How an EMI ESD Filter Works
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ESD and EMI
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ESD stands for electrostatic discharge, and EMI stands for electromagnetic interference. Both are types of electromagnetic activity that can be dangerous or disruptive for advanced electronics such as computers, cell phones and MP3 players. ESD is caused by the admittance of some kind of electric charge into semiconductors and other sensitive electronic devices--in other words, an outside source produces an electric shock that damages the system. The shock does not have to be large, and the short zap that the human body can sometimes produce is enough to overload and fry more sensitive systems. Shocks introduced from other systems can be even more powerful.
EMI is usually not as destructive, but it can still be very annoying. If there are no protections in place, other signals and electrical activity can generate EMI that enters the electrical cables of the computer system and confuses the digital relay of data. This means the system will have a difficult time recognizing data or commands, and connections will waver or be lost. Since most computer devices are used around other electrical equipment that is also producing different kinds of electromagnetic waves, EMI is always a possibility.
Casings and Filters
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There are several ways to protect against ESD, and one of the most common is building a Faraday cage around the equipment in the form of a cover, whether it be the shell of your computer tower or the case of your cell phone. These covers are made to divert the electric discharge away from the system itself, and the only weak spot is where cables and connections enter the cover. Since ESD can still sneak in through these port spaces, a filter is often used to dissipate the energy before it can do any damage. Most computer ports use a pair of conductors to maintain a flow of power when cables are connected. By placing metal shielding on and around these conductors, manufacturers can ensure the electrical current will only travel in a specific direction, and unexpected discharges will not be able to escape one way or the other.
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Inner Protection
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Additionally, to prevent any damage to the system in case a discharge does make it past the casing and filters, many manufacturers will create a simple, low-impedance voltage capacitor and a ferrite bead to draw off as much energy as possible, hopefully enough energy to keep the shock from damaging any sensitive parts. Fortunately, these protections, including the conductor filter, all work to keep EMI in as well as ESD out. Any EMI signals that are in danger of escaping the system and interfering with others first have to run the gauntlet made of the voltage capacitor, ferrite bead and conductor filters. These are usually enough to draw off and dissipate any escaping energy, keeping the computer from interfering with other systems.
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