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CAPACITORS

V. Ryan © 2002-09

 

PDF FILE - CLICK HERE FOR PRINTABLE EXERCISE BASED ON WORK BELOW

 

Capacitors are components that are used to store an electrical charge and are used in timer circuits. A capacitor may be used with a resistor to produce a timer. Sometimes capacitors are used to smooth a current in a circuit as they can prevent false triggering of other components such as relays. When power is supplied to a circuit that includes a capacitor - the capacitor charges up. When power is turned off the capacitor discharges its electrical charge slowly.

   

A capacitor is composed of two conductors separated by an insulating material called a DIELECTRIC. The dielectric can be paper, plastic film, ceramic, air or a vacuum. The plates can be aluminium discs, aluminium foil or a thin film of metal applied to opposite sides of a solid dielectric. The CONDUCTOR - DIELECTRIC - CONDUCTOR sandwich can be rolled into a cylinder or left flat

   

 

   

HOW A CAPACITOR WORKS

   

When the circuit is switched on, the LED emits light and the capacitor charges up. When the switch is turned off the LED stills emits a light for a few seconds because the electricity stored in the capacitor is slowly discharged. When it has fully discharged it's electricity the LED no longer emits light. If a resistor is introduced to the circuit the capacitor charges up more slowly but also discharges more slowly. What will happen to the light ?

   
 
   

Electrolytic capacitors are ‘polarised’ which means they have a positive and negative lead and must be positioned in a circuit the right way round (the positive lead must go to the positive side of the circuit).
They also have a much higher capacitance than non-electrolytic capacitors.

Non-electrolytic capacitors usually have a lower capacitance.
They are not polarised (do not have a positive and negative lead) and can be placed anyway round in a circuit.
They are normally used to smooth a current in a circuit.

CAPACITANCE - means the value of a capacitor.

 

Notice the electrolytic capacitors above. They all have two polarised leads, in other words they have a positive and negative leg. This type of capacitor is used with ICs such as the 555 timer chip and it is the capacitors and resistors that determine the timing sequence.

 

Look carefully at the photographs of the two types of capacitors. Can you work out which one is electrolytic and which is non-electrolytic ?

The simple circuit below is basically a switch which is connected to a computer. When the switch is pressed the computer detects that the relay is closed and then turns on a motor.
However, there is a problem. When the switch is pressed it only closes the relay for a split second and this is not enough time for the computer program to detect that it has been pressed in the first place. A time delay is the obvious answer and this can be achieved by adding a capacitor in parallel to the switch. If the relay is held closed for 3/4 seconds then the computer program will have time to detect it - A capacitor provides the time delay.

 

REMEMBER - there are polarised and non-polarised capacitors. Look for a positive and negative sign.

 
   

CLICK HERE FOR EXAMPLES - HOW CAPACITORS CAN BE USED

 
 
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