A capacitor discharges when a conductive or resistive path bridges its terminals, allowing stored electrons to flow from the negative plate to the positive plate until voltage equilibrium is reached. On paper, this is a simple exponential decay governed by the RC time constant ($\tau = R \times C$). On the bench, however, discharge behavior is complicated by equivalent series resistance (ESR), dielectric absorption (soakage), and leakage current. Understanding these physical realities is the difference between a safe, functional power supply and a shocking hazard or a failed timing circuit.
The Physics of Discharge: RC Time Constants and Dielectric Absorption
The rate at which a capacitor discharges through a resistor is defined by the equation $V(t) = V_0 \times e^{-t/RC}$. In practical workshop terms, we use the 5-tau rule: after five time constants ($5 \times R \times C$), the capacitor is considered 99.3% discharged. For a 1000µF capacitor discharging through a 100Ω bleeder resistor, $\tau$ is 0.1 seconds, meaning it reaches a safe voltage in roughly half a second.
However, if you short a large electrolytic capacitor with a screwdriver, watch the spark, and measure 0V, you have not fully discharged it. If you leave it open-circuited for an hour and measure it again, you will often read a recovered voltage of 10V to 30V. This is dielectric absorption. The dielectric material's molecular dipoles do not align and relax instantaneously. When you short the plates, the fast dipoles discharge, but the slow dipoles remain polarized. Once the short is removed, those slow dipoles relax and push charge back onto the plates. According to Vishay's application notes on dielectric absorption, this effect is severe in Class 2 ceramics (X7R, Y5V) and electrolytics, but negligible in polypropylene film and Class 1 ceramics (C0G/NP0).
Capacitor Type Comparison: Construction, Tolerance, and Discharge Traits
Selecting a capacitor requires matching its discharge and leakage profile to the circuit's demands. A timing circuit needs low dielectric absorption; a bulk filter needs high capacitance and can tolerate higher leakage.
| Type | Construction / Dielectric | Tolerance | Tempco / Stability | Discharge & Leakage Trait | Typical Use |
|---|---|---|---|---|---|
| Aluminum Electrolytic | Etched aluminum foil, liquid/polymer electrolyte | ±20% | Poor (varies widely with temp) | High leakage, high dielectric absorption | Bulk power filtering, hold-up |
| Tantalum | Porous tantalum anode, manganese dioxide cathode | ±10% to ±20% | Moderate | Moderate leakage, risk of thermal runaway on discharge | Compact DC decoupling |
| MLCC (Class 2 X7R) | Barium titanate ceramic layers | ±10% to ±20% | Voltage & temp dependent | High dielectric absorption (memory effect) | High-freq bypass, general decoupling |
| Film (Polypropylene) | Metallized polypropylene film wound/stacked | ±1% to ±5% | Excellent (stable) | Negligible absorption, very low leakage | Audio coupling, snubbers, timing |
| MLCC (Class 1 C0G) | Paraelectric ceramic (titanates) | ±1% to ±5% | Near zero (±30ppm/°C) | Zero measurable dielectric absorption | RF filters, precision integrators |
Decoding Capacitor Markings and Codes
Reading through-hole and SMD capacitor markings is a mandatory bench skill. Misreading a code by one order of magnitude will destroy your timing calculations or cause a catastrophic overvoltage failure.
Through-Hole 3-Digit Codes (Ceramics and Film)
The standard EIA 3-digit code uses two significant figures and a multiplier in picofarads (pF).
- 104K: '10' are the significant digits. '4' means multiply by $10^4$ (10,000). Result: 100,000 pF = 100 nF = 0.1 µF. The 'K' denotes ±10% tolerance.
- 473M: 47 × $10^3$ = 47,000 pF = 47 nF. 'M' denotes ±20% tolerance.
- 221J: 22 × $10^1$ = 220 pF. 'J' denotes ±5% tolerance.
SMD Electrolytic and Tantalum Codes
SMD cylindrical electrolytics often print the value directly (e.g., 100 16V), but compact SMD chip tantalums and ceramics use alphanumeric codes. A common format is a letter followed by two digits. For example, 'A7' on a tantalum chip might map to a manufacturer-specific chart, but standard EIA SMD codes often use a letter for voltage and two digits for capacitance. Always cross-reference the manufacturer's datasheet for SMD codes, as a '106' marking universally means 10 × $10^6$ pF (10 µF), but the voltage band color or letter varies by brand.
Failure Modes and Visual Symptoms
Capacitors rarely fail without leaving forensic evidence. Recognizing these visual symptoms saves hours of oscilloscope debugging.
- Aluminum Electrolytic Venting: Look at the scored cross on the top of the can. If it is popped open, domed, or bulging, the internal electrolyte boiled due to overvoltage, reverse polarity, or excessive ripple current. You will often see a brown, crusty residue on the PCB beneath it. Fix: Replace with a low-ESR, 105°C rated part and check the ripple current of the driving circuit.
- Tantalum Thermal Runaway: Tantalum capacitors fail short-circuit. If subjected to a voltage spike or high discharge current without current limiting, the internal manganese dioxide acts as an oxidizer, resulting in literal fire, smoke, and a charred black crater on the PCB. Fix: Derate tantalum voltage by 50% (use a 10V part on a 5V rail) or switch to polymer tantalums.
- MLCC Flex Cracking: Multilayer ceramic capacitors are brittle. If the PCB flexes during depaneling or connector insertion, the MLCC cracks internally. Visually, the part looks perfect, but it fails short or loses capacitance. Fix: Use X7R parts with soft-termination (flexible epoxy in the end caps) near board edges and mounting holes.
Safe Substitution and Bleed Resistor Sizing
When the exact capacitor is out of stock, you must substitute safely. The golden rules of substitution are:
- Voltage Rating: Must be equal to or greater than the original. Never substitute a 25V cap for a 50V cap, even if the rail is only 12V; turn-on transients will kill it.
- Capacitance: For bulk DC filtering, you can safely go 20% to 50% higher. Do not go lower. For timing, oscillator, or filter circuits, you must match the exact value and tolerance.
- ESR and Ripple: In switching mode power supplies (SMPS), substituting a standard electrolytic for a low-ESR polymer or high-ripple-rated electrolytic will cause the substitute to overheat and vent within hours.
To size a bleed resistor for a 400V, 100µF capacitor to discharge to a safe 50V in 5 seconds, use the rearranged decay formula: $R = \frac{-t}{C \times \ln(V_{target}/V_{initial})}$.
$R = \frac{-5}{0.0001 \times \ln(50/400)} = \frac{-5}{0.0001 \times -2.079} \approx 24,000\Omega$ (24kΩ).
Calculate peak power: $P = \frac{V^2}{R} = \frac{400^2}{24000} = 6.6W$. Use a 10W wirewound resistor, or better yet, string three 8.2kΩ 5W resistors in series to distribute the heat and provide open-circuit redundancy.
Decision Path: Picking the Right Capacitor for Your Discharge Profile
Use this decision matrix to terminate your selection process with a concrete part type based on your circuit's discharge and hold-up requirements.
| Circuit Requirement | Primary Constraint | Selected Type | Concrete Pick / Series |
|---|---|---|---|
| High-voltage SMPS bulk filtering (120V/240V AC input) | High capacitance, 105°C survival, high ripple tolerance | Aluminum Electrolytic (Snap-in) | Rubycon MXG Series (e.g., 450MXG100MEFCSN22X30) |
| Precision analog integrator or sample-and-hold | Zero dielectric absorption, no voltage memory | Class 1 Ceramic (C0G/NP0) or Polystyrene | Kemet C0G/NP0 SMD or **Cornell Dubilier PS series** |
| Audio signal coupling (blocking DC, passing AC) | Low distortion, no piezoelectric microphonics | Metallized Polypropylene Film | WIMA MKP10 or **Nichicon MUSE UES** (if polarized needed) |
| High-frequency switching node snubber | Extremely low ESL, high dV/dt survival | MMKP (Double-sided metallized film) or MLCC | Vishay 225P** MMKP or **Murata KRM** series MLCC |
| Space-constrained 3.3V rail decoupling | Maximum µF per mm², low profile | MLCC (X5R/X7R) | Samsung Electro-Mechanics CL** series (e.g., 0402 10µF) |
Default Recommendation: If you are building or repairing a standard offline AC-DC power supply and need a reliable bulk discharge/filter capacitor that balances cost, availability, and safe discharge characteristics, default to the Rubycon MXG or United Chemi-Con (UCC) KXJ series. They offer guaranteed 3,000+ hour lifespans at 105°C, predictable leakage currents for bleed-resistor sizing, and robust venting mechanisms that prevent PCB damage in the event of catastrophic failure.
