Bredhurst Receiving and Transmitting Society

3. Technical Basics

3d Capacitors

3d.1 Recall that a capacitor consists of two metal plates separated by an insulating material.

The simplest of all capacitors is two pieces of conductive material (metal plates) separated by an insulating material called a dielectric. The dielectric can be as simple as air or it could be plastic, or in fact anything that is an insulator. The dielectric does not need to surround the plates but just to separate them.

3d.2 Understand that a capacitor can store an electric charge, and that its ability to store a charge (capacitance) depends upon the area of the plates and their separation.

Even the two metal plates as a capacitor has the ability to store an electric charge and that the term given to this ability to store the charge is called capacitance.

The size of the capacitance is depended upon the area of the plates and the amount of their separation. The bigger the plates the greater the charge that can be stored and the closer together the plates are also the bigger the charge.

The electric charge can be provided to the plates by attaching a positive link to a battery to one plate and a negative link to a battery to the other plate. A current will flow but only until the potential difference on the places equates to that of the battery. Then the current stops flowing.

3d.3 Understand that capacitors block direct current but allow alternating current to pass by continuously charging and discharging.

The effect described above of the current flowing in a DC circuit until the plates are charged is used to effectively BLOCK a DC current.

However, when an AC is applied to a capacitor then the current continues to "apparently flow" due to the continuous charging and discharging of the capacitor each cycle of the AC wave form.

3d.4 Recall that larger values of capacitor may be polarised and must be correctly connected.

There are two main categories of capacitor polarised and non polarised and then subdivisions of these categories into various type according to their construction.

With the non polarised capacitor may be connected into a circuit and the capacitor does not mind which way round its terminals are connected.

Students are not picking up this important point that if a polarised capacitor is connected into a circuit and care is not taken to ensure that the positive side is connected the positive feed and the negative to the negative feed then there a risk of an explosion of the capacitor.

Large Capacitors that are polarised must be connected in the circuit with the correct polarity by reference to the circuit.

The larger value capacitors are of the polarised style and thus due to their bigger capacity pose and even greater danger if incorrectly connected into a circuit !

It is necessary to know that capacitors have 3 different circuit diagram symbols.

		Standard (general) capacitor where does not matter which way round it is connected.
		Polarised capacitor where it does matter which way round it is connected.
		Variable capacitor.