Electronic – Why pulsed DC passes through a capacitor


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.

Best Answer

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.