X7R, X5R, Y5V — why your 100nF is not 100nF

Two capacitors marked 100nF 50V can behave completely differently in circuit. The three-character dielectric code is what separates them, and it is the specification most often ignored.

Reading the code

• C0G / NP0. Essentially stable across temperature and voltage. Small values only, and the right choice for timing and filters.
• X7R. Plus or minus fifteen percent from -55C to +125C. The sensible default for decoupling.
• X5R. Same tolerance band but only to +85C. Fine in consumer, marginal in anything industrial.
• Y5V. Plus 22 / minus 82 percent. That is not a typo.

DC bias is the bigger trap

Capacitance falls as DC voltage is applied, and the effect worsens as the package shrinks. A 0402 10uF 6.3V part can lose well over half its value at 5V bias. The datasheet curve tells you; the marking does not.

Two practical rules: derate voltage generously — a 50V part on a 5V rail rather than a 6.3V one — and never substitute a dielectric to save a fraction of a penny without checking the bias curve.

Written for Circuit North Electronics — a demonstration store. See the AgenticEcom Suite.