# Totaly Stumped

Discussion in 'Electronics Homework Help' started by shorteddiode, Nov 2, 2012.

1. ### shorteddiode

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Mar 23, 2011
I attached the question right out of the book. I have re-read the entire unit and am lost, How do you figure the voltages with the information given? File size:
86.7 KB
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168

3,635
5
Apr 7, 2012
3. ### shorteddiode

41
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Mar 23, 2011
OK, I think reading the link it finally clicked. So the source voltage would be 300 volts, the voltage between A & C would be 200 volts and the voltage between B&D would be 200 volts. Is that correct?

4. ### CocaCola

3,635
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Apr 7, 2012
That is correct...

5. ### shorteddiode

41
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Mar 23, 2011
Thanks CocaCola, I don't know why it seemed so hard

6. ### CocaCola

3,635
5
Apr 7, 2012
Sometimes a different worded explanation makes all the difference...

Trust me you are not alone in being totally lost only to discover the obvious answer that causes a facepalm...

7. ### Laplace

1,252
184
Apr 4, 2010
I think that you may have lost sight of the fact that this is an AC circuit with reactance and resistance so the voltage at each point will be a phasor voltage where the inductive phasor leads the current and the capacitive phasor lags the current. So plot the AB, BC, and CD voltages on a phasor diagram then add them together to get the source voltage. Each phasor will have the same 100V magnitude, but what about phase cancellation?

8. ### shorteddiode

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Mar 23, 2011
OK< I understand the phasor voltage, but can you explain about phase cancellation
I found out the source voltage is not 300 volts, the hard way

9. ### CocaCola

3,635
5
Apr 7, 2012
Guilty as charged...

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Look at this

You have something a little more complex, but considering the driven LC case first should yield some insight.

11. ### shorteddiode

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Mar 23, 2011
I used the Pythagorean theorem Vs= the square root of Rv squared plus (Lv-Lc) squared and came up with a voltage source of 100 volts AC. Right??
So how do I figure the voltage between A,C and B,D?

12. ### Laplace

1,252
184
Apr 4, 2010
I also calculated that Vs=Vr. Another way to view it is that since Vc=Vl, the capacitor and inductor are in series resonance so the effective reactance is zero and the full source voltage appears across the resistor. This is easy to see on a phasor diagram where Vr has no phase shift in reference to the current, and Vc lags by 90 degrees while Vl leads by 90 degrees. So the resistive voltage and reactive voltage drops are two sides of a right triangle when being added together.

13. ### shorteddiode

41
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Mar 23, 2011
So Laplace are you saying the voltage across the capacitor and inductor would be 0?

14. ### Laplace

1,252
184
Apr 4, 2010
If the full source voltage appears across the resistor, then what else is left to drop across the capacitor and inductor?

15. ### shorteddiode

41
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Mar 23, 2011
nothing, That makes sense, thanks

16. ### shorteddiode

41
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Mar 23, 2011
OK Guys here are the answers I was given after the re-test, Thanks for all the help guys

1 source voltage 100 volts AC
2 Between A and C 0 volts AC
3 Between B and D 141 volts AC
4 Between A and D 100 volts AC  