Capacitors Reference
Capacitor fundamentals — formulas, series/parallel behavior, and SMD code decoding.
Reference
Core formulas
- Q = C V
- Charge on plates
- I = C · dV/dt
- Current from changing voltage
- E = ½ C V²
- Energy stored (joules)
- X_C = 1 / (2π f C)
- Capacitive reactance (Ω)
- τ = R · C
- Time constant (seconds)
- V(t) = V₀ (1 − e^{-t/τ})
- Charging through R
- f_c = 1 / (2π R C)
- RC filter −3 dB corner
Combinations
- C parallel
- C_total = C1 + C2 + … (capacitance adds like resistor parallel)
- C series
- 1 / C_total = 1/C1 + 1/C2 + … (voltages add)
- Two in series
- C = (C1 · C2) / (C1 + C2)
SMD 3-digit code
| Code | Value |
|---|---|
| 100 | 10 × 10⁰ pF = 10 pF |
| 101 | 10 × 10¹ pF = 100 pF |
| 102 | 10 × 10² pF = 1 000 pF = 1 nF |
| 103 | 10 × 10³ pF = 10 nF |
| 104 | 10 × 10⁴ pF = 100 nF |
| 105 | 10 × 10⁵ pF = 1 µF |
| 106 | 10 × 10⁶ pF = 10 µF |
| 473 | 47 × 10³ pF = 47 nF |
| 224 | 22 × 10⁴ pF = 220 nF |
Prefix shorthand
- "n47" or "470n"
- 470 nF / 0.47 µF
- "4µ7" or "4u7"
- 4.7 µF
- "p47"
- 0.47 pF
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