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    Capacitors


CONSTRUCTION & CAPACITANCE

 

A basic, parallel-plate capacitor is easy to picture.  It is two sheets of conductive foil with an insulating film in between.  The two sheets and the film are rolled up like a jelly roll to save space.

When a capacitor is put in the path of an electric current, electrons flow onto its upstream plate, making it negatively charged (see figure).

Capacitor

Because like charges repel, electrons are driven off the other plate, leaving positively charged atoms called ions.  Eventually, the capacitor fills up with charge, rebuffing additional current.

But charge is now trapped in the capacitor.  Positive ions beckon adjacent electrons that are blocked by the insulation.  The charges are held in place, creating an electric field.  The capacitor is now storing electromagnetic energy and a voltage appears across its plates.

Large capacitors have lots of storage space or "capacitance".  As a result, the charge spreads out, weakening its concentration.  A larger capacitor needs more charge to create a potential difference of one volt across it.

The unit of capacitance is the farad.  A one farad capacitor needs one coulomb of charge, 6.24 quintillion electrons, to produce one volt of voltage.  Mathematically, capacitance = charge per volt.

Farad is abbreviated capital F, for Michael Faraday; coulomb is capital C, for Charles-Augustin de Coulomb; and volt is capital V, for Alessandro Volta.  So, F = C/V

The farad is an impractically large unit.  Most capacitors are measured in one of the following subunits, starting with the smallest:

  • picofarad (pF) = a trillionth (10-12) of a farad

  • nanofarad (nF) = a billionth (10-9) of a farad, equal to 1,000pF

  • microfarad (μF or MF) = a millionth (10-6) of a farad, equal to 1,000nF

 

CAPACITORS IN POWER SUPPLIES

 

You've seen how a capacitor can store energy.  This ability is used in power supplies to help smooth out its supplied current.

Most gear runs on direct current but house current alternates.  Capacitors are used to store charge when the alternating current is high and give it back when the current is low.  This helps maintain a constant output.

 

TONE CONTROLS

 

Now that you're thinking about electrons sloshing in and out of capacitors, consider the following.

Large capacitors can accept electrons for a longer time period than small capacitors because of their higher capacity.  Small capacitors can only accept electrons for a short time before they fill up and start repelling any more charge.

Now, low-frequency signal currents have long, slow time periods compared to high-frequency signals.  Low-frequency signals can't pass through capacitors that are small and fill up quickly.

On the other hand, high-frequency signals have quick reversals of current and can pass through a small capacitor unimpeded.

Because capacitors discriminate by frequency, they're perfect for tone control circuits.

 

SERIES VERSUS PARALLEL TONE CAPS

 

A capacitor wired in series with a circuit has a tonal effect opposite to the same capacitor wired in parallel with the circuit (see figures below).


SERIES-WIRED CAPACITOR

Simplified Schematic

This guitar will sound trebly because only the higher frequencies can cross the capacitor to the speaker.


PARALLEL-WIRED CAPACITOR

Simplified Schematic

This guitar will sound bassy because high frequencies will go through the capacitor instead of the speaker.





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