Capacitors

A capacitor is a discrete component which can store an electrical charge for a period of time. The larger the capacitance the more charge it can store.

The unit of measurement of a capacitor is the farad. Often you will see capacitors of much less than a farad. These will be measured in microfarads (one millionth of a farad or 1/1,000,000) or picofarads (one million-millionth of a farad or 1/1,000,000,000,000).

There are two types of capacitor:

• polarised or electrolytic capacitors, and
• non-polarised or non-electrolytic capacitors

Polarised capacitors

• 

  Polarised (electrolytic) capacitor

  These generally have larger capacitance values. Polarised capacitors have a positive pole and a negative pole, so you have to connect them to a circuit the correct way round. The polarity and value of a capacitor are usually shown like this:

• Mounting of polarised capacitors

  

  Image shows two electrolytic capacitors. One is axially mounted, one is radially mounted.

  Electrolytic capacitors may be either axially mounted (on their side, connected at each end) or radially mounted (upright with both connections at the bottom).

Non-polarised capacitors

These are usually much smaller than the polarised type, and have smaller capacitance values ranging from a few picofarads to a few microfarads. Because they have no positive or negative poles these capacitors can be connected to a circuit either way round. There are four types of non-polarised capacitor, each named after the material they are made from:

• Polyester
• Polystyrene
• Mica
• Ceramic

Applications of capacitors

• Smoothing rectified alternating current voltages into steady direct current voltages
• Blocking direct current signals whilst allowing alternating signals to pass
• Filtering out unwanted portions of a fluctuating signal
• Timing applications
• Storing charge to keep a transistor turned on or off