Senin, 04 Juli 2011

Ceramic Capasitor

Ceramic capacitors consist of a flat plate of ceramic material with an electrode on each side. The most common shapes for ceramic capacitors are disk, for through hole mounting techniques, and rectangular monolithic or multilayer surface mount devices. Ceramic capacitors come with various temperature dependent tolerances. The lowest cost, and loosest are usually designated Y5 and Z5 (see EIA coding below). They have their nominal value centered around 25 - 40 degrees C and diminishing on either side. Having a linear relationship across temperature, the Negative Positive or NP series start out with a higher than nominal capacitance at colder temperatures and decrease below nominal at higher temperatures. NP capacitors usually have a label on them designating them as NP, followed by a number. The number designates the number of parts per million per degree C change in the capacitance. The most stable type is classified NP0. NP0 capacitors are available with tolerances as close as ± 3% of nominal. From there, the slope of the tolerance variation with temperature increases to 150, 330, 750, and 1500 ppm/°C.

Ceramic capacitors are available in values as low as 0.5 pF. However, be wary that half a picofarad of capacitance can easily be swamped by the parasitic capacitances of the printed circuit board, the resistance of the solder flux, or the inductance of the capacitor leads or circuit traces.

Ceramic capacitors in values between 10 and 220 nanofarads are used as high frequency bypass elements for digital logic circuits. For digital logic, loose tolerance values such as Z5U are used. Additionally, ceramics are very good for use in radio frequency circuits. NP0 types are generally used due to their low, predictable, temperature variance.

The Electronic Industry Association uses a code to label ceramic capacitors. The code consists of three characters. The first two characters designate the temperature range. The last character designates the percentage of change over the specified range.

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