Key Engineering Takeaways
- •A capacitor stores electrical energy in an electrostatic field between two conductive plates separated by an insulating dielectric material.
- •Capacitance (C = Q / V) is measured in Farads (F), commonly in microfarads (µF), nanofarads (nF), and picofarads (pF).
- •Electrolytic capacitors are POLARIZED. Connecting positive and negative backwards will cause internal pressure buildup and violent rupture.
- •Decoupling capacitors (0.1µF / 100nF ceramic) placed right next to microcontroller power pins eliminate high-frequency digital noise spikes.
- •The RC Time Constant (τ = R · C) determines how quickly a capacitor charges to 63.2% of supply voltage.
- • Voltage, Current and Resistance fundamentals
- • 100µF Electrolytic Capacitor
- • 0.1µF (100nF) Ceramic Capacitor
- • 10kΩ Resistor
- • LED
- • Breadboard
What is a Capacitor & How Does It Store Charge?
A Capacitor is a passive two-terminal component designed to store electrical energy in an electrostatic field.
It consists of two parallel conductive metal plates separated by a non-conductive insulating material called the Dielectric (such as ceramic, mica, plastic film, glass, or electrolyte paper).
How Charge Storage Works:
- 1When connected to a DC voltage source (battery), electrons accumulate on the plate connected to the negative terminal.
- 2The opposing plate loses electrons to the positive terminal, becoming positively charged.
- 3Because the dielectric prevents electrons from jumping across the physical gap, an electrostatic attraction builds up between the plates.
- 4When disconnected, the capacitor retains this electrical potential like a miniature rechargeable battery, discharging its energy in fractions of a millisecond when connected across a load.
Capacitance Formula & Units (Farads, µF, nF, pF)
The ability of a capacitor to store charge per unit voltage is defined as Capacitance (C):
Where Q is charge in Coulombs, and V is voltage in Volts. Capacitance is measured in Farads (F).
A 1 Farad capacitor is enormous in classic electronics. Most common circuit values are in micro, nano, or pico Farads:
- 1µF (Microfarad) = 10^-6 F = 0.000001F (Used in power supply filtering)
- 1nF (Nanofarad) = 10^-9 F = 0.000000001F (Used in audio and filters)
- 1pF (Picofarad) = 10^-12 F (Used in high-frequency radio and crystal oscillators)
Capacitor Types: Ceramic vs Electrolytic (Polarity Caution!)
1. Ceramic Capacitors (Non-Polarized)
Small yellow/orange discs or SMD chip capacitors. They have low capacitance values (1pF to 10µF), high voltage ratings, and NO POLARITY (can be inserted either way). Excellent for high-frequency bypass filtering.
2. Electrolytic Capacitors (POLARIZED ⚠️)
Cylindrical metal cans filled with conductive liquid or gel electrolyte. They offer massive capacitance (1µF to 10{,}000+ µF).
RC Time Constant (τ = R · C) Charging & Discharging Curves
When a capacitor is charged through a series resistor, it does not charge instantaneously. The rate of charging is governed by the RC Time Constant (tau, Tau):
Where R is in Ohms, C is in Farads, and tau is in seconds.
The 63.2% Rule:
- After 1tau: Capacitor reaches 63.2% of full supply voltage.
- After 2tau: Reaches 86.5%.
- After 3tau: Reaches 95.0%.
- After 5tau: Considered 99.3% (Fully Charged).
Example: With a 100 kΩ resistor and a 100µF capacitor:
It takes 5 × 10 = 50 seconds to fully charge! This predictable curve is how classic timers (555 ICs) and debounce filters work.
Frequently Asked Questions
Why do capacitors block DC but pass AC?
For steady DC, once the capacitor plates charge up to source voltage, current stops completely (infinite DC resistance). For AC, the continuous back-and-forth polarity reversals cause charge to move into and out of the plates continuously, allowing AC signals to pass through easily.
What is a decoupling or bypass capacitor?
When high-speed microcontrollers switch internal transistors millions of times a second, they create tiny brief voltage sags on the power wire. A 100nF decoupling capacitor placed millimeters away from the chip acts as an instant local energy reservoir to smooth out these spikes.