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Design & Technology



A selection of electrical capacitors

A capacitor is a discrete component that can store an electrical charge. The larger the capacitance the more charge it can store.

The unit of measurement of capacitance 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
  • non-polarised or non-electrolytic capacitors

Polarised capacitors

These generally have larger capacitance values. Polarised capacitors have a positive pole and a negative pole, so they must be connected to a circuit the correct way round.

Mounting of polarised capacitors

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

A capacitor is marked with the polarity of the wires and the capacitor value.

Radially mounted capacitors have both poles on the bottom. Axially mounted capacitors have the poles coming out of the sides.

Non-polarised capacitors

These are usually much smaller than the polarised type, and have smaller capacitance values. These might range from a few picofarads to a few microfarads. They don't have positive or negative poles so they can be connected to a circuit either way round.

Applications of capacitors

Capacitors are used to smooth rectified alternating-current voltages into steady direct-current voltages. They can also be used to filter out fluctuations in a signal.

Capacitors are often used in series with resistors to achieve a time delay. The time it takes for the capacitor to become charged is related to the size of the capacitor and the value of the regulating resistor.

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