The Design Corner
    Introduction to Capacitance Proximity Switches
    
 Updated on:  Friday, December 18, 2015 08:20 AM

 

Units of Capacitance  
Dielectric Constant Definition   
Human Body Capacitance  

Excitation Circuits
Capacitance Change Sensor Circuits 
Interface Circuits  

 
The Physics of Capacitance

A capacitor is formed by placing two metal plates in parallel with each other, separated by a distance D with each plate having an area of A.  The capacitance of the capacitor can be calculated using the equation below.  Note that the capacitance is proportional to the plate area but is inversely proportional to the distance between the plates.  When the plates are close to each other, even a small change in distance between the plates can result in a sizeable change in capacitance.  Many industrial capacitance type sensors take advantage of this relationship and can detect very small vibrations and motion.

  • C = (K)(A)(8.85 x10^-14)/D

  • C = capacitance in farads

  • K = dielectric constant

  • A = plate area in square centimeters

  • D = distance between plates in centimeters

 

 
Units of Capacitance   

The Farad is the unit of capacitance.  Smaller units include the microfarad, which is one millionth of a farad and the picofarad, which is a millionth of a microfarad.    Most of the capacitance changes discussed in this section will be listed in picofarads.

Dielectric Constant Definition

When a material, which does not conduct electricity, is slid between the two parallel metal plates the capacitance will usually increase.  The ratio of the capacitance before and after the material is placed between the two plates, is equal to the dielectric constant of the center material.  The dielectric of a vacuum is one while dry air is almost one.  The table below lists the dielectric constant of some other materials. Note that water has a high dielectric constant.  Objects containing water, such as a human body, will therefore dramatically increase the capacitance.

  • Vacuum = 1.00

  • Dry air = 1.0006

  • Bakelite plastic =  5.0

  • Dry wood = 1.5 5.0

  • Paper = 2.0 3.0

  • Teflon plastic = 2.1

  • Paraffin wax = 2.5

  • Polyethylene plastic 2.3

  • Polystyrene plastic = 2.6

  • Distilled Water = 80

  • Plexiglas plastic = 2.8

  • Rubber = 3.0 4.0

  • Celluloid plastic = 4.0

  • Quartz = 4.0 5.0

  • Formica = 4.7

  • Mica = 4.5 8.0

  • Pyrex Glass = 4.8

  • Window glass = 7.0 8.0

  • Porcelain 5.0 6.0

Human Body Capacitance

One application of a capacitance-activated circuit is to form a switch that is triggered by the touch of a human finger to a metal button.  The circuit operates by measuring the capacitance change between the metal touch button and an earth ground. 

The skin of the human finger is thin and the human blood under the skin makes a nice electrical conductor.  The outside surface area of the human body is also large.  The act of touching a human finger to a metal button will therefore form an electrical interface that will produce a sizable capacitance change, if there are any metal objects nearby that are connected to earth ground. This environmental capacitance will depend on many factors.  The most important factors are the type of floor the human is standing on and the type of shoes he or she is wearing. The capacitance change will increase with bare concrete floors and thin sole shoes. Nearby metal shelves, tables, light fixtures, heating ducts and electrical wires will also effect the total capacitance between the human body and earth ground.   Experiments have shown that seldom does the capacitance change measure less than about 30 picofarads.  At the other end of the spectrum, if the human is also in contact with an earth grounded object, the finger/button interface can produce a capacitance change of several thousand picofarads.  In most applications, a minimum capacitance change threshold can be set.  When exceeded, the detected change is used to activate an electronic switch.   With a carefully designed circuit a switch can be triggered with a capacitance change of only one picofarad.

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