Line filter

A line filter is an electrical circuit that both electrical interference from electronic devices to the public power supply network limited ( RFI ) and electromagnetic compatibility of electrical devices against interference from the power grid improves ( increase noise immunity ).

Line filters are often low-pass filters of inductors and capacitors. The application area is mainly the low-voltage range of the power grid.

Power filters are often integrated directly into sensitive electronic equipment or in -disrupting switch mode power supplies (eg computer, TV and monitor power supplies). They are installed in special outlets and power strips or are available as stand-alone module.

Construction

Power line filters for the power supply are usually passive filter without active elements such as transistors. They consist of resistors, inductors and capacitors. The throttle is constructed as a current-compensated choke often: a plurality of windings in the same direction are located on a common ferrite core. Such chokes suppress particularly asymmetrical ( common mode ) or Gleichtaktstörströme flowing in the same direction on the leads. For the operating current of the push-pull - on both lines in the opposite direction - flows, they do, but only small inductance represents the magnetic fields of a push-pull current cancel each other and hence can not generate a magnetic field in the ferrite core. This mode chokes ( common-mode chokes ) are therefore not in a position to stop mode interference. Such disorders are, for example, as harmonics in switched mode power supplies or thyristor and possess as well as the useful currents oppositely directed polarity. In order to effectively suppress interference that other individual, uncompensated chokes are often needed such network filters are therefore larger than those exclusively with current-compensated chokes.

Examples are so-called sine filter between drives and motors or passive PFC chokes.

As a special safety capacitors suppression capacitors are used in power supply filters. They are divided into two groups:

• X capacitors: For damping push- interference voltages ( noise voltage between the leads ).
• Y- capacitors: occur to suppress noise voltages that passed through the same phase on the phase conductor and neutral conductor ground ( common mode noise ).

The latter are usually smaller than the X - capacitors in order not to increase the leakage inadmissible. You must also be very safe and excess voltage to cause at large common-mode interference, such as lightning strikes nearby, a short circuit to the housing.

Both X and Y capacitors are self-healing, that is, they cause in internal electrical breakdowns no short circuit.

Interactions with other components

RCD

The two Y capacitors on the supply lines of single-phase powered equipment of protection class I ( equipment having a protective earth connection or ground wire) cause an increased leakage: at one of the two is the full mains voltage to ground or protective conductor, it may a reactive current of 0, 5 mA ( medical devices) and 3.5 mA (other appliances with protection class I) or 5 mA (industrial assemblies ) to flow to ground.

If several devices are operated with mains filter at a residual current circuit breaker, the leakage may add up to a value above the cut-off threshold ( typically 30 mA) and it triggers. This difference is particularly high current when the voltage maximum of the AC voltage, therefore GFCI offered for delayed operation.

Housing potential

Is the safety purpose of the protection class I device is interrupted, gets its housing under current to ground potential. This is a result of capacitive voltage divider, formed by the Y- capacitors, so it is approximately 230 V / 2 = 115 volts. The currents are life-threatening, but are one reason why medical devices may only have a lower leakage current ( 0.5 mA). Their line filters are therefore different dimensions; the Y- capacitors are smaller, the inductors must be correspondingly larger.

Furthermore, this voltage can be dragged unbalanced signal connections. Here is the return conductor of the signal associated with the low-voltage mass and therefore (though wrong, but frequently encountered ) also connected to the housing of the device. If, for example to a ( non-grounded ) conventional HiFi amplifier connected a (grounded ) computer, there is a probability that you both touches the chassis of the computer and the ground connection (115 V ) of the amplifier upon insertion. Furthermore, the voltage when you plug in the wrong socket can damage the computer. So get this connector just plug when the plug of the amplifier is unplugged.

Ground loops

Is a grounded device connected (for example, a computer ) via an unbalanced signal connection to a non-grounded device, such as a hi-fi amplifier, often results in over the Y- capacitors, the ground line of the signal wire and the ground of the computer from one to above the lower distribution transformer reaching ground loop, which can be clearly heard.