Lexicon DIY-Knowledge





A capacitor or condenser is an energy storage unit which can store a specific energy quantity (Q) depending on its type and version. Its storage capacity is called capacitance, its unit of measurement is farad (F). The following formula is used to calculate the stored energy:

Q = C x U(As)

From this you can calculate the capacitance 


C    =                           


For practical purposes, the unit of measurement 1 farad is much too big and only fractions of it can be used.

1 millifarad mF = 1 x 10-3 F
1 mikrofarad µF = 1 x 10 -6 F
1 nanofarad nF = 1 x 10 -9 F
1 picofarad pF = 1 x 10 -12 F

Motor starting and running capacitors operate in the µF range, suppression capacitors in the nF and pF range. Power tools are mainly equipped with broadband suppression capacitors to ensure adequate radio interference suppression.

Capacitors consist of two metal foils separated by a dielectric. Size, distance and insulation strength of the dielectric determine the capacitance. Air, oil, paper, plastic foils or ceramics are used as dielectric which leads to a wide range of capacitor designs.

Motor capacitors are often MP capacitors made of metallised paper wound up in a cup or tube shape. Metallised paper has self-healing properties, i.e. perforations are burnt out. Capacitors block direct current, yet during the charging and discharging process there is a measurable current going in and out.

However, alternating current charging and discharging processes follow the AC sinus curve, so that an ammeter connected to the feeder wire would register current as if the alternating current was flowing through the capacitor. This is a rather complex technical process and will not be further discussed here.