Capacitance and Inductance: Electronic Component Basics

Introduction to Reactive Components

Every electronic circuit relies on three fundamental passive components: resistors, capacitors, and inductors. While resistors simply oppose current flow, capacitors and inductors do something more interesting - they store energy and release it in ways that shape electrical signals.

These two components are called "reactive" because they react to changes in voltage and current rather than simply resisting them. Understanding capacitance and inductance opens the door to designing filters, power supplies, oscillators, and countless other circuits that make modern electronics possible.

What Is Capacitance?

Capacitance is the ability to store electrical energy in an electric field. A capacitor is a component designed to exhibit this property💡 Definition:An asset is anything of value owned by an individual or entity, crucial for building wealth and financial security. in a controlled, predictable way.

The Basic Structure

At its simplest, a capacitor consists of two conductive plates separated by an insulating material called a dielectric. When you apply voltage across the plates, electrons accumulate on one plate and are depleted from the other, creating an electric field between them.

The amount of charge a capacitor can store for a given voltage is its capacitance. This relationship is expressed as:

C = Q / V

Where:

• C = Capacitance (in farads)
• Q = Charge (in coulombs)
• V = Voltage (in volts)

How Capacitors Store Energy

Think of a capacitor like a spring. When you push on a spring (apply voltage), it compresses and stores potential energy. Release it, and that energy is released. Similarly, when you charge a capacitor, energy is stored in the electric field between its plates.

The energy stored in a capacitor is:

E = 1/2 CV^2

This means energy storage increases with the square of the voltage - double the voltage, and you store four times the energy. This is why high-voltage capacitors can be dangerous even with relatively small capacitance values.

Capacitor Behavior in Circuits

Capacitors have a unique property: they oppose changes in voltage. When voltage tries to rise suddenly, the capacitor absorbs current to maintain the original voltage. When voltage drops, the capacitor releases stored charge to maintain it.

This behavior is described by:

I = C (dV/dt)

In plain English: the current through a capacitor is proportional to how quickly the voltage is changing. A constant voltage means zero current - the capacitor acts like an open circuit to DC. But for rapidly changing AC signals, the capacitor readily passes current.

This frequency-dependent behavior makes capacitors essential for filtering, coupling, and decoupling in electronic circuits.

Capacitor Units: Farads and Beyond

The SI unit of capacitance is the farad (F), named after English scientist Michael Faraday. One farad is an enormous amount of capacitance - it would store one coulomb of charge at one volt.

Why Farads Are So Large

To put this in perspective, a one-farad capacitor at 5 volts would store enough charge to power a small LED for several minutes. Such capacitors exist (they are called supercapacitors or ultracapacitors) but they are large and expensive.

Most electronic circuits use capacitors measured in much smaller units:

Common Capacitance Ranges

Different capacitor types serve different purposes:

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

Use our Microfarads to Picofarads Converter to quickly convert between capacitance units.

What Is Inductance?

Inductance is the ability to store electrical energy in a magnetic field. An inductor is a component - typically a coil of wire - designed to exhibit this property.

The Basic Structure

When current flows through a wire, it creates a magnetic field around that wire. If you coil the wire, these fields overlap and reinforce each other, creating a stronger magnetic field. This is the principle behind an inductor.

The relationship between inductance, voltage, and current is:

V = L (dI/dt)

Where:

• V = Voltage across the inductor (in volts)
• L = Inductance (in henries)
• dI/dt = Rate of change of current (in amperes per second)

How Inductors Store Energy

If a capacitor is like a spring, an inductor is like a flywheel. A flywheel resists changes in rotational speed - it takes effort to speed it up, and once spinning, it wants to keep spinning. Similarly, an inductor resists changes in current.

The energy stored in an inductor is:

E = 1/2 LI^2

Notice the symmetry with the capacitor energy formula - for capacitors it is voltage squared, for inductors it is current squared.

Inductor Behavior in Circuits

Inductors oppose changes in current. When current tries to increase suddenly, the inductor develops a voltage that opposes the change. When current decreases, the inductor generates voltage to try to maintain it.

For DC current (which is not changing), an ideal inductor has zero resistance - it acts like a short circuit. For rapidly changing AC signals, the inductor resists current flow. This is the opposite of capacitor behavior, which blocks DC and passes AC.

This complementary behavior between capacitors and inductors is the foundation of filter design and many other circuit applications.

Inductor Units: Henries and Subunits

The SI unit of inductance is the henry (H), named after American scientist Joseph Henry. One henry is the inductance that produces one volt when current changes at one ampere per second.

Practical Inductor Values

Unlike the farad, the henry is a practically useful unit - many inductors have values in the henry range. However, smaller values are common for high-frequency applications:

Common Inductance Ranges

Different applications use different inductance values:

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

Use our Millihenries to Microhenries Converter for quick inductance unit conversions.

How to Read Capacitor Values

Reading capacitor values can be confusing because different types use different marking systems.

Electrolytic Capacitors

Large electrolytic capacitors usually have their value printed directly on the body:

• "100uF 25V" means 100 microfarads rated for 25 volts maximum
• "2200uF 16V" means 2200 microfarads rated for 16 volts

Always note the voltage rating - exceeding it can cause capacitor failure or explosion.

Ceramic Capacitors (3-Digit Code)

Small ceramic and film capacitors often use a three-digit code:

• First two digits: significant figures
• Third digit: multiplier (number of zeros to add)
• Result is in picofarads

Examples:

• 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

For values under 10 pF, the code may be just one or two digits (like "5" for 5 pF).

Tolerance Codes

A letter following the value indicates tolerance:

Voltage Rating Codes

On smaller capacitors, voltage may be coded as a single letter:

How to Read Inductor Values

Inductor marking systems vary by type and manufacturer.

Color Code System

Some inductors use a four-band color code similar to resistors:

The result is in microhenries. For example, red-red-brown-gold = 22 x 10 = 220 uH with 5% tolerance.

Direct Marking

Many inductors have values printed directly:

• "100uH" = 100 microhenries
• "4.7mH" = 4.7 millihenries
• "R47" = 0.47 uH (R represents the decimal point)
• "4R7" = 4.7 uH

SMD Inductor Codes

Surface-mount inductors often use a three or four-digit code similar to capacitors, with the result in microhenries or nanohenries depending on the manufacturer datasheet.

Real-World Applications of Capacitors

Capacitors appear in virtually every electronic device. Here are their most common uses:

Power Supply Filtering

Rectifier circuits that convert AC to DC produce pulsating output. Large electrolytic capacitors smooth this ripple into steady DC voltage. Smaller ceramic capacitors handle high-frequency noise that electrolytics cannot filter effectively.

Signal Coupling and DC Blocking

Capacitors pass AC signals while blocking DC. This allows stages of an audio amplifier to operate at different DC bias points while passing the audio signal between them. A coupling capacitor blocks the DC component while allowing the desired signal through.

Decoupling and Bypassing

Every digital IC needs small capacitors (typically 0.1 uF ceramic) placed close to its power pins. These "decoupling" or "bypass" capacitors provide local energy storage to handle rapid current demands and prevent voltage spikes from propagating through the power supply.

Timing Circuits

RC (resistor-capacitor) circuits create predictable time delays. The time constant tau = R x C determines how quickly the capacitor charges or discharges. This is the basis💡 Definition:The original purchase price of an investment, used to calculate capital gains or losses when you sell. of oscillator circuits, delay timers, and PWM (pulse width modulation) generators.

Motor Start and Run

Single-phase AC motors often use capacitors to create the phase shift needed for starting or continuous operation. Start capacitors (high capacitance, intermittent use) get the motor spinning, while run capacitors (lower capacitance, continuous use) improve efficiency.

Energy Storage

Supercapacitors can store enough energy to power small devices for minutes or even hours. They bridge the gap between batteries (high energy, slow discharge) and conventional capacitors (low energy, fast discharge). Applications include backup power for memory chips, regenerative braking in vehicles, and solar energy storage.

Real-World Applications of Inductors

Inductors are less common than capacitors in consumer electronics but essential in power and RF circuits:

Switching Power Supplies

Modern power supplies use high-frequency switching (typically 50 kHz to 2 MHz) to convert voltage efficiently. Inductors store energy during part of the switching cycle and release it during the other part, enabling voltage step-up (boost) or step-down (buck) conversion with over 90% efficiency.

EMI Filtering

Common-mode chokes are inductors that suppress electromagnetic interference (EMI) on power and signal lines. They allow desired signals to pass while blocking high-frequency noise that could cause interference or regulatory compliance💡 Definition:Compliance ensures businesses follow laws, reducing risks and enhancing trust. failures.

RF Circuits

Radio frequency circuits use inductors for tuning, impedance matching, and filtering. An LC (inductor-capacitor) tank circuit resonates at a specific frequency, enabling radio receivers to select one station while rejecting others.

Audio Crossovers

Speaker crossover networks use inductors (and capacitors) to divide audio signals by frequency. Large inductors pass low frequencies to woofers while blocking high frequencies that go to tweeters.

Motor Drives

Variable frequency drives and motor controllers use inductors to smooth the PWM waveforms that drive motors. This reduces motor heating and acoustic noise while improving efficiency.

Wireless Power Transfer

Inductive coupling enables wireless charging for smartphones and electric vehicles. A transmitter coil generates a magnetic field that induces current in a receiver coil, transferring power without physical contact.

LC Circuits: Capacitors and Inductors Together

When capacitors and inductors are combined, they create circuits with remarkable properties.

Resonance

An LC circuit naturally oscillates at its resonant frequency:

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

At this frequency, energy continuously transfers between the capacitor (storing it in an electric field) and the inductor (storing it in a magnetic field). This oscillation is the basis of radio transmitters, crystal oscillators, and many timing circuits.

Filtering

LC filters can be more selective than RC filters, with sharper cutoff characteristics. Low-pass LC filters pass frequencies below the cutoff while strongly attenuating higher frequencies. Band-pass filters pass a specific frequency range while rejecting everything else.

Impedance Matching

At resonance, an LC circuit can transform impedance, matching a high-impedance source to a low-impedance load (or vice versa) for maximum power transfer. This is critical in RF systems where mismatched impedances cause signal reflection and power loss.

Key Differences Between Capacitors and Inductors

Key Takeaways

1. Capacitance is the ability to store energy in an electric field, measured in farads (F)
2. Inductance is the ability to store energy in a magnetic field, measured in henries (H)
3. Practical capacitor values typically range from picofarads to millifarads
4. Practical inductor values typically range from nanohenries to henries
5. Capacitors block DC and pass AC - impedance decreases with frequency
6. Inductors pass DC and block AC - impedance increases with frequency
7. The three-digit capacitor code gives a value in picofarads (e.g., 104 = 100,000 pF = 100 nF)
8. LC circuits resonate at f = 1/(2 x pi x sqrt(LC)), enabling filters and oscillators
9. Both components are essential in power supplies, filters, timing circuits, and RF applications
10. Understanding these fundamentals enables you to design and troubleshoot real electronic circuits

Whether you are building your first Arduino project or designing professional electronics, capacitors and inductors are components you will💡 Definition:A will is a legal document that specifies how your assets should be distributed after your death, ensuring your wishes are honored. encounter constantly. Mastering their behavior and knowing how to read their values gives you the foundation to tackle any circuit challenge.