coupling capacitor, also known as electric field coupling or electrostatic coupling, is a coupling method due to the existence of distributed capacitance. Coupling capacitors make the two systems of strong and weak currents coupled and isolated by capacitors, provide high-frequency signal paths, prevent low-frequency currents from entering the weak current system, and ensure personal safety. In addition to the above functions, the coupling capacitor with voltage extraction device can also extract power frequency voltage for protection and reclosing use, and play the role of a voltage transformer.

Coupling refers to the process of signal transmission from the first stage to the second stage, and usually refers to AC coupling when it is not specified. From the circuit point of view, it can always be divided into driving power and driven load. If the load capacitance is relatively large, the drive circuit needs to charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so that the drive current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins will bounce). Compared with normal conditions, this current is actually a kind of noise, which will affect the normal operation of the previous stage. This is coupling.