am I missing?

Thanks for the help.

I was trying to figure out how the table values could be related to a circuit... so far I failed... it is like a puzzle... more than science

I keep my opinion, that the table is meaningless without knowing the circuit, and the meaning of the quantities within in.

Re: frequency and simple RLC circuit

In the sweeped frequency range, the circuit is mainly capacitive. That is, in this range the inductance reactance is much smaller than the capacitor.

That can be a student error or a bad choice of inductance by the teacher, since the goal of this work is the visualization of the resonance.

Re: frequency and simple RLC circuit

Thank you, FvM, Kerim, Eduardo. You guys have been very helpful.

Are you trying to study AC circuits on your own... this table is a rather big step ahead of the DC circuits we were working on.

Only if the voltage waveform is sinusoidal 10Vp-p => 10/2/SQRT(2) = 3.53 V rms

I had only this RLC circuit to understand a little bit.

I'm almost sure that it was sinusoidal.

In the sweeped frequency range, the circuit is mainly capacitive. That is, in this range the inductance reactance is much smaller than the capacitor.

That can be a student error or a bad choice of inductance by the teacher, since the goal of this work is the visualization of the resonance.

I'm sure that was the case. Unfortunately I wasn't able to grasp what you were saying? Like what really Irms is?

Thank you all for all the help.

Re: frequency and simple RLC circuit

Since a sinusoidal signal changes its value with time (unlike the constant DC sources), the power I^2 * R for example also varies with time.

I think you can see what Ipeak means (it is 1/2 I peak-to-peak).

Obviously a current of Ipeak passing through R will generate a power less than Ipeak*Ipeak*R, because the current is not always at I peak (or -I peak). Let us call this power as P_dissipation.
Therefore we know that P_dissipation < Ipeak*Ipeak*R

Now let us suppose there is a DC current that can generate the same P_dissipation in R.
By definition we call this equivalent DC current... Irms (rms from Root Mean Square, you will learn later the reason for this naming)

The good news is that we can always calculate Irms if the waveform of the current is known.
In case of a sinusoidal current: Irms = Ipeak / SQRT(2) ≈ Ipeak * 0.707

Kerim

Edited:
I forgot to add that in case of a sinusoidal current:

P_dissipation = Irms*Irms*R

P_dissipation = Ipeak*Ipeak*R/2

P_dissipation = Ipp*Ipp*R/8
where Ipp = I peak-to-peak

Note: The same applies for the voltage (Vpp, Vpeak and Vrms)
For example P_dissipation = Vrms*Vrms/R
where Vrms = Vpeak * 0.707

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Thank you, Kerim.

Can I use this formula to find Irms: XL = (Vrms / Irms) => Irms = (Vrms / XL), where XL = 2*pi*f*L?

Here I have these formulas to find Vr (voltage across the resistor in a R-L-C circuit) and VL (voltage across the inductor): Vr = I*R, VL = I*XL. I was wondering how to find I? Perhaps I could simple use this to find I: Irms = I / sqrt(2).

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I had only this RLC circuit to understand a little bit.
Did you refer to a particular circuit? Which one?
As far as I'm aware of, no circuit is given.

If you have reasons to assume a RLC series circuit, you forgot to mention it.

Besides not knowing the circuit, we have the problem that the table doesn't say, which are measured or calculated quantities.

P.S.: Without additional information, I would assume, that Irms is a measured quantity, e.g. acquired by an analog or digital current measuring device. It's designated "rms" because the instrument is measuring rms quantities.

If Irms is a measured quantity, then Vrms/Imrs can be calculated as an impedance. It's frequency dependancy doesn't show an inductive component however, it looks like a RC series circuit. Even if there's a certain inductive component, the shown XL numbers can't be related to it.

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man.electronics is not some kind of reverse engineering.why are u predicting circuit from observation table? Will u plz justify ?

Can I use this formula to find Irms: XL = (Vrms / Irms) => Irms = (Vrms / XL), where XL = 2*pi*f*L?
Of course you can, XL (also in Ohm) is the reactance of a coil of inductance L (in Henry). Its counterpart is XC which is the reactance of a capacitor C (in Farad) and equals to 1/(2*pi*f*C).

But the trick point is that if R and L are in series their impedance Z doesn't equal to the sum of the resistance R and the reactance XL.
Z = SQRT ( R^2 + XL^2 ) as if R, XL and Z form a rectangular triangle (its sides R and XL).

The same is for R and C in series:
Z = SQRT ( R^2 + XC^2 )

The second trick point is that if L and C are in series, their impedance (at each frequency):
Z = XL - XC
You see, XC is not added to XL and when at a frequency XL = XC, we may say that the circuit is in resonance.

So if in a branch we have R, L and C in series:
Z = SQRT[ R^2 + (XL-XC)^2 ]
At resonance, Z will be R since XL-XC will be zero. At this frequency the current reaches its maximum (lowest Z).

But R, L and C could be connected in parallel... Could you guess their impedance :grin:

Kerim

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