5P : ake your own capacitor
|In this practical, you will make your own capacitor and use it to control the frequency of an astable circuit, producing pulses. By monitoring frequency changes, you can find out what happens to the size of the capacitor when you change the area of overlap of the plates, or change the space between them.|
|About capacitors||Adding an amplifier|
|Building an astable|
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Capacitors store electric charge. A capacitor consists of two plates of a conducting material separated by a space filled by an insulator. The insulating layer is called the dielectric of the capacitor.
Capacitance is measured in units called farads, F, where:
|DEFINITION:||1 farad stores 1 coulomb of electric charge when 1 volt of potential difference is applied|
The farad is a very large unit of capacitance. Practical measurement units are much smaller:
|µF||microfarad||10 -6 F|
|nF||nanofarad||10 -9 F|
|pF||picofarad||10 -12 F|
Things about capacitors which can be changed include:
the area of overlap of the plates
the distance between the plates
the material used as an insulator
You can change all of these in the experiments which follow.
The diagram below shows you how to build a circuit in which a capacitor controls the frequency of a pulse generator, or astable, circuit:
Follow the diagram carefully. Use the colour code convertor to work out the colour code of the two resistors.
The capacitor consists of two sheets of aluminium foil. Leave the foil inside the plastic packets and make connections using crocodile clips. The crocodile clips must not touch each other.
The astable is made using an integrated circuit, the 4060 cmos. Here is the pin connection diagram for the 4060:
The 4060 contains an astable section followed by a binary counter:
To get the astable section to produce pulses, you need to add two resistors and a capacitor:
In this circuit, R1 is the 390 resistor and RT is the 47 timing resistor. CT is the capacitor made up of the two sheets of aluminium foil.
The initial frequency of the pulses can be divided by up to 214 times. Use the oscilloscope to probe the counter outputs. You will find the fastest pulses at pin 7 of the 4060, with slower pulses at the other counter outputs. When you move the probe to pin 5, the frequency of the pulses is halved compared with the frequency at pin 7, and is halved again if you move the probe to pin 4. The slowest pulses can be observed at the final output of the counter, pin 3.
Connect the probe to any one output. What happens to the frequency of the pulses on the oscilloscope screen when the area of overlap of the capacitor plates is increased?
What happens to the frequency of the pulses when the plates are pressed closer together?
It is nice to see the pulses on the oscilloscope screen, but it is more fun to hear the changes in the frequency of the pulses by adding an audio amplifier stage to your circuit:
Up to previous stage
The audio amplifier integrated circuit is an LM 380. The circuit diagram for the amplifier stage is:
Once the circuit is complete, the sound produced will be quite loud. The frequency should change when the area of overlap of the capacitor plates is changed, or when they are pressed closer together. A lower frequency means that the capacitor is taking longer to fill up and empty. In other words, lower frequencies indicate an increase in the size of the capacitor.
The effect of changing the insulator between the capacitor plates is difficult to investigate properly in this experiment. However, changing the insulator can change the size of the capacitor.
Insulators used in making non-polarised capacitors include polyester, polythene and ceramic. A thin layer of aluminium oxide forms the insulating layer in most polarised capacitors.
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