Capacitors & Diodes
Two fundamental components with very different personalities — one stores energy in a field, one enforces a one-way street.
Capacitors
A capacitor stores energy in an electric field between two conductive plates separated by an insulator (dielectric).
Unit: Farad (F). Named after Faraday. A farad is enormous — practical values are in microfarads (µF, 10⁻⁶), nanofarads (nF, 10⁻⁹), and picofarads (pF, 10⁻¹²).
Key formula:
Q = C × V (charge = capacitance × voltage)
I = C × dV/dt (current = capacitance × rate of voltage change)
The second formula matters: a capacitor only allows current when voltage is changing. Constant voltage → no current. This is why capacitors block DC but pass AC.
Charging and Discharging
Through a resistor, a capacitor charges and discharges exponentially:
Time constant: τ = R × C
After 1τ → 63% charged
After 5τ → ~99% charged (considered "full")
A 1kΩ resistor with a 100µF capacitor: τ = 0.1 seconds. It’s “full” in about 0.5 seconds.
This RC time constant is used everywhere: debounce filters, timing circuits, audio filters, signal smoothing.
Types
Electrolytic (polarized): large capacitance, small size. Has + and − terminals — wrong polarity → it fails violently. Common for power supply filtering. 1µF to 10,000µF.
Ceramic (non-polarized): small capacitance, cheap, very common. Go either way. 1pF to 1µF. Used for decoupling and filtering.
Film capacitors: stable, low noise. Used in audio and precision timing.
Key Uses
- Power supply decoupling: 100nF ceramic cap near every IC power pin. Absorbs sudden current demands to keep voltage stable.
- Smoothing: large electrolytic across a rectified AC supply smooths out the ripple.
- Timing: RC circuits set timing in oscillators and filters.
- Coupling: pass AC signal between stages while blocking DC bias.
Diodes
A diode allows current to flow in one direction only — from anode (+) to cathode (−).
Schematic symbol: triangle pointing into a bar. Arrow shows conventional current flow direction.
Forward Bias
Apply + to anode, − to cathode: above the forward voltage threshold, the diode conducts and drops a fixed voltage.
- Silicon diode: ~0.6–0.7V forward drop
- Schottky diode: ~0.2–0.3V forward drop (faster, lower loss)
- LED: 1.8–3.5V forward drop (plus it emits light)
In reverse: ideally no current flows. In practice: a tiny leakage current, negligible for most purposes.
Reverse Breakdown
Apply too much reverse voltage → breakdown. For regular diodes this is destructive. But the Zener diode is engineered to break down at a precise voltage and do so safely and repeatedly — making it useful for voltage regulation.
Common Diode Uses
Rectification: convert AC to DC. Four diodes in a bridge rectifier give full-wave rectification.
Flyback protection: motors and relays have inductors that generate voltage spikes when switched off. A diode (called a flyback or freewheeling diode) placed across the inductor gives that spike a safe path to discharge.
[Motor]
│ ← spike voltage
│ ── [Diode] ──┐ (spike loops back harmlessly)
Without this diode, the spike can destroy transistors or microcontroller pins.
Voltage reference: Zener diodes set a fixed voltage, useful for simple regulators.
Logic isolation: prevent current from flowing backward between circuits.
The LED Revisited
An LED is just a diode made from a semiconductor material that releases photons when electrons cross the junction. The color depends on the bandgap energy of the material:
- Red: GaAsP, ~1.8eV, ~700nm
- Green: InGaN, ~2.1eV, ~520nm
- Blue: InGaN, ~3.3eV, ~460nm
- White: blue LED + yellow phosphor coating