# I don't know why the analogue analysis on Proteus is not working as it should

#### Gnahore

Joined Nov 11, 2022
5
Hi, I'm a student and I wanted to make some electrical circuits for fun on Proteus, to challenge what i learned all those years.
I'm trying something really basic : a RC-circuit with a direct source. Here is a capture of it.

I know the voltage at the capacitor terminals is C1(t) = V1(1-exp(-t/RC)), so I should see the tension inscreasing to u(0)=0V to reach u(t) = V1.
But why the analogue analysis doesn't show that? RC = 1000*1*10^(-6) = 1ms.
I'm using Proteus 8 Professionnal.

#### Papabravo

Joined Feb 24, 2006
21,225
Capacitors are AC devices. There is nothing in your circuit that is changing. Maybe you could start your 5V source at 0V and ramp the voltage up to +5V. I'd be surprised if it would let you turn on the source in zero time.

#### Gnahore

Joined Nov 11, 2022
5
Capacitors are AC devices. There is nothing in your circuit that is changing. Maybe you could start your 5V source at 0V and ramp the voltage up to +5V. I'd be surprised if it would let you turn on the source in zero time.
But in class we studied different circuits and those RC circuits use a DC source and I know that I'm supposed to have something like this
So why can't I have something similar? I really don't understand

#### Papabravo

Joined Feb 24, 2006
21,225
That is what you get when the voltage source executes a step function at t=0. Your source is at 5V at t=0 and nothing is changing and the capacitor won't pass DC. Notice how in the following simulation the DC source stars at 0 and rises rapidly to +5V. That is what you need to get a capacitor charged.

#### Gnahore

Joined Nov 11, 2022
5
That is what you get when the voltage source executes a step function at t=0. Your source is at 5V at t=0 and nothing is changing and the capacitor won't pass DC. Notice how in the following simulation the DC source stars at 0 and rises rapidly to +5V. That is what you need to get a capacitor charged.
View attachment 280477
Yes this is exactly what I'm trying to have. Could you explain how to do this? Do you use Proteus?

#### Papabravo

Joined Feb 24, 2006
21,225
Yes this is exactly what I'm trying to have. Could you explain how to do this? Do you use Proteus?
Sorry I do not use Proteus. For at least the third time, you have to simulate "turning your power supply on". When you do that the voltage at the '+' terminal will start at 0V or GND and it will rise over some period of time to its final steady state output value. You cannot charge a capacitor with a fixed voltage that does not change. Look at the red trace on my simulation and notice the rapid rise of the value from 0V to +5v at a time slightly greater than time 0.0.

#### WBahn

Joined Mar 31, 2012
30,057
Consider this:

What is the voltage output of a 5 V battery ten seconds before you start your stop watch?

Most simulators consider a 5 V DC source to be an ideal voltage source, which means that it has been outputting 5 V since the beginning of time.

Your expected results are for a source that is at 0 V for all time prior to t = 0 s and 5 V for all time thereafter. If you want the simulator to simulate that, then you have to use a source that does that. Depending on the sources that Proteus supports, you have a few options, including a piecewise-linear source or a pulsed source. Your simulator might also support a "startup" option that starts all independent sources at zero output, but this is usually reserved for circuits that have problems converging on an initial operating point solution.

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#### Gnahore

Joined Nov 11, 2022
5
Consider this:

What is the voltage output of a 5 V battery ten seconds before you start your stop watch?

Most simulators consider a 5 Vc DC source to be an ideal voltage source, which means that it has been outputting 5 V since the beginning of time.

Your expected results are for a source that is at 0 V for all time prior to t = 0 s and 5 V for all time thereafter. If you want the simulator to simulate that, then you have to use a source that does that. Depending on the sources that Proteus supports, you have a few options, including a piecewise-linear source or a pulsed source. Your simulator might also support a "startup" option that starts all independent sources at zero output, but this is usually reserved for circuits that have problems converging on an initial operating point solution.
Thank you I did what you suggested and I use a pulsed source. Here is the result :
I think this is what @Papabravo was trying to tell me but I didn't understand (I don't fully understand english, I just started electronics and I'm using this device for the first time). Does that mean in pratice every time I have a capacitor, I must use a pulsed source (or something that simulate a transitional regime) or a sine source?

I'm not sure but I think both of you have asked me question about DC source, so here is a capture of the edit component :

I don't think I could do anything else with it

#### Gnahore

Joined Nov 11, 2022
5
@Papabravo is that what you were trying to tell me? (Look like you are using LTSpice btw)

#### Papabravo

Joined Feb 24, 2006
21,225
@Papabravo is that what you were trying to tell me? (Look like you are using LTSpice btw)
Yes, indeed your conclusion is spot on.

#### Papabravo

Joined Feb 24, 2006
21,225
Thank you I did what you suggested and I use a pulsed source. Here is the result :View attachment 280484View attachment 280485
I think this is what @Papabravo was trying to tell me but I didn't understand (I don't fully understand english, I just started electronics and I'm using this device for the first time). Does that mean in pratice every time I have a capacitor, I must use a pulsed source (or something that simulate a transitional regime) or a sine source?

I'm not sure but I think both of you have asked me question about DC source, so here is a capture of the edit component :
View attachment 280486
I don't think I could do anything else with it
All it has to do is change in some way and the capacitor will "see" it change.