Conductors that carry electrical signals through an enclosure or a printed circuit board are called feedthrough conductors. There is a considerable difference in pressure along the length of the conductor, and the high pressure electrical feedthrough conductor has a set minimum value of built-in capacitance, which is useful in ultra high-frequency applications.
The humble spark plug is the simplest example of a high-pressure electrical feedthrough capacitor.
The spark plug case withstands the pressure and temperature produced in the engine while transmitting a reliable electrical connection to the spark gap in the combustion chamber.
High-pressure electrical feedthrough is also used in precision instrumentation like an electron microscope. The microscope can only operate in a vacuum, but it needs the electrical current supplied through a pressured container to operate. Similarly, underwater craft needs a feedthrough connection to form a communication channel between the exterior instruments and the pressure cabin controls.
Several categories of these feedthrough capacitors are available on the markets. The two main categories are power feedthrough and instrumentation feedthrough. Power feedthroughs carry high current electrical voltage, while the instrumentation feedthroughs carry low voltage electrical signals. A more specialized type, the RF feedthrough, is designed to carry very high-frequency RF or microwave signals.
