Help with Series RLC

Discussion in 'Homework Help' started by fatboy2131, Oct 5, 2007.

Oct 5, 2007
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Hello Everyone,

I've got a question. I am taking an on-line electronics course, and we have been doing Series RLC circuits. I have a test question (attached) that I am not sure of. Basically, it is a Series RLC circuit with no values (resistor, capacitor, inductor, supply voltage, or current) other than three voltage readings. Can someone out here explain to me how I can find out this answer? I have read twice so far my lessons on this and can't figure it out. I'm sure I'm just missing something or "overthinking' it.

Thanks!

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2. techroomt Senior Member

May 19, 2004
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sorry, it didn't open for me.

3. ixisuprflyixi Active Member

Sep 16, 2007
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You must be overthinking it. All voltage drops in a series circuit add up to system voltage.

4. Ron H AAC Fanatic!

Apr 14, 2005
7,050
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Not true. You are forgetting about phase.
Here's my take: Since the voltages are equal, and all components have the same current flowing through them, then XL=XC=R.
Since XL=XC, the circuit must be at resonance. At resonance, the voltage across (XL+XC) will be zero, because voltage across L is 180 degrees out of phase with the voltage across C. Therefore, the supply voltage is 100V.

Oct 5, 2007
10
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Nice! Thanks alot!

6. cumesoftware Senior Member

Apr 27, 2007
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So the AC voltages at the inductor and capacitor cancel each other because they are 180º out of phase (the capacitor is 90º behind and the inductor is 90º ahead)?

7. Ron H AAC Fanatic!

Apr 14, 2005
7,050
656
That's correct. Below is a Bode plot of the circuit. It's a little busy, but you can see that at 1kHz (resonance), the voltage across each element is 100V, and the phase across L and C are 180 degrees apart. You can also see that the voltage across the LC combination (V(A,C)) is zero.
Note that R=100, XC=100, and XL=100 at 1kHz. They could actually have any value, so long as all 3 are equal.

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8. Ron H AAC Fanatic!

Apr 14, 2005
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I hate how this forum converts .PNG to .JPG. It screws up the quality of the images, and also shrinks them, making them difficult to read. The other forum I lurk in doesn't convert the image format.

9. ixisuprflyixi Active Member

Sep 16, 2007
52
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Thank you Ron, I overlooked that. Hope I didnt confuse anyone.