QUCS simulation of simple capacitor/resistor circuit - strange results.

Question

Hi I have just started to learn electronics. In order to understand the RC constant I have built the following very simple circuit in QUCS simulator.

According to the formula

RC time constant = R x C = 2,000,000 ohms x 0.000015 farads = 30 seconds

I would expect the graph of voltage on capacitor C1 rise steadily till it reaches 2/3 of 1.5 after 30 secs . However as it can be seen the the simulation shows straight graphs of both current and capacitor's voltage without any difference in time.

Can someone explain these results

@IgnacioVazquez-Abrams Can you please explain what is this? Have a mercy I am completely novice at this. — Boris, Oct 03 '16 at 17:12

score 5 · Accepted Answer · answered Oct 03 '16 at 17:15

5

The circuit simulator is solving for the steady-state initial condition before running the transient sim.

To get your desired behaviour you have to explicitly tell the simulator you want zero volts across the cap at t=0. To do this, double click on the part and set the initial voltage to zero - I've highlighted the line you need to change in the dialog below.

answered Oct 03 '16 at 17:15

Synchrondyne

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Worked as a charm... I cannot accept it as an answer immediately. Becuase it says to wait for 8 more minutes. Thanks!!! – Boris Oct 03 '16 at 17:17
@PeterK Dear Peter, I was wondering, would you be so kind to have a look at this post if time allows: https://electronics.stackexchange.com/questions/378429/resistance-or-conductance-of-resistor-networks Thanks in advance. – user929304 Jun 07 '18 at 07:34

score 2 · Answer 2 · answered Oct 01 '17 at 18:08

2

To simulate charging and discharging a capacitor, we use a voltage pulse.

answered Oct 01 '17 at 18:08

pak wayan di mataram

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score 1 · Answer 3 · answered Jul 14 '23 at 09:33

I have had this same issue.

I had to set the initial value of the capacitor as mentioned by Synchrondyne.

However I also had to turn off the InitialDC calculation of the Transient simulator object. To do this

Double click the "Transient Simulation" object.
Go to the properties Tab
Find the "InitialDC" property and set it to "No"

LostEcho · Answer 4 · 2023-07-09T15:29:31.847

0

use .IC Section in the SPICE specific sections can help. I found that setting the initial voltage for the capacitor doesn't work at all.

ps: The software I use is qucs-s instead of qucs.

edited Jul 09 '23 at 15:29

answered Jul 09 '23 at 15:25

LostEcho

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QUCS simulation of simple capacitor/resistor circuit - strange results.

4 Answers4