# Electronic – Why pulsed DC passes through a capacitor

basiccapacitordctheory

OK, the current flowing through a capacitor equals C*dV/dt, I'm aware of that. What I don't understand is the physics of the process: why does a capacitor pass pulsed DC (0-10V for example) when charge carriers don't change their direction?

Even if I use the "water analogy" it doesn't make sense: the flow moves in one direction, so the "diaphragm" will not be able to move back and forth.

Zero volts doesn't mean zero current. Assume your circuit looks like this: simulate this circuit – Schematic created using CircuitLab

When the switch turns on (connects to 10V), current flows to the right and charges the capacitor up to 10V. Once that happens, the current stops*. When the switch turns off (connects to ground/0V), current flows to the left and discharges the capacitor. (The capacitor acts like a voltage supply.) The current stops when the capacitor reaches 0V.

Short version: Pulsed DC is actually AC.

*The charge and discharge are actually exponential decays, so mathematically, the current never really stops. It approaches zero asymptotically at a rate determined by the resistance and capacitance.