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Passive electronic components

A passive electronic compoenent will typically absorb energy without create any active gain. These are some of the common passive electronic components. One other passive component is the diode however I've included that in the semiconductors.


Single resistor A resistor is the most simiple electronic component. I resists the flow of current through a circuit. The end result is a voltage drop (loss in potential) and power being absorbed within the device. Every electrical device, cable or connection has resistance in it, but for most circuits the resistance within the wiring can be ignored.

The resistance is measured in ohms (Ω) and varies from only a few ohms (or even less if you are considering the normal cabling) to many mega ohms (MΩ).

A resistor is normally a small cylindrical device which has different coloured bands indicating the resistance value. For more details see the resistor colour code scheme.
Strip of resistors

ISO / European symbol for resistor

ISO / European circuit symbol for resistor

US symbol for resistor

US circuit symbol for resistor

Light dependent resistor (LDR) / photoresistor

A light dependent resistor (LDR), sometimes called a photoresistor is a special kind of resistor whose resistance varies depeding upon the amount of light on it's exposed surface.

Different LDRs can have different properties, but a typical one may have a resistance measured in kΩ when in bright light to MΩ when dark.

Circuit symbol for a light dependent resistor

Circuit symbol for a light dependent resistor


Electrolytic Capacitor A capacitor is a pair of conductors seperated by a diaelectric (insulator). When a voltage is applied a field is created that stores energy between the conductors.

A capacitor can be used in a variety of different ways. Some common uses in electronic circuits are listed below:

  • Smoothing DC circuits - used to smooth the output from a bridge rectifier when converting from AC to DC
  • Power storage - like a low capacity battery to maintain voltage when power is removed for a short period of time. This is typically only a very small amount of power as capacitors have a very small capacity compared with a similarly sized battery.
  • Signal coupling - passing AC signals, blocking DC signals
  • Decoupling - removing AC signals from DC
  • Noise filtering / snubbers
  • Tuned circuits - to select a particular frequency band

The effective "size" of the capacitor is measured in farads (F). The capacity of capacitors used in electronic circuits is very small and is normally referenced as micro-farads (μF), nano-farads (nF) or pico-farads (pF).
Polyester Capacitor

Capacitors are made of different diaelectrics according to the size of the capacitor and to its intended use. In order of size (smallest capacity to highest) these are usually ceramic, polyester and electrolytic. The ceramic and polyester capacitors can be connected either way around, but the electrolytic capacitors have a positive and negative terminal that must be connected accordingly.

Circuit symbol for capacitor

ISO / European / US circuit symbol for capacitorCircuit symbol for an electrolytic capacitor
Note that the plus sign on the second example indicates the polarity for an electrolytic capacitor.

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Discrete semiconductors