How do you define Active and Passive Components?

An electrical circuit is a closed path followed by an electric current. A circuit consists of components like resistors, inductors, capacitors, diodes, current and voltage sources, transistors, vacuum tubes, switching devices etc. These components are basic building blocks of an electrical or electronic circuit. A circuit contains following types of components:

1. Energy sources like voltage and current generators,
2. Transistors, vacuum tubes, amplifiers which modify or reshape the input signal/ voltage / current to suit the circuit requirements,
3. Resistors, inductors, capacitors, transformers etc., which can receive and dissipate energy or store it in magnetic or electric field,
4. Switches, relays, connectors, cables and wires, circuit breakers etc., which conduct the current, or control and ease the current path. These are electromechanical components.

Active and passive components are found in everyday common things like lighting, ovens, batteries, inverters, amplifiers, mobile phones, torches, lifts etc.

All circuit components can be broadly divided in three categories.

1. Sources of electrical energy like current or voltage generators, which supply power / energy to circuit, or provide voltage/current or power gain in a circuit.  These are called Active Components. An active component can supply power to a circuit, or control power to a circuit, and thus makes the circuit function buy doing so. Examples of active devices are generators, batteries, fuel cells, vacuum tubes, transistors, thyristors, amplifiers etc. They rely on a source of energy, or are a source themselves.
2. Elements which only absorb, dissipate it in the form of heat (or work), or store the absorbed energy in electric or magnetic field. These work only in the presence of an energy source, and do not supply energy. These are called Passive Components. Resistors, inductors, capacitors, transformers fall under this category. Even though they may increase or decrease a voltage / current (as in transformers or resonant circuits), they do not increase the power in a circuit.
3. Switches, relays, contactors, connectors, cables, wires etc. are necessary in circuit to carry or break a circuit, and do not otherwise take part in functioning of circuit. These are Electromechanical Components.

Active Components	Passive Components	Electromechanical Components
Battery Fuel Cells Generator Dynamo Transistor Thyristor Vacuum Tube Amplifiers	Resistor Capacitor Inductor Transformer Oven / griller Heaters Lamps Oscillators	Switches Isolators Relays Contactors Motor starters Cables / wires Connectors MCB / MCCB

Active Components

All circuit components can be broadly divided in three categories.

Active components are essentially energy donors in a circuit. They may be from two main categories.

1. Independent Source: These devices produce energy in the form of voltage or current. An ideal voltage source will produce any amount of current while maintaining a constant voltage. Similarly, an ideal current source will maintain a constant current for any magnitude of voltage. These sources may even be able to absorb energy like in the case of batteries.
2. Dependent Sources: These are the sources having the ability to control the current in a circuit electrically, I.e. electricity controlling electricity. Either current or voltage may be used to control the current or voltage in a circuit. These in turn could be from 4 different types. These sources have gain more than 1, meaning they amplify a signal.

Vacuum tubes are voltage-controlled devices, whereas transistors are current controlled devices. Vacuum tubes, transistors, triacs, thyristors, photovoltaic devices are common active components. Saturable reactors (magnetic amplifiers) may be considered as active devices.

There is some confusion among people about diodes being passive or active components. In fact, diodes are active devices, which pass current only in one direction, and block in the reverse direction. Thus they alter the waveform of input, and hence nature of input. They act as switches, and control current passing through them.

Passive Components

Passive components use the power or energy in a circuit, or store in electric / magnetic field. They cannot control current or voltage by means of another signal. They cannot provide gain or amplification. They may filter a signal, and reject some frequencies, can handle complex functions, but cannot generate power.

Basic Passive Components

There are three basic components in any electrical or electronic circuit. Their characteristics are tabulated below.

Resistors	Capacitors	Inductors
Power loss component	Stores energy in electric field	Stores energy in magnetic field
Represents work being done / heat dissipation	Opposes change in voltage	Opposes change in current
Unity power factor for AC	Offers leading power factor to AC	Offers lagging power factor to AC
Resists all current AC / DC	Dampens surge voltage	Dampens surge current
Follows Ohm’s Law	Short circuit for DC	Open circuit for DC