When is a smoothing capacitor primarily used in a circuit?

A smoothing capacitor is primarily used in a circuit after current has been passed through a rectifier, typically in the power supply section of electronic circuits. Its main function is to reduce the ripple voltage that results from converting alternating current (AC) to direct current (DC). This process creates a pulsating DC output, where the voltage level varies instead of being smooth.

The smoothing capacitor charges when the voltage from the rectified output is at a peak and discharges to maintain the output voltage when the rectified voltage drops. By doing so, it helps to "smooth out" the fluctuations in voltage, providing a more stable DC output that is essential for the proper operation of electronic devices which require a constant voltage supply.

While other options mention contexts around AC conversion and transformers, these do not directly relate to the primary function of smoothing capacitors within circuits, which is specifically to enhance the DC output quality after rectification.