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How a Capacitor Works as an EMI/EMC Filter

The capacitor is a component/device that stores or “holds up” an electrical charge into the electronic circuitry. This applied charge and discharge method greatly reduces the magnetically driven electrical impulses; plus can turn an electrical flow constant into a series of electrical impulses, reducing or eliminating the culprit RFI / EMI.

The Simple Process of Capacitance

For a capacitor to work, there needs to be at least two or more parallel metallic plates separated by a material called a dielectric. The dielectric must be made of a non-conductive insulating material. There are several types of dielectrics such as paper, plastic, ceramic, glass and even an air gap.

When a capacitor is placed into a circuit with active current, the electrons from the negative side of the cap builds up on the closest plate which, in this case, is the negative plate. Once the collector plate can no longer hold the stored energy, that energy is forced past the dielectric material and onto the positive plate, thus displacing the stored electrons back into the circuit. The process of stored energy release is simply called a capacitive discharge.

Another way to visualize the action of a capacitor is to imagine it as a water tower hooked to a water pipeline. A water tower “stores” water and when the water system produces more water than a building or town requires, the excess water is then stored within the water tower until needed.  Then, at time of high water demand, the excess stored water is released out of the tower keeping the water pressure up, and flowing the water into the building or town.

A capacitor stores the electrons in the same way and releases the stored energy at a later time when the demand for current flow arises within the electronic circuitry.

What are EMI / EMC Filters and why are they used?

Electronic devices, such as circuit card assemblies, contain both wanted and unwanted radio frequency (RF) interference.  Therefore, some type of electronic filtering technique must be applied to separate the two types of RF signals, thusly, separating the culprit noise (the creator of the unwanted signal on the PCB) from the victim (the electronic components that are being adversely affected by the unwanted RF signal).

Every electronic design produces some form of RF signals that will in turn require some form of EMI filtering; either in the data processing section, signal amplification, or for more complex digital-signal processing, which also known as (DSP).

All electronic systems, whether they are Analog or Digital, are sources of unwanted RF oscillations, either from clocks or other timing devices, as well as sources for radiated emissions from switching devices which can emanate and couple onto other un-protected components or external equipment.

Electronic Circuit Designers must base their filter design selections on the desired bandwidth and accuracy required for the target system while assuring that the unwanted RF noise, which is the main source for EMI / EMC interference, is virtually eliminated.

Important Note: ALL CAPACITORS ARE FREQUENCY RESPONDENT; which simply means that by just placing any capacitor of incorrect value into a culprit circuit you may not have the required electromagnetic filtering effect. Thus, choosing the wrong capacitance values will have a severe adverse effect on the culprit circuitry, if not carefully applied.

Why filtering at the connector can have better EMI / EMC performance controls, than just filtering at the PCB level for EM & RF compliance, (food for thought).

Filtered connectors generally have solid ground planes surrounding their filter components that provide the lowest impedance path for the filter, plus, provide increased RF shielding throughout the entire connector area.

Additionally, by using filtered connectors as opposed to filtering at the PCB level; RFI and EMI is eliminated at the source noise that will be picked up from the contacts / leads between the on board filter components. In addition since PCBs are getting smaller & smaller the filtered connector will free up the much needed circuit board space normally required to implement the filtering on the PCB itself.


We hear the words Milli-farad, Micro-farad, Pico-farad, and Nano-farad when engineers talk about capacitive values. So, why exactly is that unit of measured capacitance called a farad?

The term “farad” was originally coined by Latimer Clark and Charles Bright in 1861 in honor of Michael Faraday; as a unit of a quantity of a stored electrical charge within a vessel. In 1881 at the International Congress of Electricians in Paris, the name farad was officially used for the unit of electrical capacitance.

A single (1) farad capacitor can store one coulomb (pronounced coo-lomb) of charge at 1 volt.

A coulomb is expressed as: 6.25e18 (6.25 * 10^18) or more simply put, 6.25 billion, billion, electrons.

One amp (or ampere) represents a rate of electron flow of 1 coulomb of electrons per second, so a (1) farad capacitor can hold a 1 amp-second of electrons at 1 volt.

A single (1) farad capacitor is pretty large, and it can be as big as a can of tuna, or even a 1-liter soda bottle, depending on the voltage it is required to handle.

To get some perspective on how big a farad is, think about this:

A typical alkaline AA battery of 1.5 VDC holds about 2.8 amp-hours.

That means that typically a AA battery can produce 2.8 amps for an hour at 1.5 volts, or about 4.2 watt-hours.

Using a 1.5 volt DC battery as an energy source, in order to store one AA battery’s entire energy into a capacitor, it will require 3,600 * 2.8 amps or 10,080 farads to hold / store that energy.

Note: An amp-hour is measured as 3,600 amp-seconds.

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