Capacitance and Inductance: Electronic Component Basics

The Capacitor That Saved Apollo 13

When an oxygen tank explosion crippled Apollo 13 in April 1970, the crew had to power down nearly every system to conserve the remaining batteries. Among the components that had to work flawlessly during the desperate power-up sequence before reentry: a bank of capacitors in the command module's signal conditioning equipment. If those capacitors had degraded during the cold soak of the powered-down spacecraft, the guidance computer would not have initialized. The crew would not have come home.

Capacitors and inductors are the two "reactive" passive components in electronics -- they store energy and release it, unlike resistors which simply burn it off as heat. Every power supply, radio receiver, motor controller, and audio amplifier depends on them. If you have ever built a circuit, soldered a board, or even changed a car stereo, you have handled these components.

Capacitance: Storing Energy in an Electric Field

A capacitor is two conductive plates separated by an insulator (the dielectric). Apply voltage across the plates, and electrons pile up on one side and drain from the other, creating an electric field. The amount of charge stored per volt is the capacitance:

C = Q / V

Where C is capacitance in farads, Q is charge in coulombs, and V is voltage.

Think of it like a spring. Push on it (apply voltage), and it stores potential energy. Release it, and the energy comes back. The energy stored is:

E = 1/2 CV squared

Double the voltage, and you store four times the energy. This is why high-voltage capacitors are dangerous even when small -- a camera flash capacitor at 300V can deliver a painful jolt despite holding only a few hundred microfarads.

How Capacitors Behave in Circuits

Capacitors resist changes in voltage. When voltage tries to spike, the capacitor absorbs current to hold the voltage steady. When voltage drops, the capacitor releases charge to compensate.

The math: I = C (dV/dt)

A constant voltage means zero current -- the capacitor looks like an open circuit to DC. But for rapidly changing AC signals, current flows freely. This frequency-dependent behavior is why capacitors filter noise, couple audio signals between amplifier stages, and smooth rectified power supplies.

Capacitor Units: Farads and Subunits

The farad (named after Michael Faraday) is enormous. A one-farad capacitor at 5V stores enough energy to run a small LED for several minutes. Supercapacitors reach hundreds of farads but are physically large and expensive. Everyday electronics use much smaller values:

Typical ranges by application:

• Picofarads (1-1000 pF): RF circuits, oscillators, high-frequency filters
• Nanofarads (1-100 nF): Signal coupling, decoupling, timing circuits
• Microfarads (1-1000 uF): Power supply filtering, audio circuits, motor start capacitors
• Millifarads to farads: Energy storage, backup power, supercapacitors

Convert between these units with our farads to microfarads converter.

Inductance: Storing Energy in a Magnetic Field

An inductor is a coil of wire. When current flows through it, a magnetic field forms around the coil. The inductor resists changes in that current -- try to increase current suddenly, and the inductor pushes back. Try to cut the current, and the inductor generates voltage to keep it flowing.

V = L (dI/dt)

Where L is inductance in henries, and dI/dt is the rate of current change.

If a capacitor is a spring, an inductor is a flywheel. A flywheel resists changes in rotational speed -- it takes effort to spin up and keeps spinning when you stop pushing. An inductor does the same thing with electrical current.

Energy stored in an inductor: E = 1/2 LI squared

Notice the symmetry with capacitors: voltage squared for caps, current squared for inductors.

DC and AC Behavior

For steady DC current (not changing), an ideal inductor has zero impedance -- it is just a wire. For rapidly changing AC, the inductor blocks current flow. This is the exact opposite of capacitor behavior. That complementary relationship is the foundation of filter design.

Inductor Units: Henries and Subunits

The henry (named after American physicist Joseph Henry) is a more practical unit than the farad -- many inductors sit in the henry range.

Typical ranges:

• Nanohenries (1-100 nH): RF circuits, EMI suppression
• Microhenries (1-1000 uH): Switching power supplies, RF chokes
• Millihenries (1-100 mH): Audio crossovers, motor control, power line filters
• Henries (1-10 H): Audio equipment, analog instrumentation

Our henrys to millihenrys converter handles quick unit translations.

Reading Capacitor Values

Electrolytic Capacitors

Large electrolytics print the value directly: "100uF 25V" means 100 microfarads, maximum 25 volts. Exceeding the voltage rating can cause failure or explosion -- electrolytic capacitors contain liquid electrolyte that boils at overvoltage.

Ceramic Capacitors (3-Digit Code)

Small ceramics use a three-digit code where the first two digits are significant figures and the third is a multiplier (number of zeros). The result is in picofarads:

• 104 = 10 + 0000 = 100,000 pF = 100 nF = 0.1 uF
• 473 = 47 + 000 = 47,000 pF = 47 nF
• 102 = 10 + 00 = 1,000 pF = 1 nF
• 330 = 33 + 0 = 33 pF

A trailing letter indicates tolerance: J = 5%, K = 10%, M = 20%.

Inductor Markings

Some inductors use a four-band color code like resistors, with the result in microhenries. Others print values directly: "100uH" or "4R7" (4.7 uH, where R marks the decimal point). SMD inductors often use three or four-digit codes similar to capacitors.

Where Capacitors Show Up

Power supply filtering: Rectifiers that convert AC to DC produce pulsating output. Large electrolytics smooth the ripple; small ceramics handle high-frequency noise the electrolytics miss.

Signal coupling: A capacitor between amplifier stages blocks DC bias while passing the audio or RF signal. Without coupling caps, the DC operating point of one stage would crash the next.

Decoupling: Every digital IC needs a 0.1 uF ceramic capacitor within a few millimeters of its power pin. These local energy reservoirs handle the rapid current spikes that occur when millions of transistors switch simultaneously, preventing voltage dips from propagating through the power supply.

Timing circuits: The time constant tau = R x C governs how fast a capacitor charges through a resistor. This creates predictable delays for oscillators, debounce circuits, and PWM generators.

Motor starting: Single-phase AC motors use a high-capacitance start capacitor to create the phase shift needed for initial rotation, then switch to a smaller run capacitor for efficient steady-state operation.

Where Inductors Show Up

Switching power supplies: Modern DC-DC converters toggle a transistor at 50 kHz to 2 MHz. The inductor stores energy during the on-phase and releases it during the off-phase, stepping voltage up or down with 90%+ efficiency.

EMI filtering: Common-mode chokes block high-frequency electromagnetic interference on power and data lines. They are mandatory in virtually all commercial electronics for regulatory compliance (FCC, CE marking).

RF circuits: An inductor-capacitor (LC) tank circuit resonates at a specific frequency, letting radio receivers tune into one station while rejecting everything else.

Speaker crossovers: Inductors and capacitors split audio into frequency bands. Inductors pass low frequencies to woofers; capacitors pass highs to tweeters.

Wireless charging: The charging pad in your phone charger is a transmitter coil (inductor). It generates a magnetic field that induces current in a receiver coil inside the phone.

LC Circuits: When Caps and Coils Combine

An LC circuit oscillates at its resonant frequency:

f = 1 / (2 x pi x sqrt(L x C))

Energy bounces between the capacitor (electric field) and the inductor (magnetic field) indefinitely in an ideal circuit. Real circuits have resistance that dampens the oscillation, but the principle drives radio transmitters, crystal oscillators, and frequency filters.

LC filters offer sharper cutoff than RC filters. A well-designed LC bandpass filter can select a narrow frequency range while rejecting everything above and below -- exactly what you need for radio reception or signal processing.

Capacitors vs. Inductors: Summary

These two components are mirror images of each other. That symmetry is what makes LC circuits, filters, and power converters possible. Master how each one behaves alone, and their combined behavior follows naturally.