I want to find how long a capacitor takes to discharge from 12 volts to 2 volts using Qucs. I can use a voltmeter to measure the voltage across it when it charges up, but I cannot turn the power source off during simulation.
Suggestions? Thanks
Electronic – Simulate capacitor discharge in Qucs
capacitorqucssimulation
Related Solutions
The biggest capacitor I saw was one at 300 Farads, similar to the one below: -
At full charge, the voltage is 2.7 volts. This means it can store an energy of: -
Energy = \$\dfrac{CV^2}{2}\$ - plug in 2.7 volts and 3000F and you get 10935 joules.
This is equivalent to consuming 10.9 watts for 1003 seconds. In reality this won't be achievable due to non-ideal power convertors. You might get half this time before things are pretty much spent and the robot won't move.
This is just an example calculation for you to plug in your own values.
The more general form of the first equation is
$$v_C(t) = v_C(0)\cdot e^{\frac{-t}{RC}}$$
This must be so since, for \$t=0\$
$$e^{\frac{-0}{RC}} = 1$$
So, assuming the capacitor has charged up to the source voltage before you disconnect the source at \$t=0\$, we have
$$v_C(0) = V_S$$
and then
$$t(v_C) = - RC \ln \frac{v_C}{V_S} = RC \ln \frac{V_S}{v_C} $$
Best Answer
One of the really cool lumped components in Qucs is a time controlled switch. You can specify the initial value as open or closed (see below), as well as a list of times when the switch should change state. Example of how it's used.
Actually, the time-controlled switch is really a time-controlled two-state resistor with default values [Ron=0 Ω; Roff=1e12 Ω].
You can choose any value for Ron and Roff. Combined with the list of times when the state changes, this a powerful and easy-to-use time-controlled element in simulations.