Forums » CircuitLab Support Forum

Does CircuitLab somehow ignore capacitor charge times? I made a very simple circuit charging a 1000uF capacitor with a 9v battery through a 3k resistor and it shows it instantly being fulling charged. Using T = 5RC as the amount of time to (approximately) fully charge the capacitor, is should take 15s.

Here is the circuit: https://www.circuitlab.com/circuit/6f7mjpfpwy9f/capicator-charge-time/

The problem is the capacitor starts out charged up. Put a time controlled switch to discharge the cap, and open it at t=0. Like in:

(Yes, that's kind of weird)

Dealing with this sort of problem (finding a time constant) I generally use a square wave generator rather than straight dc. This gives an indication of both the initial transient and the steady state if you let it run long enough

Great idea. I was just trying to give the OP the picture he requested, but showing him more is better.

Thanks for the info guys (although, wdparker3, your image doesn't seem to be showing up for me).

That's odd. I just now made it public. See if it's there now. I would expect a post of a private circuit to work, but maybe not. This site is not long on documentation.

It's there now! Thanks!

I had the same issue - it seems counter-intuitive that the capacitor doesnt start out at zero volts, like it would in real life. But the timed switch seems to be inflexible - it starts open, and then it closes. Is it also possible to configure it the other way around, to start closed, and then to open?

Hi @rdesalis, yes, there are actually three good options here:

Option 1: Time-Controlled Switch

We simply move the switch SW1 to a place where it interrupts the circuit. As far as starting closed or open, note that SW1 has three terminals (Normally Open, Normally Closed, and Common - where "Normal" means "before the specified switching time") so you may choose which behavior you like.

Option 2: Voltage Step Source

This is my favorite personally. A Voltage Step Source, which you can find in the toolbox under Voltage Signal Sources, is set to step from 0 to a specified voltage at a specified time:

Option 3: Skip Initial = Yes

My least favorite option is to set "Skip Initial = Yes" in the configuration of the Transient simulation. It tells the simulator to completely ignore any initial DC solution of the problem as it starts the time-domain simulation. For a simple circuit like this it works, but for circuits of any real complexity, it has unintended consequences as transistors etc are not starting from any equilibrium point. That's why it's my least favorite option, and I would recommend Option 1 or 2 instead. But you may try it here:

You may try all three simulations above and see they produce the same RC charging curve. Hope that helps!

that's great - thanks!