Capacitors
A capacitor stores electrical energy in an electric field between two conductive plates separated by an insulating material called a dielectric. Unlike a resistor which dissipates energy as heat, a capacitor stores and releases energy — making it one of the most versatile components in electronics and a critical building block in every radio circuit.
How Capacitors Work
A capacitor consists of two conductive plates facing each other with a dielectric (insulator) between them. When a voltage is applied, electrons accumulate on one plate and are repelled from the other, creating an electric field across the dielectric. This field stores energy. When the voltage source is removed, the capacitor discharges, releasing the stored energy back into the circuit.
Capacitance is measured in farads (F). One farad is the capacitance that stores one coulomb of charge when one volt is applied. Practical capacitors are much smaller:
- Microfarad (µF) — 10−6 F, common in power supplies and audio circuits
- Nanofarad (nF) — 10−9 F, common in RF and timing circuits
- Picofarad (pF) — 10−12 F, common in RF tuning circuits
The capacitance of a capacitor depends on the plate area, the distance between plates, and the dielectric constant of the insulating material. A larger plate area, shorter gap, or higher dielectric constant all increase capacitance.
Key formula: The charge stored is Q = CV, where Q is charge in coulombs, C is capacitance in farads, and V is voltage in volts.
Capacitor Types
Common capacitor types: electrolytic (cylindrical), ceramic disc, film, and variable air-gap capacitors used in RF tuning.
View Larger| Type | Typical Range | Polarized? | Key Characteristic |
|---|---|---|---|
| Electrolytic (aluminum) | 1 µF – 100,000 µF | Yes | High capacitance, low voltage rating, power supply filtering |
| Tantalum electrolytic | 0.1 µF – 1,000 µF | Yes | Compact, stable, used in audio and signal circuits |
| Ceramic (MLCC) | 1 pF – 100 µF | No | Very common, cheap, good for bypass and RF decoupling |
| Film (polyester, polypropylene) | 1 nF – 10 µF | No | Stable, low loss, excellent for audio and timing circuits |
| Silver mica | 1 pF – 1,000 pF | No | Very stable, low loss, precision RF circuits and filters |
| Variable (air / trimmer) | 1 pF – 500 pF | No | Adjustable capacitance for antenna tuners and oscillators |
Electrolytic capacitor polarity markings: the longer lead is positive; the stripe running down the can marks the negative lead. The voltage rating printed on the body must not be exceeded.
View LargerTo identify polarity on a through-hole electrolytic: the positive lead is longer and the body is usually unmarked on that side. The negative lead is shorter and a prominent stripe with minus signs runs down the can next to it. On PCBs, the footprint silk-screen shows a plus sign or a filled half-circle on the positive pad. SMD tantalum capacitors mark the positive terminal with a stripe or a plus sign on top of the body.
Reading Capacitor Values
Small capacitors use a three-digit code similar to resistor color codes. The first two digits are significant figures, and the third is a multiplier (power of ten in picofarads).
Code 104 → 10 × 104 pF = 100,000 pF = 100 nF = 0.1 µF
Code 472 → 47 × 102 pF = 4,700 pF = 4.7 nF
Code 220 → 22 × 100 pF = 22 pF
A trailing letter gives the tolerance: J = ±5 %, K = ±10 %, M = ±20 %.
Large electrolytics print their value directly in µF along with the voltage rating (e.g., 1000 µF 25 V). The negative lead is marked with a stripe or minus signs down the can.
Series and Parallel Capacitors
Capacitors combine differently from resistors. When capacitors are connected in parallel, capacitances add directly — they share the same voltage. When connected in series, the total capacitance is less than the smallest individual value — they share the same charge.
Series rule: 1/Ctotal = 1/C1 + 1/C2 + 1/C3 + …
Capacitors in Series & Parallel
Enter up to four capacitor values (leave unused fields blank). Select the circuit configuration.
RC Time Constant
When a resistor and capacitor are connected together, the capacitor does not charge or discharge instantly. The time it takes to charge to approximately 63.2 % of the supply voltage (one time constant) is given by:
The RC charging curve: voltage reaches 63.2 % of supply at τ, 86.5 % at 2τ, and is considered fully charged after 5τ.
View LargerAfter five time constants (5τ) the capacitor is considered fully charged (99.3 %). The same curve applies in reverse during discharge — after 5τ the capacitor is considered fully discharged.
RC Time Constant Calculator
Enter resistance and capacitance to find τ (tau) and the time to full charge (5τ).
Capacitive Reactance
In a DC circuit, a fully charged capacitor blocks current — it acts like an open circuit. In an AC circuit, the capacitor repeatedly charges and discharges as the voltage alternates, so current does flow. The opposition a capacitor presents to AC current is called capacitive reactance (XC), measured in ohms:
Capacitive reactance decreases as frequency increases — a capacitor passes high-frequency signals more easily than low-frequency ones. This is the opposite behavior to an inductor and is fundamental to filter design.
Capacitive Reactance Calculator
XC = 1 / (2π f C) — enter frequency and capacitance to find reactance in ohms.
Capacitors in Ham Radio
Capacitors appear throughout radio equipment in several critical roles:
- Power supply filtering — large electrolytics (1,000 µF and up) smooth the pulsating DC from a rectifier into steady voltage. A bigger capacitor means less ripple.
- Bypass and decoupling — ceramic capacitors (0.1 µF) placed across power supply pins of ICs bypass high-frequency noise to ground, preventing it from reaching sensitive circuits. Multiple values in parallel cover a wider frequency range.
- Coupling (blocking DC) — a capacitor in series with a signal path passes AC while blocking DC bias. It couples one stage to the next without disturbing operating points.
- RF tuning (tank circuits) — a variable capacitor paired with an inductor forms a resonant LC circuit that selects a specific frequency. The classic variable air capacitor in a valve transmitter or antenna tuner works this way.
- Timing and oscillators — the RC time constant sets the frequency of oscillators, timers, and keyer circuits.
- Antenna tuners — switched or variable capacitors adjust the impedance matching network to present 50 Ω to the transmitter regardless of antenna impedance.
Hands-On Experiment
Experiment: RC Charge and Discharge Timing
Observe the RC time constant in action using a resistor, capacitor, LED, and a 9 V battery. You will see how the capacitor charges through the resistor and then discharges through the LED, producing a visible fade.
- 9 V battery with clip connector
- 1,000 µF electrolytic capacitor (16 V or higher rating)
- 1 kΩ resistor (for charge path)
- 470 Ω resistor (current limit for LED)
- LED (any color)
- Breadboard and jumper wires
- Multimeter (optional — to observe voltage)
- Connect the 1 kΩ resistor in series with the positive terminal of the capacitor (observe polarity — long lead is positive). Connect the negative lead of the capacitor to the battery negative. Connect the 1 kΩ resistor to battery positive. τ = 1,000 Ω × 0.001 F = 1 second.
- Touch the battery clip to the battery and allow the capacitor to charge for about 5 seconds. If you have a multimeter, watch the voltage rise from 0 V toward 9 V.
- Disconnect the battery. Now connect the LED (in series with the 470 Ω resistor) across the capacitor terminals, observing polarity.
- Watch the LED glow and then fade over several seconds as the capacitor discharges through the LED circuit. The fade follows the same exponential curve you saw in the diagram above.
- Repeat with a larger resistor (e.g. 10 kΩ) in the charge path and observe how much longer it takes to charge to the same voltage.
Frequently Asked Questions
What happens if I connect an electrolytic capacitor backwards?
Reverse polarity destroys the oxide dielectric layer inside the capacitor, causing it to draw excessive current, overheat, bulge, and potentially rupture or explode. Always match the positive terminal (longer lead, marked + or unmarked stripe) to the higher voltage point in the circuit. Replace any capacitor that shows signs of bulging immediately.
Why does adding capacitors in parallel increase total capacitance?
Parallel capacitors share the same voltage but their plate areas effectively add together. More plate area means more charge can be stored at the same voltage, so total capacitance is the sum of all individual values. This is the opposite of resistors in parallel, which reduce the total value.
What is a decoupling capacitor and why does every IC need one?
A decoupling capacitor (typically 100 nF ceramic) is placed as close as possible to the power supply pins of an IC. When the IC switches rapidly it draws short bursts of current that the PCB traces cannot supply fast enough. The decoupling capacitor acts as a local energy reservoir, supplying that current instantly and preventing voltage spikes from coupling into other circuits. Without it, ICs can malfunction or introduce noise.
Why do capacitors pass high frequencies but block low frequencies?
Capacitive reactance XC = 1/(2πfC) falls as frequency rises. At high frequencies the reactance is very low so the capacitor presents little opposition to current — it passes the signal easily. At low frequencies and especially DC the reactance is extremely high (infinite at DC) so virtually no current flows. This frequency-dependent behavior makes capacitors essential for filters and RF coupling circuits.
How do I read a three-digit capacitor code like 104?
Take the first two digits as the base value (10) and raise 10 to the power of the third digit (4), giving 10 × 104 = 100,000 pF = 100 nF = 0.1 µF. The units are always picofarads. A trailing letter gives tolerance: J = ±5 %, K = ±10 %, M = ±20 %. This same convention applies to many ceramic and film capacitors.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.