# Purpose of series resistor in parallel (R')RLC circuit

## Main Question or Discussion Point

Hi, I've understood the basics of an RLC circuit but it seems that when actually putting them into practice I'm advised to put an additional resistor in before the (RL) and C that are in parallel with one another with no explanation. What's the purpose of this resistor and what does altering its resistance do?

Thanks

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sophiecentaur
Gold Member
An LC circuit will resonate at its natural frequency but, if you don't know the Q of the circuit, you could be very embarrassed by how it actually performs for you when off-rsonance. If you put in a significant value resistor (to swamp the inbuilt one), you will be defining the Q. It will be lower than without the resistor but you'll have defined how the circuit will behave off-resonance.

vk6kro
If a parallel L/C circuit is connected across a constant voltage signal source, the voltage across the L/C circuit will stay the same at all frequencies, even though the current may vary.

Putting in a series resistor forms a voltage divider so that the impedance of the L/C circuit compared with the resistance of the resistor controls the output voltage.

For example, the following circuit shows a constant voltage signal source connected to two identical tuned circuits. One, via 10 ohms and the other via 1000 ohms.

You can see that the 1000 ohm resistor produces a much more selective output.

[PLAIN]http://dl.dropbox.com/u/4222062/LC%20R%20comparison.PNG [Broken]

If there was no resistor, the output would just be a flat line across the top of the graph.

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Ah, makes a lot of sense now, thank you very much!