Why the nameplate value is not the working value
A ceramic capacitor's rated capacitance is quoted under specific conditions. Once the part sits in a circuit, three separate mechanisms push the effective value away from it: temperature, applied DC voltage, and time. For the most common high-density dielectrics, the shift is not a rounding error. It is one of the leading causes of circuits that work on the bench and behave differently in production.
The two dielectric classes
Ceramic dielectrics divide into two families, and the difference is not one of quality but of purpose.
- Class 1 (C0G, also written NP0). Very stable. Its temperature behaviour is specified in parts per million per degree rather than in percent, it has no meaningful voltage dependence, and it does not age. It cannot achieve high capacitance in a small case, and it costs more per microfarad. Used where stability matters: oscillators, filters, timing, precision analogue.
- Class 2 (X5R, X7R, Y5V and relatives). High capacitance density at low cost, which is why almost all decoupling is done with these. The trade is that capacitance varies with temperature, with applied voltage, and with time.
Reading the letter code
In a Class 2 code, the first character gives the low temperature, the second the high temperature, and the third the permitted change in capacitance over that range. So:
- X7R — rated from -55 to +125 degrees C, with capacitance permitted to change by 15 percent.
- X5R — rated from -55 to +85 degrees C, also with 15 percent.
- Y5V — a much narrower temperature range and a far wider permitted change, which is why its effective capacitance is unpredictable.
Note what the number does not cover. The permitted change is defined over temperature only. It says nothing about voltage, and it says nothing about aging.
DC bias: the effect that surprises people
Apply DC voltage to a Class 2 ceramic and the effective capacitance falls. The higher the voltage relative to the part's rating, the more it falls, and the effect is worse in the physically smallest case sizes carrying the largest values. It is not unusual for a Class 2 part to lose a large fraction of its rated capacitance at its own rated voltage.
In practice this means a capacitor chosen for its nameplate value may deliver substantially less. The consequences range from a filter with a corner frequency well off its design value, to a regulator that is under-decoupled and unstable, to a timing constant that quietly differs from calculation.
The available mitigations are all trade-offs:
- Choose a higher voltage rating. A part rated well above the working voltage operates further from the steep part of the curve. It costs board area and money.
- Use a larger case size. All else equal, a bigger body loses proportionally less.
- Design with margin in the value. Accept that the fitted capacitance will be lower and specify accordingly.
- Where stability is essential, use Class 1. No voltage coefficient, at the cost of capacitance per unit volume.
The curve is published. When the value actually matters, the manufacturer's data for that specific case size and rating is what to check, not the headline number.
Aging, and why the board measures differently
Class 2 dielectrics lose capacitance as they age after manufacture, following a roughly logarithmic decay. The process is reversed by heating the part above its Curie temperature, which re-establishes the original value — and this happens during soldering. So a part can measure high before reflow, drop after assembly, and continue to drift slowly downward over the following months. Where a design is tight, that drift belongs in the budget.
Derating as practice, by family
Derating means operating a part below its rated stress, and how much is conventional depends on the family and on what happens when it fails.
- Ceramic. Derating on voltage mainly buys reliability and reduces the bias loss described above. There is usually no dramatic single-point failure mode, which is why ceramic derating is moderate.
- Tantalum. Derated substantially as a matter of routine, because the failure mode is severe: reverse bias or overvoltage can produce a short circuit rather than a benign open, and in some conditions a failure can be energetic. Tantalum is also polarity sensitive, and the stripe marks the positive terminal — the opposite convention to an aluminium electrolytic, as our guide to reading SMD markings on passives sets out.
- Aluminium electrolytic. Not operated above its rated voltage, and not reverse biased beyond a small permitted amount. Its service life is another matter entirely.
Electrolytic life, ESR and ripple current
An aluminium electrolytic wears out, and its life is dominated by temperature. The widely used rule of thumb is that life roughly doubles for every ten degrees of reduction in temperature, which is why placement away from heat sources is worth more than a slightly larger part. The heating comes from ripple current passing through the internal resistance, so the ripple current rating matters alongside capacitance and voltage. A part that is electrically adequate but placed next to a hot component can fail years before an inferior part placed in cool air.
Equivalent series resistance also matters for a different reason: at high frequency it dominates the impedance, so a large capacitance with poor ESR can perform worse than a smaller, better part for decoupling or output filtering.
Matching the family to the job
- Decoupling and high-frequency bypass — ceramic, close to the pin, with short return paths. Package inductance matters as much as capacitance at high frequency.
- Bulk energy storage and smoothing — aluminium electrolytic, sized on ripple current and life as well as capacitance.
- Compact mid-value, space constrained — tantalum, with proper derating and attention to polarity.
- High voltage, high stability, AC or pulse duty — film capacitors, which handle voltage and current stress that ceramics cannot.
Common selection errors
- Specifying a Class 2 part by its nameplate value with no account of DC bias
- Reading the temperature band from the code but ignoring the permitted change
- Assuming capacitance stays where it started, with no allowance for aging
- Using Y5V where the value has to be predictable
- Tantalum fitted with the polarity convention of an aluminium electrolytic
- Sizing an electrolytic on capacitance and voltage while ignoring ripple current and temperature
- Comparing parts on capacitance alone, with ESR and package inductance left out
Ordering capacitors
At YILUDA TECH every listed part is 100% original, new and unused, with no refurbished or counterfeit material, held in stock at our Shenzhen warehouse and shipped worldwide by DHL, FedEx or UPS. Incoming inspection and traceability documentation are available on request.
Send the full ordering code with quantity and destination and we will confirm availability, pricing and lead time. Dielectric, voltage rating and tolerance all sit in the suffix, so our guide to reading a part number is worth checking before specifying. Browse the full component inventory, or start from KEMET, Murata or Vishay.