Capacitors

Capacitors store energy in an electric field. At first glance they look like resistors — two-terminal passive components — but their behavior is fundamentally different. A capacitor blocks DC and passes AC, which makes them essential for filtering, coupling, decoupling, and timing circuits.

How They Work

A capacitor is two conductive plates separated by an insulating material (the dielectric). Apply voltage and charge builds up on the plates, creating an electric field across the dielectric. The formula for capacitance:

C = Q / V

C — capacitance in farads (F)
Q — charge in coulombs (C)
V — voltage in volts (V)

Real capacitors are measured in microfarads (µF), nanofarads (nF), or picofarads (pF) — a 1F capacitor is enormous. The key relationship for circuit behavior is the voltage-current relationship:

I = C × dV/dt

Current flows into a capacitor only when voltage is changing. Steady-state DC? No current flows. Rapidly changing voltage? High current. This is why capacitors block DC but pass AC.

Types

Ceramic — small, cheap, non-polarized. The default for decoupling (0.1µF ceramic on every IC power pin) and high-frequency bypass. Values up to a few µF. Don't use for precision timing — their capacitance varies with voltage.
Electrolytic (aluminum) — polarized, large capacitance, relatively low cost. 10µF to thousands of µF. Used in power supplies for bulk energy storage. Must be installed with correct polarity or they fail, sometimes violently.
Tantalum — polarized, smaller than electrolytic for the same capacitance, better high-frequency performance. More expensive, and also must be installed correctly — reverse voltage or overcurrent can cause them to catch fire. Used in portable electronics where size matters.
Film (polyester, polypropylene) — non-polarized, stable over temperature, low loss. Used in audio circuits, filters, and anywhere precision matters.
Supercapacitors — capacitance in the farads range, used as backup power sources or to handle pulse loads.

Charging and Discharging

Through a resistor, a capacitor charges and discharges exponentially:

Charging:   V(t) = V₀ × (1 - e^(-t/RC))
Discharging: V(t) = V₀ × e^(-t/RC)

The time constant τ = RC. After one time constant, the capacitor is at 63% of the final voltage when charging, or has dropped to 37% when discharging. After 5τ it's considered fully charged or discharged.

Example: R=10kΩ, C=100µF
τ = RC = 10,000 × 0.0001 = 1 second

Combining Capacitors

Opposite rules to resistors:

Series:   1/C_total = 1/C1 + 1/C2 + ...   (total is smaller)
Parallel: C_total = C1 + C2 + ...          (total is larger)

Common Uses

Decoupling — place a 100nF ceramic cap between VCC and GND, as close to each IC as possible. It supplies burst current during fast switching, preventing voltage dips that cause glitches. This is one of the most important good habits in PCB design.
Power supply filtering — large electrolytic caps after a rectifier smooth the ripple in DC power supplies.
AC coupling — a capacitor in series blocks the DC offset but passes the AC signal. Common in audio circuits to connect stages with different DC operating points.
Timing — RC circuits with a comparator or timer IC form the basis of oscillators and one-shot timers. The 555 timer IC is the classic example.

What to Watch For

Electrolytic capacitors dry out over time — this is the most common failure mode in old electronics. Swollen tops or leaked electrolyte are the obvious signs. Voltage rating: always use a cap rated for more than the maximum voltage it will see, with margin. A 16V rated cap on a 12V rail is cutting it close; 25V gives comfortable headroom.