Capacitors Complete Guide: Types, Values, Codes, ESR, and How to Choose
What Is a Capacitor?
A capacitor is a passive electronic component that stores electrical energy in an electric field. It consists of two conductive plates separated by an insulating material called a dielectric. Capacitors block direct current (DC) while allowing alternating current (AC) to pass, making them essential for filtering, coupling, decoupling, timing, energy storage, and power conditioning. The fundamental relationship is C = Q/V, where capacitance (C) equals stored charge (Q) divided by voltage (V). Capacitance is measured in Farads (F), with most practical values ranging from picofarads (pF) to millifarads (mF).
Capacitor Fundamentals
Key Equations
FormulaSolves ForExample C = Q ÷ VCapacitance from charge and voltage1 coulomb at 10V = 0.1F Q = C × VStored charge100μF at 12V stores 1.2mC E = ½ × C × V²Stored energy in joules1000μF at 50V = 1.25 joules XC = 1 ÷ (2πfC)Capacitive reactance (AC resistance)10μF at 60Hz: XC = 1÷(2π×60×0.00001) = 265Ω τ = R × CRC time constant10kΩ + 100μF: τ = 1 second I = C × (dV/dt)Current through capacitor1μF with 1V/μs change: I = 1ACapacitors in Series
1/Ctotal = 1/C1 + 1/C2 + 1/C3 + ...Two equal capacitors in series: Ctotal = C/2. Voltage rating adds: two 100μF/25V caps in series = 50μF/50V.
Capacitors in Parallel
Ctotal = C1 + C2 + C3 + ...Voltage rating stays the same (use the lowest rated cap's voltage).
Reading Capacitor Values
Electrolytic Capacitors (Large Cylindrical)
Values printed directly on the body. Example: "100μF 25V" = 100 microfarads, 25 volts maximum. The negative lead is marked with a stripe and minus signs. The positive lead is usually longer.
Ceramic Disc Capacitors (3-Digit Code)
CodeValue 10210 × 100 = 1000pF = 1nF 10310 × 1000 = 10,000pF = 10nF = 0.01μF 10410 × 10,000 = 100,000pF = 100nF = 0.1μF 10510 × 100,000 = 1,000,000pF = 1μF 22122 × 10 = 220pF 22222 × 100 = 2.2nF 47247 × 100 = 4.7nF 47347 × 1000 = 47nF 47447 × 10,000 = 470nFUnits: 3-digit codes are always in picofarads (pF). Letters after the code indicate tolerance: J=±5%, K=±10%, M=±20%. Voltage rating is often printed separately.
MLCC SMD Capacitors (Ceramic Chip)
CodeValue 1051μF 10610μF 2252.2μF 47647μF 107100μF 4754.7μFSame 3-digit system but values tend to be larger. Some use explicit μF markings.
Tantalum Capacitors
Values printed directly: "106" = 10μF, "475" = 4.7μF. Voltage indicated by color or explicit marking. Stripe indicates positive terminal (opposite of electrolytics).
Film Capacitors
Values often in μF with explicit markings: "0.1μF 100V" or "100nJ100" (100nF, ±5%, 100V). The J indicates ±5% tolerance.
Capacitor Types Compared
1. Ceramic Capacitors (MLCC)
PropertyValue Capacitance range0.5pF to 100μF (typical: up to 10μF) Voltage rating6.3V to 500V+ Polarized?No — non-polarized ESRVery low (2-20mΩ typical) Price$0.01-0.50Best for: High-frequency decoupling, filtering, timing, coupling. Use 100nF (0.1μF) ceramic caps next to every IC power pin — the most common capacitor application in digital circuits.
Dielectric types: C0G/NP0 (best stability, low capacitance), X7R (good general purpose), Y5V (high capacitance, poor stability — avoid for precision).
2. Aluminum Electrolytic Capacitors
PropertyValue Capacitance range0.1μF to 1,000,000μF (1F) Voltage rating6.3V to 450V+ Polarized?Yes — polarity must be observed ESRModerate to high (0.01Ω to 10Ω) Price$0.10-5.00Best for: Power supply filtering, bulk energy storage, audio coupling, motor starting. Cheap high capacitance per volume.
Lifespan concern: Electrolytics dry out over time. Rated for 1,000-10,000 hours at rated temperature. Rule of thumb: every 10°C below rated temperature doubles life. A 105°C cap running at 45°C can last 15+ years. A 85°C cap running at 65°C might only last 3-5 years.
3. Tantalum Capacitors
PropertyValue Capacitance range0.1μF to 1000μF (typical: up to 220μF) Voltage rating2.5V to 50V Polarized?Yes — observe polarity ESRLow (0.05Ω to 2Ω) Price$0.30-5.00Best for: Compact designs needing stable capacitance, military/aerospace (high reliability grades available). Excellent volumetric efficiency (more capacitance per cubic mm than aluminum electrolytic).
Warning: Tantalums can fail short-circuit violently if reverse voltage is applied or if voltage surge exceeds rating. Use a current-limiting resistor in series during power-up. Never exceed rated voltage — derate to 50% for reliability.
4. Polymer Aluminum Capacitors
PropertyValue Capacitance range2.2μF to 2700μF Voltage rating2V to 125V Polarized?Yes ESRVery low (3-50mΩ) Price$0.50-5.00Best for: Switching power supplies, high-ripple-current applications, VRMs on motherboards. Replaces tantalum in many applications with better safety characteristics. Much lower ESR than traditional electrolytics.
5. Film Capacitors (Polyester/Mylar, Polypropylene, Polystyrene)
PropertyValue Capacitance range100pF to 100μF Voltage rating50V to 2000V+ Polarized?No — non-polarized ESRVery low Price$0.20-10.00Best for: Audio circuits (excellent sound quality), high-voltage applications, motor run capacitors, precision timing, snubber circuits. Self-healing dielectric (small shorts burn clear rather than failing catastrophically).
6. Mica Capacitors
PropertyValue Capacitance range0.5pF to 10,000pF (10nF) Voltage rating100V to 1000V+ Polarized?NoBest for: RF circuits, high-frequency applications requiring extreme stability. Excellent temperature coefficient. Expensive and relatively large.
7. Supercapacitors (EDLC / Ultracapacitors)
PropertyValue Capacitance range0.1F to 3000F+ Voltage rating2.3V to 3.0V per cell (series for higher voltage) Polarized?Yes ESRLow to moderate (10mΩ to 100Ω depending on size) Price$1.00-50.00+Best for: Energy storage, backup power, regenerative braking, burst power. NOT for filtering — high leakage current. Series connection requires active balancing circuits (unlike regular capacitors).
Capacitor Comparison Summary
TypeCap RangeVoltageESRPolarized?Best Use Ceramic (C0G/NP0)0.5pF-10nFHighUltra lowNoRF, precision timing, oscillators Ceramic (X7R)100pF-10μFHighVery lowNoDecoupling, general purpose, coupling Ceramic (Y5V)1nF-100μFMediumLowNoBulk decoupling (price sensitive) Aluminum Electrolytic0.1μF-1FMediumModerateYesPower filtering, bulk storage, audio Polymer Aluminum2.2μF-2700μFLow-MedVery lowYesSwitching supplies, VRMs, high ripple Tantalum0.1μF-1mFLowLowYesCompact, stable, high reliability Film (polypropylene)100pF-100μFHighVery lowNoAudio, high voltage, motor run Mica0.5pF-10nFHighUltra lowNoRF, precision, high frequency Supercapacitor0.1F-3000F2.5-3VLow-MedYesEnergy storage, backup powerCommon Capacitor Applications
1. Power Supply Decoupling / Bypass
Place a 100nF (0.1μF) ceramic capacitor as close as physically possible to each IC power pin. This provides a local reservoir of charge for switching transients:
- Digital ICs: 100nF ceramic per power pin, plus 10μF-100μF electrolytic per rail for bulk
- Microcontrollers: 100nF on each VCC pin, plus 10μF bulk
- Op-amps: 100nF on power pins, sometimes a 10μF for low-frequency stability
Placement rule: The capacitor lead/trace should be shorter than 10mm to the IC pin for effective high-frequency decoupling. Poor placement makes the capacitor useless above a few MHz.
2. Power Supply Filtering
After a rectifier, large electrolytic capacitors smooth the pulsating DC into steady DC:
C = Iload ÷ (2 × f × Vripple)Example: 1A load, 60Hz mains, 1V ripple allowed: C = 1÷(2×60×1) = 8,333μF → use 10,000μF
3. Timing Circuits (RC Oscillators, 555 Timer)
555 astable frequency: f = 1.44 ÷ ((R1 + 2×R2) × C)Example: R1=1kΩ, R2=10kΩ, C=10μF: f = 1.44÷((1000+20000)×0.00001) = 6.86 Hz
4. Audio Coupling
A capacitor blocks DC while passing audio AC signals:
C = 1 ÷ (2π × f × R) where f is lowest frequency and R is input impedanceExample: Coupling to a 10kΩ input, lowest frequency 20Hz: C = 1÷(2π×20×10000) = 0.8μF → use 1μF or 2.2μF
Cap choice: Film capacitors (polypropylene, polyester) are preferred for audio coupling due to low distortion. Electrolytics are acceptable for budget designs but introduce slight distortion.
5. Audio Filters (Crossover Networks)
Capacitors in series with speakers block low frequencies (high-pass filter). Capacitors in parallel shunt high frequencies to ground (low-pass filter when combined with inductors).
Example: 8Ω tweeter with 3kHz crossover: C = 1÷(2π×3000×8) = 6.6μF → use 6.8μF film capacitor
6. Motor Starting / Run Capacitors
AC induction motors need a phase shift to start:
- Start capacitors: 50-400μF, 125-330V AC. Electrolytic type. Only engaged during startup (centrifugal switch disconnects).
- Run capacitors: 5-100μF, 370-440V AC. Film or oil-filled. Permanently in circuit.
Safety: Motor capacitors store lethal voltages. Always discharge with a 10kΩ 5W resistor before handling.
7. Snubber Circuits
A resistor-capacitor network absorbs voltage spikes from inductive loads (relays, motors, transformers):
- Typical values: 100nF film cap + 10-100Ω resistor
- Place directly across the inductive load or switching element
- Prevents arcing, EMI, and voltage transients
8. Switching Regulator Input/Output
Switching regulators (buck, boost, buck-boost) need specific capacitors:
- Input capacitor: Handles ripple current from switching. Use low-ESR ceramic or polymer. Place right at regulator input pins.
- Output capacitor: Determines output ripple and transient response. Low ESR ceramic for modern regulators, sometimes electrolytic for older designs.
- Ceramic output caution: Some regulators become unstable with only ceramic output caps. Check the datasheet — it may specify minimum ESR or require a small series resistor.
ESR: Equivalent Series Resistance
ESR is the internal resistance of a capacitor. It's the single most important parameter for power applications:
Why ESR Matters
- Heat: Power dissipated in ESR = Iripple² × ESR. High ESR + high ripple current = hot capacitor = short lifespan.
- Voltage ripple: Vripple = Iripple × ESR. Lower ESR = cleaner DC output.
- Transient response: Lower ESR means the capacitor can supply current faster during load steps.
ESR by Capacitor Type
TypeTypical ESR Ceramic X7R2-20mΩ Polymer aluminum3-50mΩ Tantalum50mΩ-2Ω Aluminum electrolytic (general)0.1Ω-10Ω Aluminum electrolytic (low ESR)10-500mΩ Film5-50mΩMeasuring ESR
- Use an ESR meter ($15-50 for basic models) — tests in-circuit without desoldering
- Capacitance meter with ESR function
- General rule: ESR should be under 1Ω for small electrolytics, under 0.1Ω for power supply caps
- High ESR is the #1 sign of an aging electrolytic capacitor
Capacitor Markings and Packages
SMD Ceramic Capacitor Packages
PackageDimensionsMax Cap (X7R)Common Use 02010.6mm × 0.3mm10nF/16VMobile devices, ultra-compact 04021.0mm × 0.5mm100nF/16VHigh-density designs 06031.6mm × 0.8mm1μF/16V or 100nF/50VDefault choice for most SMD decoupling 08052.0mm × 1.25mm4.7μF/16V or 1μF/50VHigher value, still easy to solder 12063.2mm × 1.6mm10μF/16V or 4.7μF/50VPower decoupling, larger values 12103.2mm × 2.5mm22μF/16VPower input filtering 18124.5mm × 3.2mm100μF/6.3VBulk decouplingElectrolytic Capacitor Markings
Example marking: "1000μF 25V 105°C"
- 1000μF = capacitance
- 25V = maximum working voltage
- 105°C = maximum operating temperature (lifespan rated at this temp)
- Negative stripe with (-) marks the negative lead
Tantalum Capacitor Case Sizes
Case CodeDimensions (L×W×H) A (3216)3.2mm × 1.6mm × 1.6mm B (3528)3.5mm × 2.8mm × 1.9mm C (6032)6.0mm × 3.2mm × 2.5mm D (7343)7.3mm × 4.3mm × 2.8mm E (7343-43)7.3mm × 4.3mm × 4.1mmCapacitor Safety
Working Voltage Derating
Never operate a capacitor at its maximum rated voltage. Derating improves reliability:
- General rule: Use capacitor rated for at least 1.5× the expected voltage
- Aluminum electrolytic: Derate to 80% of rated voltage for long life
- Tantalum: Derate to 50% of rated voltage (mandatory for safety)
- Ceramic: Derate to 70% of rated voltage (capacitance drops under DC bias)
Discharging Capacitors
Capacitors store energy and can deliver dangerous shocks:
- Power supply capacitors: Can hold hundreds of volts for minutes after unplugging
- Discharge method: Place a 10kΩ 5W resistor across the terminals for 10 seconds
- NEVER short with screwdriver: Creates massive current spike, molten metal, fire hazard
- Flash capacitor (camera): 300V+ even in a disposable camera — can be lethal
- Supercapacitors: Can deliver hundreds of amps briefly — risk of burns and fires
Capacitor Failure Modes
TypeCommon FailureCause ElectrolyticDried out (high ESR, low capacitance)Age, heat, overvoltage ElectrolyticVented/burst (top domes or explodes)Overvoltage, reverse voltage, extreme ripple TantalumShort circuit (can catch fire)Overvoltage, reverse voltage, surge current CeramicMechanical crackBoard flex, impact, thermal shock CeramicMicrophonic (acts as microphone)Class 2 dielectrics (X7R, Y5V) — normal behavior FilmSelf-healing puncturesOvervoltage transients (usually recovers)How to Test Capacitors
With a Multimeter (Basic)
- Set to capacitance mode (many multimeters have this)
- Discharge the capacitor first
- Connect leads (observe polarity for electrolytics)
- Reading should be within tolerance of marked value
- For electrolytics: also check with resistance mode — should show charging (resistance rises from low to high)
With an ESR Meter
- Can test in-circuit (desoldering not required if parallel resistance is high)
- Compare reading to known good values or manufacturer specs
- ESR over 2× the rated value indicates end of life
Visual Inspection
- Bulging top = vented electrolytic, definitely failed
- Leaked electrolyte (brown crust) = failed
- Blown safety vent = catastrophic overvoltage
- Cracked ceramic body = mechanical damage, suspect
- Scorch marks = short circuit occurred
Where to Buy
- Assortment kits: 400-1000 piece kits with common values ($10-20 on Amazon)
- Digi-Key / Mouser: Massive selection, filter by ESR, ripple current, size
- JLCPCB Basic Parts: Extremely cheap SMD caps when assembling PCBs (ceramics from $0.001)
- Recycling: Old power supplies and motherboards are excellent sources of high-quality electrolytics and ceramics