Help with a CircuitLab graph question, simple RC circuit.

Thread Starter

timbaker0365

Joined Aug 11, 2020
9
I am working on an example from a book, which is: a 1uf capacitor connected in series with a 1500 ohm resistor, AC 60V 60Hz PS. So I'm experimenting with CircuitLab at the same time. From the calculations I did (and from the book example), the readings should be approx 20 mA, 30V across the resistor, and 52V across the capacitor. I ran this in CircuitLab, and everything checks out - the amperage, voltage across the resistor, EXCEPT the voltage across the capacitor - which is showing the full 60V instead of the approx. 52V? Am I doing something wrong or is this the way it should be displayed?

You can see in the top right of the image the voltage legend for the cap & res.

This is not really a HW problem, just something I'm working on on my own, trying to get used to learn CircuitLab and figuring out RC circuits.

Thanks for any help.

RC1.png
 

KeithWalker

Joined Jul 10, 2017
1,216
We don't have your circuit diagram but I will make a guess that you are making the capacitor voltage measurements using the common side of the input as the reference instead of measuring from the input to the junction of the R and C.
Regards,
Keith
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
9
Nope Keith, that wasn't it. I probed both sides (see below) same result as last - showing 60V going through cap (V(C1.na), then 30 going to resistor (V(C1.nb)) ? That's about all I can try with it I guess?

rc2.png
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
9
V(C1.na) and V(C1.nb) are both being measured with reference to circuit common so V(C1.na) will be equal to the supply voltage.
Yeah, well I don't know of any other method to get "that" measurement/result (52V) - those are the only spots (nodes) available to attach the probes to in the app. I wonder if I switched the cap & res around - I might try that out of curiosity.

But I was thinking this might be a useful resource for if I was ever to TS a live circuit ( but it looks like that might be limited) - which is one reason why I was practicing with CL. I was even thinking of setting something up on a breadboard so I could test it and see the results for myself but my practice lab is only DC, so that's out too. But thanks for your help Keith, I'll see what I can come up with.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
9
Well, that did do it - I flipped the res & cap around and got my ~52V on the cap, but now I get the full source V at the resistor, so again, I guess this is a limited feature if I'm ever trying to decipher a live circuit - better to know this stuff now.

RC3.png
 

KeithWalker

Joined Jul 10, 2017
1,216
I recommend that yo reinforce what you learned with some real measurements. Do you have a transformer laying around that has a low voltage secondary winding, 3 or 6 volts? That would be a good source for AC measurement experiments.
Regards,
Keith
 

crutschow

Joined Mar 14, 2008
25,392
I guess this is a limited feature
Not really.
You just need to read the documentation.
According to the Circuitlab manual, you can plot the voltage difference between two nodes like this:

Expressions support the basic mathematical expressions +,-,*, and /. For example, to plot the difference between the voltage a node named V_Plus and the voltage at a node named V_Minus, do:

V(V_Plus)-V(V_Minus)
 
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Thread Starter

timbaker0365

Joined Aug 11, 2020
9
I recommend that yo reinforce what you learned with some real measurements. Do you have a transformer laying around that has a low voltage secondary winding, 3 or 6 volts? That would be a good source for AC measurement experiments.
Regards,
Keith
I don't know I might be able to come up with something, I'll poke around for one maybe when I get a chance - good idea, thanks.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
9
Not really.
You just need to read the documentation.
According to the Circuitlab manual, you can plot the voltage difference between two nodes like this:

Expressions support the basic mathematical expressions +,-,*, and /. For example, to plot the difference between the voltage a node named V_Plus and the voltage at a node named V_Minus, do:

V(V_Plus)-V(V_Minus)
Thanks. Yeah, I've watched a couple of quick tutorials to get started and read a little bit, and may have even come across that before or missed it in my haste - but I certainly haven't read all of the documentation - most of it I won't need, for now anyway. I'll take a look see if it works, thanks.
 

Thread Starter

timbaker0365

Joined Aug 11, 2020
9
@crutschow
So working on a different circuit below:
rlc1.png

by my calculations, the inductor should read ~47.4 V going across it. I added the expression V(L1.nA)-V(L1.nB) and I'm pleased to say that it indeed did work, see below. After the wave settled down I get a reading of ~47V so I'm happy with that. Thanks!

rlc2.png
 
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