CAPACITORS- WHAT IS A CAPACITOR?

It’s almost impossible not to talk about capacitors when talking about electricity. A capacitor is an electrical component used to store energy in an electric field. It has two electrical conductors separated by a dielectric material that both accumulate charge when connected to a power source. One plate gets a negative charge, and the other gets a positive charge.

Unlike a resistor, a capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates. Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass.

Categories of capacitors

There are all sorts of capacitor with certain features. Capacitors can be categorized by:

Size: Although capacitors could be the largest component in a circuit, they can also be small. The more the capacitance; the larger the capacitor. The size of a capacitor could be in terms of its volume or capacitance.

Leakage current: Every cap is prone to leaking some tiny amount of current through the dielectric, from one terminal to the other. This tiny current loss is called leakage. Leakage causes energy stored in the capacitor to slowly, but surely drain away.

Tolerance - Capacitors also can't be made to have an exact, precise capacitance. Each cap will be rated for their nominal capacitance, but, depending on the type, the exact value might vary anywhere from ±1% to ±20% of the desired value.

Maximum voltage - Each capacitor is rated for a maximum voltage that can be dropped across it. Some capacitors might be rated for 1.5V, others might be rated for 100V. Exceeding the maximum voltage will usually result in destroying the capacitor.

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COMMON USES OF CAPACITORS

• AC coupling/DC blocking – the component allows only AC signals to pass from one section of a circuit to another while blocking any DC static voltage. They are commonly used to separate the AC and DC components of a signal. In this method, it is necessary to ensure that the impedance of the capacitor is sufficiently low. The capacitor rated voltage must be greater than the peak voltage across the capacitor. Usually, the capacitor will be able to withstand the supply rail voltage with some margin to ensure reliability.

• Power supply decoupling – The capacitor is used to decouple one part of a circuit from another. Decoupling is done when an incoming line signal is taken through a transformer and a rectifier; the resulting waveform is not smooth. It varies between zero and the peak voltage. If applied to a circuit, this is most unlikely to operate because a DC voltage is typically needed. 

• Filter AC noise from DC circuits – Any AC signals that may be on a DC bias point, power rail, or other nodes that need to be free of a particular varying signal should be removed by the capacitor. It must also be able to withstand the supply voltage while supplying and absorbing the levels of current arising from the noise on the rail.  

• Audio signal filtering – it is necessary to consider the capacitor’s RF performance. This performance can be different at lower frequencies. Ceramic capacitors are usually used here since they have a high self-resonant frequency, specifically the surface mount capacitors that are very small and have no leads that can cause any inductance.

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