Consider a circuit that only has a constant current source and a capacitor. The voltage across a capacitor is proportional to the integral of the current I, times time. Since the current is constant it may be taken outside the integral. Based on this, the voltage across the capacitor will continue to increase as time goes by. Now let's say I turn the source on for X seconds and then turn it off. Would the time constant for the charging phase, and the time constant for the discharging phase be the same? I'm not sure if the input resistance of the source has any effect on the time constant. Thank you for any advice!
Different time constants for charging and discharging of a circuit with a constant current source and a capacitor
capacitorcapacitor chargingcurrent-sourcetime constant
Related Solutions
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
$$t(v_C) = - RC \ln \frac{v_C}{V_S} = RC \ln \frac{V_S}{v_C} $$
At one extreme, RC is exactly what it is for the example without L or a very small value of L. As L becomes more dominant, things change: -
So, the blue curve is a small value of L. The pink/magenta curve is the case when the value of L is sufficient to cause a little overshoot and the green condition is when L and C are more dominant than R. It's all about ratios.
This might make useful reading on the topic.
Best Answer
Here's the circuit I think you're describing:
simulate this circuit – Schematic created using CircuitLab
There's no time constant in this situation. The current source makes the capacitor voltage rise linearly. Time constants only appear when there's exponential decay.
When the current source is disconnected from the capacitor, there's no way for the capacitor to discharge. You can't just connect both ends to ground since the voltage across a capacitor is not allowed to change instantaneously in circuit theory.
Sources don't have input resistance since they have no inputs. They do have output resistance. If you include an output resistance with the current source, that will get you an exponential decay with a time constant, and the resistance will affect the time constant.
In general, it is possible to have different time constants for charging and discharging if you switch other components in or out of the circuit.