What is the effect of using a resistor instead of an inductor in a power supply pi filter?

Using a resistor instead of an inductor in a power supply pi filter indeed leads to several negative effects, which illustrates why the correct choice is that it reduces efficiency and potentially increases voltage.

In a typical power supply pi filter, the inductor is used to smooth out the fluctuations in the rectified voltage. Inductors resist changes in current, thus they help to maintain a steady output voltage by filtering out ripple. When an inductor is replaced with a resistor, the ability to store energy and resist changes in current is lost.

As a result, the filter's efficiency decreases because resistors dissipate energy in the form of heat, rather than storing it. This energy loss leads to less effective voltage regulation, which can manifest as an increase in the output voltage ripple. Therefore, the overall efficiency of the power supply is compromised, and the performance is not optimal compared to using an inductor.

Thus, using a resistor negatively impacts both efficiency and voltage stability, confirming that selecting an inductor serves critical functions in this application that resistors cannot fulfill effectively.