Have you ever had to call the air conditioner repair man and all he had to do was replace a capacitor?  A friend recently experienced that with a unit at his business and afterwards asked me just what is a capacitor?  I started to describe how a capacitor behaves in the electrical system but he quickly stopped me and clarified his question.  He wanted to know what it physically is and how it works.  I thought for a second, but I then had to admit some ignorance on the matter.  I have since done a little digging and hope to answer the question now.

So what is a capacitor?  There are many types, but most contain at least two metal plates separated by a dielectric material (an insulator like glass, ceramic, paper, etc).  The positive and negative leads of the capacitor then each connect to one of the conductive plates as shown in the image below.

Capacitor Construction Example

The capacitor works by developing an electric field between the plates when it is connected to a power source.  This causes a potential difference between the plates resulting in one plate have a positive charge and the other plate having a negative charge.  If the power source on the circuit is steady the capacitor will charge up, match the voltage of the power source, and current through the circuit will stop (in a simple circuit with no other loads).  In a system where the power source is variable or unpredictable, the capacitor will charge and discharge providing a “smoother” voltage for the circuit downstream (more on this below).

As a mechanical engineer, I tend to think about hydraulic analogies (links to Wikipedia) when I want to explain how electronics work.  In a hydronic system, the capacitor would be a membrane blocking the flow of water.  If water pressure (voltage) increases on one side, the membrane would stretch and affect the pressure on the other side.  Water cannot flow (current) through the membrane but does move a little as the membrane stretches.  It took me awhile to understand the last point because I remembered from my electronics classes in school that our equations treated capacitors as if current flows through them.  While electric current does not flow through, the equations are still correct because of something called displacement current.  For more on the differences between practical electronics versus theoretical physics in capacitors, I recommend this video from EEVblog.

As I hinted at a paragraph above, a capacitor is used to filter and regulate electrical energy and current flow in a circuit.  It does this by storing excess energy coming through the circuit in an electric field.  This extra capacity can “soak up” small spikes and “fill in” dips in the the voltage.  However, if it receives more energy than it is designed for, it can easily burn out resulting in it needing to be replaced.  Many times when an HVAC system fails, it’s this little (but critical) electronic component that has failed.

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