Capacitors act like open circuit

Capacitors and Inductors
Their responses to changes in voltage

A sudden change in voltage can occur when a switch is closed or opened. It can also occur when a squarewave is present. (The following properties don't apply to gradual changes, such as occur when there is a sinewave present.)

• Right when the sudden change occurs, the inductor acts like an open circuit.
• Immediately after the sudden change is over, the voltage across the inductor starts to decrease.
• If the voltage across the inductor remains constant for a long time, the inductor will act like a short circuit.

• When a sudden voltage change across an inductor is such that the inductor's magnetic field will be reduced, the inductor will react by creating a voltage opposing the change. Sometimes very large voltages can be produced by this reaction, and other components can be damaged.
  • In DC circuits, a diode is often connected in parallel with the inductor to prevent voltage spikes.
  • In AC circuits, a capacitor is often connected in parallel with the inductor to prevent voltage spikes.
  • Right when the sudden change occurs, the capacitor acts like a short circuit (a piece of wire).
  • Immediately after the sudden change is over, the capacitor starts charging up.
  • If the voltage across the capacitor remains constant for a long time, the capacitor will act like an open circuit.
  • Large capacitors can absorb large amounts of current when there is a sudden change in voltage. In some circumstances, these large currents can damage other components (especially semiconductors).
  • Capacitors can supply large amounts of current when there is a sudden change in voltage. Often this is a desireable feature. However, it can also damage other components.