# Help with a CircuitLab graph question, simple RC circuit.

#### 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.

#### 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

#### timbaker0365

Joined Aug 11, 2020
9
Thank you I will take a look at that.

#### 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?

#### KeithWalker

Joined Jul 10, 2017
1,216
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.

#### 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.

#### 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.

#### 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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#### 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.

#### 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.

#### timbaker0365

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

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!