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

  • \$\begingroup\$ Did you try delaying the turn-on of the voltage source? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 3 '16 at 17:11
  • \$\begingroup\$ @IgnacioVazquez-Abrams Can you please explain what is this? Have a mercy I am completely novice at this. \$\endgroup\$ – Boris Oct 3 '16 at 17:12

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.

Qucs Capacitor Dialog box

  • \$\begingroup\$ Worked as a charm... I cannot accept it as an answer immediately. Becuase it says to wait for 8 more minutes. Thanks!!! \$\endgroup\$ – Boris Oct 3 '16 at 17:17
  • \$\begingroup\$ @PeterK Dear Peter, I was wondering, would you be so kind to have a look at this post if time allows: electronics.stackexchange.com/questions/378429/… Thanks in advance. \$\endgroup\$ – user929304 Jun 7 '18 at 7:34

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

  • \$\begingroup\$ How does this answer the question? \$\endgroup\$ – Oskar Skog Oct 1 '17 at 18:15

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