RC Circuit Simulation in Simulink Shows Incorrect Charging Time

Hello,

I am simulating a simple RC circuit in Simulink (MATLAB R2025b) to observe the capacitor charging time. The circuit consists of a 1V DC voltage source, a 1-ohm resistor, and a 1-µF capacitor.

According to theory, the time constant (τ = R*C) is 1 µs. This means the capacitor voltage should reach approximately 63% of the source voltage (about 0.63V) at t = 1 µs.

However, my simulation shows a significant delay. The capacitor voltage only begins to rise noticeably between 2 µs and 3 µs, and does not match the theoretical curve at all.

I have attached my model file for reference. Could you please help me understand why the simulation results do not align with the theoretical expectations? I suspect it might be related to my solver configuration.

Thank you for your help.

[Torsten]

Can you set a start stepsize "InitialStep" ? Then use 1e-2*R*C [s] , e.g.

Answer 1

[Paul]

Hi Xiaolong,

Disclosure: I did not actually open your model.

If you know the fastest system dynamics are on the order of 1e-6,consider changing the Max Step Size in the solver from auto to 1e-7, at least to see if that gets things going in the correct direction.

load_system('untitled');
c = getActiveConfigSet('untitled');
get_param(c,'Solver')

ans = 'VariableStepAuto'

get_param(c,'MaxStep')

ans = 'auto'

You can change the the Max Step Size in the Solver Pane, which can be acessed as shown at Set Model Configuration Parameters for a Model.