# Simple RC question about a simple step response

#### mike _Jacobs

Joined Jun 9, 2021
122
I have two circuits here and im looking at the step response of a simple RC circuit.

One of them has an additional series resistor and a small voltage drop before ground.

When i simulate the first one, you get the response you would expect just based on the RC constant.
When i simulate the second one, i get 50mV on the cap steady state. Can someone explain why that is.
The current path through the cap still exists even in the circuit with the additional resistor.
I do not understand why the additional resistor would effect the step response of C13
It still has its path to ground without any difference

The excitation is just a dc source

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#### AnalogKid

Joined Aug 1, 2013
10,777
Your schematics have unconnected nets and components, and no indication of where your measurement points are. Please update the drawings.

Also, the time constants involves are over 10 seconds. Are you sure your "steady" states are in fact steady?

Have you built these circuits, or are all of your results from simulations?

ak

#### WBahn

Joined Mar 31, 2012
29,489
No, the current path does not still exist in the second case. Remember, unless there is leakage current, if current is flowing through a capacitor, then the voltage across it is changing. If the voltage across it is not changing, then the current through it is zero.

Why won't you show the entire circuit? Why do we have to guess?

In order for the voltage across the cap to be 50 mV in steady state, a current of (500 mV - 50 mV) / 1.5 MΩ, or 300 nA is flowing, right to left, in R57.

Assuming the capacitor model has no leakage current, then all of that current must flow through R56 and the voltage on the other side of it would need to be -100 mV.

If the voltage on the other side of R56 is 0 V, then it would have 100 nA flowing through it, requiring the capacitor to have a leakage current of 200 nA, or an equivalent shunt resistance of 250 kΩ.

But that's all a bunch of guessing because I have no idea what is to the left of R56 because you, for some reason, didn't feel like giving us the entire circuit.

#### mike _Jacobs

Joined Jun 9, 2021
122
No, the current path does not still exist in the second case. Remember, unless there is leakage current, if current is flowing through a capacitor, then the voltage across it is changing. If the voltage across it is not changing, then the current through it is zero.

Why won't you show the entire circuit? Why do we have to guess?

In order for the voltage across the cap to be 50 mV in steady state, a current of (500 mV - 50 mV) / 1.5 MΩ, or 300 nA is flowing, right to left, in R57.

Assuming the capacitor model has no leakage current, then all of that current must flow through R56 and the voltage on the other side of it would need to be -100 mV.

If the voltage on the other side of R56 is 0 V, then it would have 100 nA flowing through it, requiring the capacitor to have a leakage current of 200 nA, or an equivalent shunt resistance of 250 kΩ.

But that's all a bunch of guessing because I have no idea what is to the left of R56 because you, for some reason, didn't feel like giving us the entire circuit.
What are you talking about show the entire circuit?
That is the entire circuit? I wrote right in the post its a simple step response from a dc source excitation. How is that guessing?
Why do you have to constantly act like bully? Don't answer me if you dont like my questions. There is nothing wrong with the question. I even added a schematic although its not warranted because you constantly complain i don't.

#### mike _Jacobs

Joined Jun 9, 2021
122
No, the current path does not still exist in the second case. Remember, unless there is leakage current, if current is flowing through a capacitor, then the voltage across it is changing. If the voltage across it is not changing, then the current through it is zero.

Why won't you show the entire circuit? Why do we have to guess?

In order for the voltage across the cap to be 50 mV in steady state, a current of (500 mV - 50 mV) / 1.5 MΩ, or 300 nA is flowing, right to left, in R57.

Assuming the capacitor model has no leakage current, then all of that current must flow through R56 and the voltage on the other side of it would need to be -100 mV.

If the voltage on the other side of R56 is 0 V, then it would have 100 nA flowing through it, requiring the capacitor to have a leakage current of 200 nA, or an equivalent shunt resistance of 250 kΩ.

But that's all a bunch of guessing because I have no idea what is to the left of R56 because you, for some reason, didn't feel like giving us the entire circuit.

Is this what you want to keep from belittling me and internet bullying me? here is your DC source just like i said

#### mike _Jacobs

Joined Jun 9, 2021
122
This is very simple.
during the initial transit, the cap acts like a short at DC
The capacitor should start to charge like you suggest. When it finally reaches the voltage of the input source, it will no longer conduct. The total current flow will then be reduced to nothing more then the source divided by the series resistance.
But that is not what i see in simulation.

#### WBahn

Joined Mar 31, 2012
29,489
What are you talking about show the entire circuit?
That is the entire circuit? I wrote right in the post its a simple step response from a dc source excitation. How is that guessing?
Why do you have to constantly act like bully? Don't answer me if you dont like my questions. There is nothing wrong with the question. I even added a schematic although its not warranted because you constantly complain i don't.
As you wish, I will no longer attempt to assist you.

#### mike _Jacobs

Joined Jun 9, 2021
122
As you wish, I will no longer attempt to assist you.
You can do whatever you want but i dont need to be the subject of your grouchiness. There is no reason for it.
This is a simple question and yet you cant answer it without animosity towards me. So why be on a HELP forum then?

#### mike _Jacobs

Joined Jun 9, 2021
122
Your schematics have unconnected nets and components, and no indication of where your measurement points are. Please update the drawings.

Also, the time constants involves are over 10 seconds. Are you sure your "steady" states are in fact steady?

Have you built these circuits, or are all of your results from simulations?

ak
They are just in a simulation

#### mike _Jacobs

Joined Jun 9, 2021
122
If the capacitor was fully open, you would have nothing but a voltage divider. About 6 volts or so.
So how is the steady state cap voltage 50mv?

#### AnalogKid

Joined Aug 1, 2013
10,777
If you open the properties for the capacitor, does it have a value for leakage current, parallel resistance, etc?

ak

#### sghioto

Joined Dec 31, 2017
5,088
How do you figure 6 volts?

#### mike _Jacobs

Joined Jun 9, 2021
122
How do you figure 6 volts?
because a voltage divider with those two resistors with no cap is ~6V
The cirucit behaves as expected. I found the issue finally.
Apparently, pspice doesnt know what 500k or 1.5M is
When i told the R values to be 500000 and 1500000
Everything worked find. Nothing like wasting a day over the simulator.

#### mike _Jacobs

Joined Jun 9, 2021
122
If you open the properties for the capacitor, does it have a value for leakage current, parallel resistance, etc?

ak
thanks for offering advice. I found the issue and posted below. Thank you

#### sghioto

Joined Dec 31, 2017
5,088
Glad you figured it out but I never saw a voltage divider in any schematic with the two resistors R56 and 57 which was why I asked about the 6 volts

#### BobTPH

Joined Jun 5, 2013
8,075
I never saw a voltage divider in any schematic
Nor a source for a voltage step.

#### mike _Jacobs

Joined Jun 9, 2021
122
Nor a source for a voltage step.
Thanks for your support as always!

#### DickCappels

Joined Aug 21, 2008
10,104
@mike_Jacobs

Relax. People here are trying to help you, provided you adequately explain the problem. The know what they need better than the rest of us - give them what they ask for and I imagine they will take care of you