Capacitors

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Contents

Capacitors

Basics

You can think of a capacitor as sort of a rechargeable battery, connect it to a voltage and a current will flow into it charging it up, later it will have a voltage of its own that can push the current back out. The key equation for a capacitor is:

c=q/v

Where:

  • C = capacitance ( units: farad = volts /coulomb )
  • V = voltage ( unit: volts )
  • Q = charge in the capacitor ( unit: coulomb )

A simple application of the formula above occurs when a constant or near constant current flow into a capacitor: then the voltage across it increases at a constant rate, the graph of the voltage against time will be a line rising ( or falling for a negative current ) from the left to the right. If the current is constant the line is straight.

Another way of thinking of a capacitor ( which is only partly correct ) is that it is a resistor with infinite resistance for DC and lower and lower resistance as the frequency goes up. This is a fair model for sinusoidal voltages and currents, but is less useful for signals such as square waves.


Uses:

  • ac coupling –- blocking -- isolation
  • timing
  • Time for a capacitor to charge or discharge is very roughly RC where R is the resistor in series with the capacitor.
  • Filter ( often power supply filter )
  • decoupling
  • tuned circuits

All capacitors are formed by having 2 conductors ( or plates ) connected to the two terminals of the capacitor. The conductors are separated from each other by insulator, typically very thin. Adding charge to one side forces like charge off the opposite plate because like charges repel. The larger the plates and the closer together they are the less voltage it takes to force in the charge. If the insulator ( called the dielectric ) is too thin it will be pierced by the charge and the capacitors becomes a conductor. Typically the basic specifications for a capacitors are its capacitance and the maximum voltage that can be used without causing the dialectic to break down.
There are many diferent technologies for manufacturing capacitor each with its own advantages and disadvantages.

External Links

  1. Capacitor From Wikipedia, the free encyclopedia
  2. RC circuit From Wikipedia, the free encyclopedia
  3. RC Timers and Timing Circuits
  4. Decoupling capacitor From Wikipedia, the free encyclopedia
  5. Components: Capacitors
  6. Electronics and radio components See the section called Capacitors
  7. Capacitors
  8. Capacitors: A Bucket Full of Charge


Parameters

The most basic parameter of the capacitor is of course its capacitance. However in practice there are several other other parameters that should normally be considered:

  • Tolerance, the degree to which the capacitor has its labeled value.
  • Peak voltage: the highest voltage that can be safely applied to the capacitor. If this voltage is exceeded, bad things can happen. Often the capacitor suddenly become a low value resistor, and shorts out the circuit is connected to.
  • Equivalent Series Resistance ( ESR ) All capitators act as if they have a resistor in series with a more or less perfect capacitor. The value of this is the ESR. Low ESR is normally considered to be good.
  • Maximum Ripple Current. If you calculate the current in and out of the capacitor it should be less than this value.
  • Leakage: This is the dc current that flow through a capacitor that is ( typically ) due to the DC value across it. Best value is 0. Another way of thinking of it is that is the current from a resistance modeled as being in parallel with the capacitor. ( Here best value is infinity ).
  • more...

Issues

  • Sometimes people restoring old equipment just replace all the capacitors in old gear:

Links: to be completed.....

  • In a power supply ( or the rectifier section of it ) too low a value of capacitance can cause ripple in the output, too high a value can cause the rectifier diode to fail.
  • more issues.....


Making Your Own

Why would you want to make your own: often to save money on high voltage caps, or perhaps to get a special value, or just learn about them:

Types of Capacitors

Capacitors come in many types based on several different criteria: fixed vs adjustable, electric parameters, enviromental parameters, size, cost, and the type of dielectric.

Capacitor Data esp for bypass use?

Type of Dielectric

see: Types of capacitor Section: Types of dielectric -- this is generally more complete that what we want to attemp here. Our old content is:

Electrolytic

In electrolytic capacitors the insulating layer is formed by electro chemical action between the plates and other chemicals in the capacitor. This forms a very thin layer which allows large capacitance in a small package. Typically this works for one polarity and not another so electrolytic capacitors are marked with their polarity. There are some non-polarized electrolytics, but they are not common.

Some electrolytic capacitors manufactured between 1999 until today are made with bad electrolyte. Avoid these, see capacitor plague for more info.

Aluminum
Tantalum
Ceramic
  • Ceramic capacitors Ceramic capacitors - an overview, information or tutorial about the basics of the ceramic capacitor: its construction, technical information, properties and the uses of the ceramic capacitor.
Film
Polyester (Mylar)
Polystyrene
Polycarbonate
  • Polycarbonate capacitors Polycarbonate capacitors - an overview, tutorial about the basics of the polycarbonate capacitor or polycarbonate film capacitor: its construction, properties and general data and information.
Polypropylene
Teflon
Exotics
Mica
  • Silver Mica Capacitor Silver Mica Capacitor - an overview or tutorial about the basics of the silver mica capacitor, its construction, properties and the uses of silver mica capacitors particularly in RF circuits.
Glass
  • Glass capacitors Glass capacitors - an overview or tutorial about the basics of the glass capacitor, its construction, properties and the uses of glass dielectric capacitors particularly in RF circuits.
Oil
Vacuum

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