A capacitor is an electrical device that stores energy in the electrical charge on closely spaced conductors (called 'plates').
These (usually metal) 'plates' are separated by a non-conducting material called a dieletric. The 'plates' may be flat or rolled up like a swiss roll. The dieletric may be air, paper, mica, plastic, glass, or ceramic depending upon the type of capacitor and its intended use. In electrolytic capacitors the dielectric is a liquid 'the electrolyte'. Air is a poor dieletric and is most often used in variable tuning capacitors within older style radios.
Capacitors (or condensers as Americans call them) have many uses.
- Timing - for example with a 555 timer IC controlling the charging and discharging.
- Smoothing - for example in a power supply.
- Coupling - for example between stages of an audio system and to connect a loudspeaker.
- Filtering - for example in the tone control of an audio system.
- Tuning - for example in a radio system.
- Storing energy - for example in a camera flash circuit.
The unit of capacitance is the farad, in most circuits capacitances will be tiny fractions of a farad, more commonly used is the microfarad (uF) which is one millionth of a farad, the picofarad (pF)- one thousandth of a microfarad or the nanofarad (nF)- one thousandth of a picofarad .
When capacitors are color coded they are marked with 3 or 4 spots or bands. The colors are the same as those used for Resistors, the first two bands being digits and the third band the number of noughts. A tolerance mark is not always present.
Some manufacturers indicate by means of an arrow which way to red the marks and others place a blob of paint to indicate which is the left hand end.
When selecting (or replacing) a defective capacitor, remember that electrolytic types can only be used with d.c., and that the voltage rating of capacitor is the maximum d.c voltage that it can withstand (in an a.c application you must allow for the fact that the potential difference to be considered is the peak voltage of a sinusoidal wave and is equal to 1.4 times its root mean square value). In mains applications at 240V, the peak potential difference is about 336 volts and a 350V working cpacitor is the lowest practical rating which can be chosen. As long as the storage value of a capacitor is correct, the voltage rating is unimportant as long as it exceeds any circuit voltage. The only penalty of using components of higher voltage rating than necessary is that they are more expensive, and sometimes bulkier, than the optimun component.
Failing capacitors can lead to various symptoms in equipment;
- Absence of Signals - (defective or failed filter capacitors or else try turning the volume up!)
- Distortion, crackles and whistles - (Leaky coupling capacitors or your audio equipment is on fire)
- Hum - (Faulty electrolytic capacitor. A Severe Hum indicates that group or artist doesn't know the words of the song!)
Non-polarised capacitors almost never fail because the currents they handle are generally so small. The most common method of identifying failed electrolytic capacitors is visual inspection. Such a capacitor will show one or more of these symptoms:
- Bulging of the vent on the top of the capacitor
- Sitting crooked on the circuit board as the bottom rubber plug is pushed out
- Electrolyte (a crusty brown substance) leaked onto the motherboard from the base of the capacitor
- Venting from the top of the capacitor, visible as rust-like brown deposits, or a visible hole in the vent - The build up of H2gas causes the hermetically sealed aluminum cans to burst, or partially blow out the rubber end cap
A capacitor that "explodes" while under use is something to be experienced!!
As the electrolytic capacitor ages, its capacitance decreases while its equivalent series resistance (ESR) increases. When this happens, the capacitors may no longer adequately serve their purpose.
While faulty Capacitors can affect any type of circuit, computer motherboards appear to be more prone to the problem.
To replace a blown capacitor in a circuit you should proceed as follows; DO NOT attempt this unless you really know what you are doing.
- Prepare your soldering iron, solder, utility knife (or something else to cut the capacitor legs short). Start heating your soldering iron...
- De-solder the old capacitor(s)! Check the circuit board to see if the polarity is indicated (usually with a samll "+" sign) otherwise pay attention to polarity when you are de-soldering the faulty component.
- Reshape the legs of the new capacitor if necessary; ie.,trim the long legs, so you'll have legs only long enough to sit on the circuit board firmly.
- Solder the capacitors on the circuit board. Pay attention to the polarity on all electrolytic capacitors. Put them on the board the correct way round, otherwise they may explode.
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