# RCL Circuits

Hi all!

I'm a student in APU University, Cambridge, England. One of my modules is Analogue Electronics, Exams are a few weeks away and i wanted to get an idea of how well ive grasped some of the concepts. We have been given a sheet to work through but have no one to mark it as all lectures are off site.

just wanted to make sure i was on the right track. heres part of it:-

Circuit Diagram - http://www.mwdata.plus.com/diagram.JPG

a) Show that the capacitor C1 has a capacitance of about 13nF.

so C1= 1/(2*Pi*f*Xc)

1/(2*Pi*2000*6000)

=13nF? i think thats ok

b) Calculate the magnitude of the combined impedance of the capacitor C1 in series with R1.

so Z total= Zr1+Zc1?

Z total = 14kOhm?

c) Calculate the current I1 in the circuit.

I1 = Ir1+Ic1?

Ir1=v/r
=3/8000
=3.75mA?

Ic1=3/6000
=5mA?

total = 8.75mA?

d) Calculate the voltage Vc1 across the capacitor C1.

so Vc1= Ic1 * Xc
=0.0005*6000
=3v?

Jack Wood.

a) is correct. d) is correct except you got the wrong value of Ic.

b) Not quite. If you draw a vector diagram of the voltages in the circuit you will have something like this:
Code:
|   /
|  /
| /
|/____
Where the vertical vector is the voltage of the capacitor, the horizontal vector is the voltage of the resistor and the diagonal vector, which represents the vector addition of the other two, is the voltage of the source. Thanks to Ohm's law you can divide the magnitude of those vectors by I, which is the current throughout the circuit (it's the same for all components). Now the orthogonal vectors represent the impedance of the capacitor and the resistance of the resistor and the last vector represents the impedance of the whole circuit, which is what you are looking to find. Through Pythagoras you see that Z2 = Xc2 + R2 and from here you find Z (10kOhm).

c) VRMS = IRMSZ. You have both VRMS and Z, so you can see how IRMS = 0.3mA.

Sorry for the crude drawing, I'm not on my main computer right now.

#### Attachments

• diag.jpg
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Excellent. yes that rings some bells now.

now on to the next sheet, any problems with that and ill be back :).

Thanks

Jack

EDIT: thanks for the diagram back again :)

diagram http://www.mwdata.plus.com/diagram2.JPG

a) Calculate the Inductance (H) of the inductor L1

can't seem to figure out how to do it.

L1 = VL1 / XL1?

It's late at night and my brain hurts, ill be seeing these equations in my sleep. lol.

Thanks

Jack

No, VL1 / XL1 is the current through the inductor. This is what you're looking for:

$$X_L = \omega L = 2\pi fL$$

And you want to extract L. ok, im going to sound like a total n00b here but, what do you mean by extracting L? i understand how the do the calculation but what is wL? and how do i get L to do 2*Pi*f*L?

sorry to be thick :)

Jack

$$\omega L$$ is $$X_L$$ (definition). $$\omega$$ is also equal to $$2\pi f$$, so $$X_L = 2\pi fL$$. By "extract L" he means "solve for L", i.e. $$L = \frac{X_L}{2\pi f}$$.

Last edited:
Sorry, I don't study in English so I don't always get the terms right. Thanks Muzza.

perfect, thanks man!

Jack

If you find any more interesting questions feel free to post them, I myself have a final exam about this in about a week's time so I could use the exercise. Oddly enough, I also have an exam concering RCL-circuits (among other things) coming up in a week.

It's that time of year I guess.

Chen said:
If you find any more interesting questions feel free to post them, I myself have a final exam about this in about a week's time so I could use the exercise. Yeah i got one more to get through.

here goes:-

Diagram:
http://www.mwdata.plus.com/diagram3.JPG

not knowing too much about diodes i can't do much of it so here it is. cheers once again for the help :).

a) Calculate the value of the DC supply Voltage (V5) in the circuit/

b) Calculate the current through the zener diode in the circuit .

c Calculate the power dissipated in both the zener diode and the resistor the of the circuit.

using P=I*R?

Jack

Sorry, no idea. I replaced electronics with a technology project so I never learned about diodes either. (We were taught RCL in physics class.)