Filter capacitor


Filter capacitor

Filter capacitors are any capacitors used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:
* Glitch removal on dc power rails
* Radio frequency interference (RFI) removal for signal or power lines entering or leaving equipment
* Capacitors used after the reservoir capacitor to further smooth dc power supplies
* capacitors used in audio, IF or RF frequency filters (eg low pass, high pass, notch, etc)
* arc suppression

Filter capacitors are not the same as reservoir capacitors, the tasks the two perform are different, albeit related.

Types

Many filtering tasks have specific types of capacitor that are required or typically used for the task.

Power rail filtering

Electrolytics are usually used due to high capacity at low cost and low size. Smaller non-electrolytic caps may be paralleled with these to compensate for electrolytics' poor performance at high frequencies

RF work

Ceramic plate capacitors are usually favoured due to extremely low inductance and low cost. Where precision is needed, silver mica capacitors offer superior precision and stability. Where manual tunability is required, plastic film trimmers are sometimes used, though it has long been more popular to adjust the inductor to achieve tuning.

UHF

Disc and plate ceramic capacitors are used due to particularly low inductance. Often a ceramic disc sits in a notch cut in the PCB, with the tracks soldered directly to the disc for best performance.

Microwave

At microwave frequencies, the (FR4) PCB material acts as a capacitor, and the PCB track has inherent inductance. The nett result depends on the ratio of capacitance and inductance present.

Increasing PCB track width causes additional capacitance, and thus a nett capacitance. Narrowing track width reduces capacitance, produces a nett inductance.

Computer power rail filtering

Computers use large numbers of filter caps, making size an important factor. High currents and low voltages also make low effective series resistance (ESR) important. Tantalum capacitors meet all these requirements and are thus commonly used. Tantalums have an additional issue which must be addressed during the design stage, namely that if their current rating is exceeded they are liable to explode. Tantalums are a type of electrolytic capacitor, but when the word 'electrolytic' is used it normally refers to aluminium electrolytics.

Lowest cost

Ceramic (for values <0.47uF) and electrolytic (for 0.47uF and up) are normally the preferred types where their performance is sufficient, since these are the lowest cost types of caps. Hence they are very popular in filters of many types.

Mains filtering

Mains filter caps are usually encapsulated wound plastic film types, since these deliver high voltage rating at low cost, and may be made self healing and fusible. The additional safety requirements for mains filtering are:
* Live to neutral capacitors are flame retardant, and in Europe are required to use use class X dielectrics
* Live or neutral to earth: The requirements for L to N use apply, plus the dielectric must be self healing and fusible. In Europe these are class Y capacitors.

Contact suppression

A wound plastic film mains rated capacitor plus series resistor are incorporated into a single component envelope for convenience and robustness. This reduces switch arcing and RFI. The most common combination of values is 0.1uF + 100 ohms.

DC motor suppression

Ceramic disc capacitors are usually used on low voltage motors for their low inductance and low cost.

Speaker crossover networks

Nonpolar aluminium electrolytics combine sufficient capacitance with low cost. In high end systems they may be parallelled with small non-electrolytic capacitors to improve their performance.


=Switched mode power supply filtering=

Low ESR (effective series resistance) electrolytics are often required to handle the high ripple current.


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