A capacitor is a two-terminal passive component that stores energy in an electric field by accumulating charge on two conductive electrodes separated by a dielectric (commonly described as parallel plates, though real-world geometries vary). In circuits, capacitance is measured in farads (F); practical values in embedded designs typically range from a few picofarads (pF) to several thousand microfarads (µF), though applications such as precision RF or high-current power circuits can fall outside this range.
In practice
Capacitors appear throughout embedded hardware in several distinct roles. As bypass or decoupling capacitors, they are placed close to an IC's power pins to suppress high-frequency noise and supply instantaneous current during switching transients. A common practice is to use multiple capacitor values in parallel — for example, a 100 nF ceramic alongside a 10 µF bulk capacitor — because each value is most effective over a different frequency range. The blog post "Two Capacitors Are Better Than One" covers this technique in detail, and "The Other Kind of Bypass Capacitor" discusses less obvious placement strategies.
In power electronics and motor drive circuits, capacitors serve as DC link energy storage and ripple filters. Electrolytic capacitors are often chosen for their high capacitance density, but they have significant equivalent series resistance (ESR) and inductance (ESL) that limit their effectiveness at high frequencies. "Lost Secrets of the H-Bridge, Part IV: DC Link Decoupling and Why Electrolytic Capacitors Are Not Enough" and "Lost Secrets of the H-Bridge, Part II: Ripple Current in the DC Link Capacitor" go into detail on these limitations and how to handle ripple current ratings.
Capacitors are also core components in analog signal conditioning. RC low-pass filters on ADC inputs, timing circuits, oscillator tank circuits, and AC-coupling stages between amplifier stages all rely on capacitor behavior. The post "R1C1R2C2: The Two-Pole Passive RC Filter" covers cascaded RC filter design. The cutoff frequency of a simple RC stage is f = 1 / (2πRC), a formula that comes up constantly in embedded analog work.
Key pitfalls include DC bias derating of ceramic capacitors: an X5R or X7R MLCC rated at 10 µF / 10 V may lose 50–80% of its capacitance at its rated voltage on some parts, though the actual loss depends heavily on the specific capacitor, package size, and manufacturer — so the effective capacitance in circuit can be far lower than the label. Temperature coefficients also matter — C0G/NP0 ceramics are stable, while Y5V types can vary by over 80% across temperature. Electrolytic capacitors have polarity requirements and limited lifetimes tied to operating temperature and ripple current.
Discussed on EmbeddedRelated
Frequently asked
Why are multiple capacitor values placed in parallel on power rails instead of a single large capacitor?
Each capacitor type and value is effective over a limited frequency range. A large electrolytic handles bulk charge storage and low-frequency ripple, while a small ceramic (e.g., 100 nF) handles high-frequency switching transients. The ceramic's low ESL and ESR make it far more effective at high frequencies than the electrolytic, so combining values covers a broader spectrum. 'Two Capacitors Are Better Than One' covers this in detail.
What is ESR and why does it matter?
Equivalent Series Resistance (ESR) is resistive loss in series with the ideal capacitance. It causes voltage droop during fast current pulses and generates heat from ripple current. Electrolytic capacitors typically have higher ESR (tens to hundreds of milliohms) than ceramic MLCCs (milliohms or less). In DC link and decoupling applications, high ESR limits how quickly a capacitor can respond to a load transient.
What is DC bias derating and how does it affect ceramic capacitor selection?
Class II ceramic capacitors (X5R, X7R, X7S, Y5V) lose capacitance as the applied DC voltage approaches their rated voltage — a phenomenon called DC bias derating. A 10 µF / 10 V X5R MLCC might measure only 2–4 µF at 9 V DC. Always check the manufacturer's DC bias curve and select a capacitor whose rated voltage is comfortably higher than the operating voltage to retain the needed effective capacitance.
When should C0G/NP0 ceramics be chosen over X7R or X5R?
C0G (NP0) capacitors have a near-zero temperature coefficient and negligible DC bias derating, making them the right choice for timing circuits, oscillator components, precision RC filters, and anywhere the capacitance value must stay stable across voltage and temperature. Their tradeoff is lower capacitance density; they are typically available up to a few hundred nanofarads in small packages. X7R and X5R are preferred when higher capacitance is needed and some variation is acceptable.
What is ripple current rating and why is it important for capacitors in power applications?
Ripple current rating is the maximum RMS AC current a capacitor can handle continuously without exceeding its thermal limits. Excess ripple current heats the capacitor through its ESR, degrading the dielectric or electrolyte and shortening its life. In switch-mode power supplies and H-bridge DC links, ripple current can be substantial. Electrolytic capacitors have explicit ripple current ratings in their datasheets and must be derated based on operating temperature. 'Lost Secrets of the H-Bridge, Part II: Ripple Current in the DC Link Capacitor' covers how to calculate and manage this.
Differentiators vs similar concepts
Capacitors are sometimes loosely compared to batteries because both store energy, but a capacitor stores energy in an electric field with very low energy density and releases it almost instantaneously, while a battery stores energy electrochemically with much higher density and slower discharge. Capacitors are also distinct from
inductors: as a useful rule of thumb, a capacitor opposes changes in voltage and tends to pass AC signals while blocking DC, whereas an inductor opposes changes in current and tends to pass DC while impeding AC — though both components have frequency-dependent, non-ideal behavior in real circuits. These complementary behaviors are why LC circuits and RC filters appear together throughout analog and power design.
