A coupling capacitor, sometimes known as a blocking capacitor, is used in electrical circuits to connect the signal from one segment of the circuit to another. Coupling capacitors typically serve the purpose of blocking direct current (DC) while letting alternating current (AC) go through. The main purpose for using a coupling capacitor is to block interference, make sure the signal is carried through to the next stage, and that the two connecting signals are comparable.
Capacitors are made up of two conductors that are separated by an insulation component known as a dielectric. The capacitor not only acts as a bridge and filter, as is the case with a coupling capacitor, but also as a battery. When voltage is passed through a capacitor, an electric field is created in the dielectric area and the energy created can be stored. In this way, capacitors can act like batteries. They are often used in this way to store power while batteries for a device are being recharged or used.
The capacitor can also act like a buffer to take power away from a circuit so that any fluctuations in the signal are reduced. This makes for a smoother signal and causes fewer problems with the circuitry. They can also be used to modify and correct power signals so that the maximum amount of power created is used. This is known as power factor correction.
In the case of a coupling capacitor, the signal is altered so that the AC current passes through while the DC current is blocked. The interference that occurs is therefore minimized. This helps to balance what is known as the DC bias. This DC bias is a signal used to balance out the polarity of the overall signal of the circuit, and even though it is blocked through the capacitor it is still necessary for the signal to be moved through the circuit. The coupling capacitor keeps the signals from the two circuits separated so that the signals are not off-setting or otherwise conflict with each other.
Occasionally, within a circuit there will be an unintentionally created coupling capacitor. One example of this would be found in two wires that are too close together. The signal from one wire can couple with another wire and produce interference, or signal noise. This is usually taken care of by putting some type of grounding material between the wires to break up, or ground, the signal. This type of problem is seen in items like printed circuit boards, where the signals and wires can get quite close.