Electronic Components – Capacitor

Both capacitors and inductors are “reactive” components.  Unlike resistors, capacitors and inductors store electricity on a short term basis but in different ways.  For me, the capacitor is a little easier to understand so let’s start with it.

For all practical purposes, capacitors consist of two pieces of conductive material (think aluminum foil) separated by an insulator (think plastic film or waxed paper).  You can, by the way, build a capacitor in your home workshop using these two materials.


Capacitor construction can involve conductive surfaces of considerable size to provide the proper characteristics for use.  Several ways have been utilized to provide large surface areas without taking up a lot of space.  Among these are winding the assembly like a roll of paper towels or interleaving the conductive layers with sheets of insulation and fusing the conductors together along the exposed edges to provide a terminal for connection.

In application, the conductors accumulate a charge when a voltage is applied and release the charge when the electrical source is removed (think static electricity).  Simply, if you connect a battery to a capacitor as shown below, the capacitor becomes “charged” as electrons move from one plate to the other.

A = Capacitor in neutral state with charge randomly distributed

B = With a voltage source attached, charges migrate to their appropriate plates as an electrical current flows into the capacitor

C = When all charges have been distributed within the capacitor, current flow stops and charges maintain position

D = When the voltage source is removed, charges stay distributed to maintain the capacitor in a charged condition (see note below)

E = When a load is connected to the charged capacitor the charges return to random distribution as current flows out of the capacitor

Note – Charged capacitors provide a considerable safety risk to those working on electronic circuits.  They can store high voltage charges which can persist for days despite the lack of connection to an electrical source of any kind!  They are the only component, save batteries, that can do this.  It is good practice to discharge any capacitors in a circuit by shorting the leads together before working on the circuit.  Assume any capacitor is charged unless the leads are shorted together.  Many deaths can be attributed to the unexpected discharge of capacitors!

A capacitor is an open circuit except for the flow of electrons from one conductive plate to the other.  In use, capacitors serve to store small amounts of charge (usually) for short periods of time.  One common use of capacitors is to smooth out the ridges and valleys of a varying voltage source to provide a constant voltage.


Capacitors are rated in farads based on the amount of charge that they can accumulate and store.  The more farads, the more charge can be accumulated.   It turns out that a farad is pretty huge so the majority of capacitors are in the microfarads range.  Capacitors also have a voltage rating based on the insulating value of the insulation layers.  Although many of today’s larger capacitors still have the classic round or rectangular shape, the push to miniaturization has resulted in a wide range of shapes and sizes.

 – JF –



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