Metal Filter End Caps With Size 50mm to 800mm are called the filter capacitors (aka smoothing caps) and sit in the high-voltage rail. Their job is to smooth out or filter any ripple in the rectified high voltage that feeds the valves. It is a type of pipe fitting which is used on the end of a pipe/tube or equipment nozzle. The main purpose of the end cap is to protect the leakage from the pipe end or isolate the pipe end as well.