How to find impedance across the capacitor

Thread Starter

Sunshine2023

Joined Jul 12, 2023
1
Hello All,
I have a 1.8V circuitry with two parallel capacitors and please let me know how can I setup or find out the impedance across the capacitor C420.

1689146031119.png
 

MrAl

Joined Jun 17, 2014
11,272
Hello All,
I have a 1.8V circuitry with two parallel capacitors and please let me know how can I setup or find out the impedance across the capacitor C420.

View attachment 298167
Hi,

Taking your question at face value, you could solve for the frequency and then knowing how current splits in a parallel circuit you could solve for the current going through just the C420 cap, then calculate the impedance. Alternately, once you get the frequency just solve for the impedance with that.

I didn't check yet, but there could be a way to solve for that without knowing the frequency. I'll check that out and let you know later.
 
Last edited:

Papabravo

Joined Feb 24, 2006
21,018
Hello All,
I have a 1.8V circuitry with two parallel capacitors and please let me know how can I setup or find out the impedance across the capacitor C420.

View attachment 298167
At DC the impedance across an ideal capacitor is infinite. That is, it completely blocks DC current. The impedance of two ideal capacitors in parallel is still infinite. For any finite frequency f, the reactance of the capacitor is given by:

\( X_C\;=\;\cfrac{1}{2\pi fC} \)

A real capacitor will also have an equivalent series resistance, and so the sum of the ESR and the capacitive reactance would be:

\( Z_C\;=\; R_{esr} + jX_C \)

For two capacitors in parallel you need to combine the impedances as you would for two resistors in parallel.
 
Last edited:

MrAl

Joined Jun 17, 2014
11,272
Hello again,

Ok i tried it and you don't even have to know the frequency to calculate the impedance of C420.
 

BobTPH

Joined Jun 5, 2013
8,671
I assume you are reading the @4A as an actual current reading. Then yes, you can calculate the impedance without knowing a frequency, but that is not the way I read it.
 

MrAl

Joined Jun 17, 2014
11,272
I assume you are reading the @4A as an actual current reading. Then yes, you can calculate the impedance without knowing a frequency, but that is not the way I read it.
Hi Bob,

Well, i took the information given and the information not given and went with that. If there is information missing or information that is not correct i cant control that.

What we are given:
1. Schematic.
2. Operating voltage.
3. Total current.

Of course there will be questions about all of these:
1. Is the schematic complete (wire running off the left side of the diagram).
2. Is the operating voltage in peak or RMS units, or a trick question it's DC.
3. Is the total current only for the two caps and nothing else running off the left of the diagram.

Given the following assumptions:
1. The schematic is complete because that is all that is given.
2. The operating voltage is in RMS, but easy to work with peak also.
3. The total current is only for the two caps because otherwise we would not be able to solve the problem unless we start to assume an unknown impedance for the missing components.

Part of the reasoning here is that the instructor would not give them a problem with missing information but we all know that's not always the case (ha ha). For the voltage, in AC circuits usually the RMS value is given. For example, "My appliances all work at 120vac", which means 120vac RMS.

So these are reasonable assumptions that should be true but there's always the possibility that something could change if we get more information. Unfortunately it doesn't look like that's going to happen. Also unfortunate is that these problems always have a number of interpretations when the information is incomplete.

I can still give a hint as i think you already guessed though. The hint is, imagine how easy this would be if the two caps were really two resistors and the voltage source was a constant DC source.
 

BobTPH

Joined Jun 5, 2013
8,671
Oh, I know what you did. I just am not sure about your assumptions. I worry that the poster has not given us everything the prof gave them.
 

MrAl

Joined Jun 17, 2014
11,272
Oh, I know what you did. I just am not sure about your assumptions. I worry that the poster has not given us everything the prof gave them.
Hi Bob,

Yes, unfortunately that's always a possibility, and in the homework section it is almost a given (ha ha).
I outlined my reasoning process so everyone would know why i gave the results i gave, and why i mentioned that fact that we may not have enough information yet.
I have a feeling the OP has gone through.

Yes, I figured you did know. Because we are given the voltage and the current and we know how current splits in a parallel circuit, with the assumptions I gave we can calculate it without even knowing the frequency.

We could go on to list all the ways to do this with an associated list of different assumptions, but I'd rather wait and see if the OP comes back and provides a little more info or verifies what has been said already.

Thanks for your ideas and suggestions as well.
 
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