Master the Basics: Capacitors vs. inductors

Capacitors and inductors are both reactive components used in electronic circuits to store energy and oppose changes in electrical signals. While they share some similarities, they function in fundamentally different ways and are suited for different applications.

How They Store Energy

• Capacitors store energy in an electric field between two charged plates.

• Inductors store energy in a magnetic field surrounding a coiled wire.

Response to Voltage and Current Changes

• Capacitors resist sudden voltage changes by absorbing or releasing charge, maintaining a constant voltage across their terminals.

• Inductors resist sudden current changes by expanding or collapsing their magnetic field, attempting to maintain a steady current.

Frequency Response

• Capacitors have low impedance at high frequencies, allowing high-frequency signals to pass while blocking low-frequency and DC signals.

• Inductors have high impedance at high frequencies, blocking high-frequency signals while allowing low-frequency and DC signals to pass.

Low-Pass Filter Design Using Capacitors vs. Inductors

Both capacitors and inductors can be used to build low-pass filters, but they work differently:

1. RC Low-Pass Filter (Resistor + Capacitor)

   • The capacitor acts as a short circuit for high frequencies, draining them to ground.

   • More efficient for DC signals, as capacitors block DC energy from dissipating as heat.

2. LR Low-Pass Filter (Inductor + Resistor)

   • The inductor blocks high-frequency signals, preventing them from reaching the circuit.

   • More efficient for high-frequency signals, as it reduces wasted energy dissipation.

Choosing the Right Component

• Use capacitors when you need to filter out AC noise from a DC power source (e.g., decoupling capacitors in power supplies).

• Use inductors when you need to block high-frequency noise before it enters a circuit (e.g., inductive filters in RF applications).

• Some applications, like ignition coils in gasoline engines, require inductors because they can generate high-voltage surges—a function capacitors cannot perform.

Key Takeways

While capacitors and inductors sometimes perform similar filtering functions, they operate on different principles. Selecting the right component depends on the frequency characteristics, energy storage needs, and efficiency requirements of your circuit.