# Understanding a RL - RC circuit

1. Aug 24, 2007

### null_null_

I´m trying to understand a circuit that is build like this:

----$#R1$#---$#C$#----
$#---------$#----------$# -----$#L$#---$#R2###---

How am I supposed to find what frequency (not f=0) the impedance is pure resistive?

What I´ve read has it something to do with the resonant frequency. Is that correct?

And the last question is how big are the impedance when that happens?

I´m totaly lost in this jungle.

//Null_Null

2. Aug 24, 2007

### Staff: Mentor

Welcome to the PF, nul_nul_

It's a little hard to interpret the circuit drawn like that. Would it be possible for you to draw it in Paint or Visio or something, and save it as a JPEG and upload it as an attachment?

Also, do you know a bit yet about how to express the impedances as complex quantities, and add them using the regular series and parallel combination rules?

3. Aug 24, 2007

### null_null_

Thank you!

Yes, I see that now. So I´ve made up one with my Paint skills :rofl:

I know a bit about that. So we say we have the values of L = 5 mH, C = 22 uF, R1 = 300 ohm and R2 = 500 ohm. Then the R1 is 300 L 0 degrees, R2=500 L 0 degrees (L=Phasor) and the X(L)=2(pi)fL and phasor 90 degrees, X(C)=1/(2(pi)fC) and phasor -90 degrees.

Is this a good start and how about the frequency?

//Null_null

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4. Aug 24, 2007

### Staff: Mentor

Yes, that's an excellent start. Now use parallel and series combination rules to come up with one equation for the impedance across the whole circuit. It will be a complex equation, with real and imaginary parts that depend on frequency.

EDIT -- And remember that the impedance will look "purely resistive" when there is no imaginary impedance, right? What can you say about the relative magnitudes of the capacitive (-j) and inductive (+j) impedances when this condition is met?