Capacitor Fundamentals
The foundational relationship governing capacitor behaviour relates stored charge to applied voltage and capacitance.
C = Q ÷ V
Q = C × V
V = Q ÷ C
C— Capacitance measured in farads (F), microfarads (µF), nanofarads (nF), or picofarads (pF)Q— Electric charge stored in the capacitor, measured in coulombs (C)V— Voltage potential difference across the capacitor plates in volts (V)
Understanding Three-Digit Capacitor Codes
Capacitors rated below 100 µF typically use a compact three-digit marking system rather than printed values. The first two digits represent significant figures, while the third digit indicates the multiplier—the power of ten by which to multiply.
Example: A code 104 means 10 with 4 zeros = 100,000 pF = 100 nF.
The resulting value depends on your reference unit:
- Picofarads (pF): First two digits directly, third digit as power-of-ten multiplier
- Nanofarads (nF): Divide the pF result by 1,000
- Microfarads (µF): Divide the pF result by 1,000,000
Capacitors rated 100 µF or higher usually display their capacitance and voltage directly, such as "220 µF 25 V", eliminating the need for code interpretation. Always verify the unit marking on the component itself.
Tolerance Codes Explained
A tolerance letter indicates acceptable deviation from the printed or coded value, critical for precision-dependent applications. Common tolerance markings appear in this table:
| Letter | Tolerance Range |
|---|---|
| B | ±0.1 pF |
| C | ±0.25 pF |
| D | ±0.5 pF |
| F | ±1% |
| G | ±2% |
| J | ±5% |
| K | ±10% |
| M | ±20% |
| Z | +80%, −20% |
For example, a capacitor marked 104K has a nominal value of 100 nF with a ±10% tolerance, meaning the actual capacitance ranges from 90 nF to 110 nF. Higher-tolerance components (K, M) suit general-purpose filtering and timing circuits, while tighter tolerances (B, C, F) are essential for audio, RF, and precision timing applications.
Common Pitfalls and Practical Considerations
Avoid these frequent mistakes when working with capacitor codes and calculations.
- Confusing the multiplier digit — The third digit in a three-digit code represents the exponent, not a simple digit to append. Code 103 means 10 × 10³ = 10,000 pF, not 103 pF. Multiplying by the wrong power of ten is one of the most common field errors.
- Ignoring tolerance and voltage margins — Real-world capacitors drift from nominal values with temperature, age, and applied stress. Always calculate upper and lower limits using the tolerance code, and never operate a capacitor at its rated voltage continuously—design for at least 20% voltage headroom to extend lifespan and prevent premature failure.
- Mixing units during calculations — Charge-voltage-capacitance calculations require consistent units. If capacitance is in microfarads and voltage in volts, charge emerges in microcoulombs, not coulombs. Convert early or use a single unit system (e.g., all SI base units) to avoid orders-of-magnitude errors.
- Overlooking voltage rating mismatches — A capacitor's voltage rating is not negotiable—exceeding it causes dielectric breakdown and catastrophic failure, sometimes explosively. An 100 µF 25 V capacitor cannot safely replace a 100 µF 50 V unit in the same circuit, even if the capacitance matches perfectly.