Practical Considerations for Selecting and Using Capacitors

Capacitors, like every electronic component, have operational limits that must be observed to ensure circuit reliability and performance.

Capacitor Working Voltage

Working voltage: A capacitor is essentially two conductors separated by a dielectric. Exceeding the dielectric’s breakdown voltage causes internal short‑circuiting and irreversible damage. Always choose a capacitor with a working voltage at least 20 % higher than the maximum voltage it will encounter in the circuit.

Capacitor Polarity

Polarity: Certain capacitors, notably electrolytic types, are fabricated with a thin insulating layer deposited on one electrode during manufacture. This construction makes them sensitive to voltage polarity; reversing the polarity destroys the dielectric and shorts the device. Electrolytic capacitors are clearly marked with polarity arrows or a plus sign. Non‑polarized types—ceramic, film, mylar, air—lack such markings and can be connected in either direction.

Capacitor Equivalent Circuit

Equivalent circuit: Real capacitors exhibit a finite series resistance (ESR) and a parallel leakage resistance (R_L). These parasitic elements affect ripple handling, heat dissipation, and frequency response. Modern manufacturing techniques allow ESR values in the sub‑milliohm range and leakage resistances exceeding 10¹² Ω, making capacitors virtually ideal for most applications.

Capacitor Physical Size

Minimizing component footprint is a key goal in modern electronics. However, size is constrained by the trade‑off between working voltage and capacitance. To increase voltage rating, the dielectric thickness must grow, which reduces capacitance unless the plate area is expanded—adding to the size. Consequently, two capacitors of identical dimensions can have vastly different capacitance and voltage ratings depending on dielectric choice.

For example, a 2 µF capacitor rated for 2000 V occupies a relatively large package. If the dielectric were thinned, its capacitance could rise by up to a hundred‑fold, but the voltage rating would drop sharply. In contrast, a similarly sized electrolytic capacitor can provide 20 000 µF at only 35 V continuous (45 V intermittent) due to its ultrathin dielectric.

Below are representative samples of various capacitor types, all smaller than the examples above:

Electrolytic and tantalum capacitors are polarized and carry clear polarity markings. Electrolytic units show a negative lead arrow; some polarized capacitors indicate the positive terminal. Non‑polarized types—ceramic, film, mylar, air—lack polarity indicators.

Capacitors are ubiquitous on printed circuit boards. On the board below, every component labeled “C” is a capacitor:

The board features standard electrolytic capacitors (C₃₀, C₃₆) and tantalum capacitors (C₁₄, C₁₉, C₂₄, C₂₂), which offer high capacitance in a compact form.

Even smaller surface‑mount capacitors appear on the board below, illustrating the shift toward dense, surface‑mount design:

All surface‑mount devices, including these capacitors and the resistors, are selected to conserve board real estate.

RELATED WORKSHEETS:

• Capacitors Worksheet