Figure 3.3 Construction of Plastic Film Capacitor. Construction of a capacitor begins with a 4 layer sandwich as follows. The bottom layer is
A thin film of plastic, then a layer of aluminum foil, next is another layer of the plastic insulation, then another layer of aluminum foil. This sandwich is made in the form of a strip about 2 or 3 inches wide and many feet long. The two layers of plastic film are aligned side to side along the width dimension but the aluminum foils are not. The second layer which is aluminum foil sticks out a little on the right. The top layer which is also aluminum foil sticks out a little on the left. In the drawing most of the many feet of length has already been rolled up with the bottom insulating layer on the outside. When the roll is complete one of the foils is sticking out on the right and the other one on the left. Contact is made by the same nail like objects as used in carbon composition resistors. The manufacturers know how long and wide to make the original sandwich to obtain the desired capacitance value. The whole assembly is slipped into a plastic tube and the ends sealed with epoxy. Capacitance value, voltage rating and manufacturer's name are printed on the outside of the tube. This completes the verbal description.