Passive components are electrical devices that do not require any form of electrical power to operate. They cannot generate, oscillate or amplify an electrical signal, but the can attenuate it. Passive components are critical in electronic circuit control and stability.
Resistors are among the most common passive components, used to reduce current flow, adjust signal levels, divide voltage, and more. Varistors are special type of resistors that provide variable electrical resistance depending on the voltage applied. Potentiometers and rheostats are special resistors that are used to manually control the resistance in a circuit.
Other examples of passive components include capacitors, which store and release electrical charge, and transducers, which generate convert one signal into another. Capacitors are most useful for blocking DC power while allowing AC power to pass, and for filter power supply lines. Transducers are used in most automation systems to measure variables and actuate equipment.