Capacitors#

Two plates separated by a dielectric. Simple in concept, remarkably complicated in practice. The capacitor chosen — not just the capacitance value, but the type, voltage rating, and dielectric — determines whether the circuit works, how long it lasts, and what surprises it has in store.

The Ideal vs. Reality Gap#

An ideal capacitor has a fixed capacitance, zero ESR, zero leakage, infinite voltage breakdown, and no frequency dependence.

Real capacitors break every one of these assumptions:

  • Capacitance varies with voltage (DC bias effect) — Ceramic capacitors, especially high-K types (X5R, X7R, Y5V), lose capacitance when DC voltage is applied. A “10 µF” X5R cap rated at 10 V might have only 4-5 µF at 10 V. This is the single most surprising capacitor behavior for those coming from textbooks
  • Capacitance varies with temperature — Y5V can lose 80% of its capacitance at temperature extremes. X7R stays within ±15% over its rated range. C0G/NP0 is essentially flat
  • ESR (Equivalent Series Resistance) — Real capacitors have internal resistance. ESR determines ripple current capability and self-heating. Electrolytic caps have the highest ESR; MLCC ceramics have the lowest
  • ESL (Equivalent Series Inductance) — Lead and body inductance. At high frequencies, a capacitor becomes inductive. The self-resonant frequency (SRF) is where capacitance and inductance cancel, and above it the “capacitor” behaves as an inductor
  • Leakage current — DC current through the dielectric. Electrolytics have the highest leakage. Important for hold-up circuits, sample-and-hold, and anything with long time constants
  • Aging — Some ceramic dielectrics lose capacitance over time (logarithmic aging). Class II ceramics (X5R, X7R) can lose 2-5% per decade of time after last heating above Curie temperature

Dielectric Types and When They Matter#

C0G / NP0 (Class I Ceramic)#

Stable, precise, no DC bias effect, no aging. Available in small values (pF to low nF). The go-to for timing circuits, filters, and anything where the capacitance value must not change.

X5R / X7R (Class II Ceramic)#

High capacitance density. Available up to 100 µF in small packages. But capacitance varies with voltage, temperature, and time. Good for bulk decoupling and energy storage where exact value isn’t critical.

Electrolytic (Aluminum)#

Large capacitance, polarized, high ESR, limited life. Standard for bulk energy storage on power rails. Lifetime depends on temperature and ripple current — the electrolyte dries out over time.

Tantalum#

Smaller than aluminum electrolytic for the same capacitance. Lower ESR. But failure mode is short circuit (potentially violent). Sensitive to voltage surges and reverse polarity. Derate significantly — never run at rated voltage.

Film (Polyester, Polypropylene, etc.)#

Stable, low loss, self-healing (can survive minor dielectric breakdowns). Larger than ceramics but better behaved. Common in audio, AC mains filtering, and snubber circuits.

Practical Patterns#

Decoupling#

Place a small capacitor close to each IC power pin. The capacitor supplies instantaneous current demands faster than the power supply can respond. Typical values: 100 nF ceramic per pin, plus a bulk electrolytic per supply rail.

The capacitor’s ESR and ESL determine how well it decouples at different frequencies. Multiple capacitor values in parallel cover a wider frequency range.

Filtering#

Capacitors with resistors form RC low-pass and high-pass filters. The cutoff frequency is f = 1 / (2πRC). But above the cap’s SRF, the filter stops working as designed because the capacitor becomes inductive.

Energy Storage and Holdup#

Capacitors provide short-term energy during supply interruptions. Energy = ½CV². Size the cap for the energy needed, and ensure the ESR can handle the discharge current without excessive voltage drop.

Tips#

  • Always check the DC bias curve for ceramic caps — the actual capacitance at operating voltage may be much lower than the rated value
  • Use multiple smaller caps in parallel rather than one large cap for lower ESL and better high-frequency performance
  • Place decoupling caps as close as possible to the IC power pins

Caveats#

  • 10 µF is not always 10 µF — Check the DC bias curve for ceramic caps. A 10 µF 6.3 V cap at 5 V might only give 5-6 µF. Oversize the voltage rating or the capacitance to compensate
  • Polarity matters for electrolytics and tantalums — Reverse voltage destroys them. Tantalums can fail violently (short, smoke, fire)
  • Ceramic caps are microphonic — Mechanical vibration causes voltage on the cap due to the piezoelectric effect of the ceramic dielectric. This matters in audio and precision analog circuits. C0G is less microphonic than X7R
  • ESR is not always bad — In voltage regulator output caps, some ESR helps stability. An LDO designed for electrolytic output caps (which have ESR) might oscillate with ultra-low-ESR ceramic caps. Check the datasheet
  • Capacitors in series halve the capacitance but double the voltage rating — Useful for AC mains work. But the voltage division is only equal if the capacitors are identical. Mismatched caps get unequal voltage sharing, and one might see more than its rating
  • Temperature and aging effects are almost always larger than expected — a design that works on the bench at 25°C may fail in the field at 60°C, and the failure will trace back to a component whose parameter shifted outside the assumed range

In Practice#

  • A swollen or bulging electrolytic cap is failing — replace it
  • An electrolytic that measures correct capacitance but high ESR is at end of life
  • Oscillation in an LDO circuit often traces to wrong capacitor type on the output — check what the datasheet specifies
  • Decoupling-related noise issues show up as high-frequency glitches or ringing on power rails near ICs
  • A capacitor that gets hot under AC ripple current has ESR losses — it may be undersized for the application
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