RLC parallel resonance circuit

[SimonTHK]

Hello Electro tech online and the great members

I am looking into RLC circuits but I do have some problems with it.

Ill try and go through what I know and maybe you can correct me when it is wrong. Ill also ask few quistions.

Does RLC circuit physically consist of all three component? R, L and C
What is the difference between an RLC and LC circuit?


What I know is: If I send low frequence into my rlc circuit then the capacitor will block the frequenzes and the inductor will let them through (what does the resistor do, I suppose it lets all frequenzes through)?

When the frequenze increse, the resistant through inductor increase (tus incresing impedance) until it reach its maximum of unlimited resistant. When it reach this maximum, the capacitor will also be at maximum resistant. If the circuit was ideal, a continous resonance would happen between the inductor and capacitor.
I am still not sure what the resistor do?

If the frequenze is furtherly increased, the capacitor will let them through, tus making the impedance fall.

The wave of this circuit looks like a very thin high mountain
How do I increase the hight of this "mountain"? In the beginning I thought it was the resistor, but I have my doubts....

I probably have some more questions, but these are the basic ones that I cannot figure out using quickipedia. Alot of it am I quite sure on, but I miss something and some points that will let me work further into it.

Mostly, I cannot figure out what the resistor does.

Thanks in advance

 

[MikeMl]

It dissipates energy. It is part of any practical LC circuit, mostly as core losses and wire resistive losses in inductor, and less so in the capacitor. A theoretical LC (with no dissipative R), when excited, would oscillate forever. With any practical (lossy) circuit, the oscillations die out due to the energy being converted to heat in the resistor.

 

[MikeMl]

Here is an LC circuit with a variable dissipative Resistor, Rs. Note the effect of varying Rs from 1Ω to 100Ω. The loss resistance can be placed in parallel with LC, or in series with L and C.

 

[SimonTHK]

Thanks you very much, this is great and helpfull.

Ok I see that there is this resistance in L and C but is R used to indicate this resistance or does it actually exist? physically in the circuit. (you say it is a part of any practical LC circuit, so I guess it doesnt exist) If it does exist physically in the circuit, can you tell me in which case it could be used? I do get the reason why you would build this circuit without a resistor, but with a resistor I dont get.

So if the circuit was supplied with a continous AC voltage, it would keep on oscillating with no damping at frequenzy resonance?

NEW QUISTION

Sooo another quistion. Imagine I was to make a radioreceiver and I have made this circuit to receive only the frequenzes I want, as good as possible. Then I go the -3db down to find my bandwith. Is there any circuit that filter anything away that is less than -3db? So that the antenna would receive any frequenze it could , the LC circuit would filter most away, and there is yet another "box/circuit" that filters everything away that is under the -3db? It is easier for me to understand it if I see where it is used practically.

It isnt the LC circuit that give us only the frequenzies between the bandwidth but another circuit afterwards? Can anyone confirm this?

Again thanks in advance, I love this forum and when I get rich I pay to it.

 

[crutschow]

Suggest you read some tutorials on the subject such as this and this.

 

[SimonTHK]

Suggest you read some tutorials on the subject such as this and this.

Well it aint easy. There is both an LC and an RCL circuit on wikipedia. But what I can understand (believe me, with alot of reading) there is no LC the LC circuit is non existing and cant be build. If thats the answear, it wasnt that hard for anyone that knew it to tell it in here.
It aint easy to read up on everything yourself, its not much I am asking, just few quistions.

 

[JCLrd]

The LC circuit is used for first order mathematical aproximations, such as filter design, and tuning circuits.
Such as you want to find the value of capacitor needed to resonate with a specific inductor.
Where you would rearange the resonat equation, to solve for that value.

However when you need more specific parameters, such as bandwidth, and narrow band tuning, you then need to include all the resisance values into the equation, this is known as the Q, of the circuit.

 

[tom_pay]

Hi

The way I see it is that the LC circuit is a hypothetical, ideal world scenario. However it would be incredibly hard to make, if not, impossible. A RLC is what you get when trying to build a LC circuit. The resistance that stop the oscillations continuing on forever comes from the inductor, capacitor leads, PCB tracks and various other unavoidable things. This is how I interpret the information given on Wikipedia and previous research.

The parallel LC (or in practice RLC) circuit will provide high resistance to all frequencies except the resonant frequency and a very small band around it. It will offer a low resistance to the resonant frequency, so it can be picked up by an amplifier. See the diagram.

There are many ways to receive only certain radio waves, that is just one. I have used it and it works.

Hope this helps

Tom

 

[SimonTHK]

The LC circuit is used for first order mathematical aproximations, such as filter design, and tuning circuits.
Such as you want to find the value of capacitor needed to resonate with a specific inductor.
Where you would rearange the resonat equation, to solve for that value.

However when you need more specific parameters, such as bandwidth, and narrow band tuning, you then need to include all the resisance values into the equation, this is known as the Q, of the circuit.

In my head, this quality has to be high. So if the resistance is great between the capacitor and inductor at resonance frequenzy we have a high quality filter, but what can make it high quality or low? Is it the components itself?

 

[SimonTHK]

Hi

The way I see it is that the LC circuit is a hypothetical, ideal world scenario. However it would be incredibly hard to make, if not, impossible. A RLC is what you get when trying to build a LC circuit. The resistance that stop the oscillations continuing on forever comes from the inductor, capacitor leads, PCB tracks and various other unavoidable things. This is how I interpret the information given on Wikipedia and previous research.

The parallel LC (or in practice RLC) circuit will provide high resistance to all frequencies except the resonant frequency and a very small band around it. It will offer a low resistance to the resonant frequency, so it can be picked up by an amplifier. See the diagram.

There are many ways to receive only certain radio waves, that is just one. I have used it and it works.

Hope this helps

Tom

Thanks Tom, it does help alot.

First Ill add this link, it is very usefull if anyone else fall over this thread **broken link removed**

I myself do not agree of what you say, atleast not in an ideal circuit where current will flow from inductor to capacitor and the impedanze will be infinite. I do believe that around this resonance frequenze, any other frequenze is let through, but not at the exact resonance. The other frequenzies is then defined by the -3db and all frequnzies that is beneath -3db will be filtered away.
A high quality filter is when the inductor and capacitor fits VERY well, and the impedance will go high in the sky, tus making the bandwidth smaller.
But I can be very wrong

 

[JCLrd]

In my head, this quality has to be high. So if the resistance is great between the capacitor and inductor at resonance frequenzy we have a high quality filter, but what can make it high quality or low? Is it the components itself?

The Q of a coil is ( L / R ), so the smaller the coil winding resistance, the sharper the resonance curve.

 

[MikeMl]

...
First Ill add this link, it is very usefull if anyone else fall over this thread **broken link removed**
...

The link unfortunately covers only the theoretical perfect (unrealistic) case where there is no R in the resonant circuit, so does not shed any light into the OP's question.

 

[Ratchit]

JCLrd,

The Q of a coil is ( L / R ),...

At the risk of being pedantic again, let me say the Q of a coil is ωL/R, not L/R.

Ratch

 

[tom_pay]

What should the best Q value? Is is higher, lower or in the middle?

I have been told so many conflicting answers by so many different people.

Thanks

Tom

 

[MrAl]

Hello there,

The Q value depends on the application to some degree. For a tuning filter for example, if you want good selectivity then you want a high Q, but if you want wide bandwidth then you want not so high Q. Also, if you can not guarantee that your component values will not drift then you can not use too high of a Q or else the frequency will drift and then the filter might not work at all in the given application.

 

[tom_pay]

Ok then,

How do I lower the Q? To make it accept more frequencies. I know I have to add resistance, but do I put a resistor in series with the inductor or parallel to it.

Thanks

 

[crutschow]

How do I lower the Q? To make it accept more frequencies. I know I have to add resistance, but do I put a resistor in series with the inductor or parallel to it.

If its a series LC circuit then you put the resistor in series, and if it's a parallel LC the you put the resistance in parallel.

 

[MRCecil]

Ok then,

How do I lower the Q? To make it accept more frequencies. I know I have to add resistance, but do I put a resistor in series with the inductor or parallel to it.

Thanks

I think there is a mixing of what Q is regarding the Quality of a tuned circuit and the Q of an inductor bringing about some confusion for you.

The Q of an inductor is a figure of merit of the inductor expressed as Q = ωL/R as Ratchit has already pointed out.

The Q of a tuned LC circuit is a figure of merit of the circuits bandwith expressed as Q = fo/BW, where fo is the resonant frequency of the circuit and BW is the actual 3db bandwidth. The Q of a resonant circuit is a measure of the shape/sharpness of the response over a given spectrum, which is determined by the needs of the design.

Among the first steps in your design is to determine exactly what you need to accomplish and proceed from there. There is no need to automatically introduce resistance in a LC tuned circuit, but there may be a need from time to time to increase the bandwidth such as in microwave receiver IF's for one example.

 

[SimonTHK]

For the dummies like me





The Q is infinite in an ideal circuit, but because there is a small resistance in L the Q will fall and not be infinite. This resistance inside L can be set in parallel with the LC so it is an RCL parallel circuit (but in real life, it isnt), just to make it easier to calculate.
IF the resistance in L is low, THEN the resistor will be high when it is put in parallel and cause a high quality factor because it is high resistance. Means that high quality inductor with low inside resistance is = high quality factor.

IF you want low quality, you can add a resistor in parallel yourself, so it would more look like and RRCL circuit. The quality will fall more but the bandwidth will be higher.

The reason why you want high quality factor is because you want a small bandwidth.

 

[Gary B]

Hi, I think that you may be getting confused by terminology. Let me attempt to consolidate some of what has already been said in a way that you might find easier to understand.
1) LC circuit will resonate at a specific center frequency. RLC is the same thing; it just does not ignore the resistance that will be in the wire and, in most cases, intentionally added to broaden the response.
2) Q defines the bandwidth and yes, it is perfectly permissible to look at it the other way around and say bandwidth defines Q. The more resistance in the RLC filter circuit, the lower the Q, the wider the bandwidth and the lower the output voltage and the more uniform over the entire bandwidth until Q= 1. At that point we have what is called a maximally flat response or Butterworth filter. To widen the bandwidth past that point, you have to add additional sections to the filter tuned above and/or below the original center frequency. The previous sentence is only practically correct, you can lower below 1 for wider bandwidth but, gain falls of so the circuit looks more and more like just a resister and you lose frequency rejection outside the desired range.