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|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 (follow link 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.|
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