Paper in oil capacitor

Paper in oil capacitors are electronic components made from paper, oil, and aluminum or tin foil. They are used in antique electronic equipment, electric guitars and basses, and intercontinental ballistic missile (ICBM) guidance systems.

Paper (film) capacitors (MP) and mixed film capacitors

• Power film capacitors using metallized paper as carrier of the electrodes, their different configurations and their usual abbreviated designations
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  MP capacitor, single-sided metallized paper (additional layer of paper to cover pinhole defects), windings impregnated with insulating oil

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  MKP power capacitor, single-sided metallized paper and polypropylene film, (mixed dielectric), windings impregnated with insulating oil

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  MKV power capacitor, double-sided metallized paper (field-free mechanical carrier of the electrodes), polypropylene film (dielectric), windings impregnated with insulating oil

Historically, the first “film” type capacitors were paper capacitors of film/foil configuration. They were fairly bulky, and not particularly reliable. As of 2012, paper is used in the form of metallized paper for MP capacitors with self-healing properties used for EMI suppression. Paper is also used as an insulating mechanical carrier of metallized-layer electrodes, and combined with polypropylene dielectric, mostly in power capacitors rated for high current AC and high voltage DC applications.

Paper as carrier of the electrodes has the advantages of lower cost and somewhat better adherence of metallization to paper than to polymer films. But paper alone as dielectric in capacitors is not reliable enough for the growing quality requirements of modern applications. The combination of paper together with polypropylene film dielectric is a cost-effective way to improve quality and performance. The better adhering of metallization on paper is advantageous especially at high current pulse loads, and the polypropylene film dielectric increases the voltage rating.

However, the roughness of a metallized paper surface can cause many small air-filled bubbles between the dielectric and the metallization, decreasing the breakdown voltage of the capacitor. For this reason, larger film capacitors or power capacitors using paper as carrier of the electrodes usually are filled with an insulating oil or gas, to displace the air bubbles for a higher breakdown voltage.

However, since almost every major manufacturer offers its own proprietary film capacitors with mixed film materials, it is difficult to give a universal and general overview of the specific properties of mixed film capacitors.

This type of capacitor may contain PCB's, which are hazardous to human health.