Line filter
A line filter is an electrical circuit that both limits electrical interference from electronic devices in the public power supply network ( radio interference suppression ) and improves the electromagnetic compatibility of electrical devices against interference from the power network (increased interference immunity).
Line filters are often low-pass filters made from inductors and capacitors . The area of application is mainly the low-voltage area of the power grid .
Line filters are often integrated directly in sensitive electronic devices or in switching power supplies that cause interference (e.g. computer, television and monitor power supplies). They are also built into special sockets and socket strips or are available as a stand-alone module.
construction
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Line filters for the line voltage are mostly passive filters without active elements such as transistors. They consist of resistors , chokes and capacitors . The choke is often designed as a current-compensated choke : several windings in the same direction are accommodated on a common ferrite ring . Such chokes suppress asymmetrical ( common mode ) or common-mode interference currents that flow in the same direction on the supply lines particularly well . For the operating current that flows in push-pull - on both lines in opposite direction - they represent only a low inductance. The magnetic fields of a push-pull current cancel each other out and consequently cannot generate a magnetic field in the ferrite core. This mode chokes ( common-mode chokes ) are therefore not in a position mode noise to stop. Such disturbances arise z. B. as harmonics in switched-mode power supplies or thyristor controllers and, like the useful currents, have mutually opposite polarity. In order to effectively suppress such disturbances, further individual, uncompensated chokes are often necessary, such line filters are therefore larger than those exclusively with current-compensated chokes.
Examples are so-called sine filters between frequency converters and motors or passive PFC chokes .
Special safety interference suppression capacitors are used as capacitors in line filters . They are divided into two groups:
- X capacitors: For attenuating push-pull interference voltages (interference voltage between the supply lines).
- Y capacitors: To suppress interference voltages that occur in the same phase on the outer conductor and neutral conductor with respect to earth ( common-mode interference ).
The latter are usually smaller than the X capacitors in order not to increase the leakage current impermissibly. They must also be particularly safe and surge-proof in order to deal with large common-mode interference, e.g. B. Lightning strike nearby, not to cause a short circuit to the housing.
Both X and Y capacitors must not cause a fire in the event of failure, so X capacitors in particular are self-healing; In other words, they do not cause a short circuit in the event of internal electrical breakdowns , but only lose capacitance. A short circuit of the X-capacitors, which are designed as film capacitors, could otherwise set the dielectric or the insulating housing on fire.
Interactions with other components
FI switch
The two Y capacitors on the supply lines of single-phase powered devices of protection class I (devices with protective earth connection or protective conductor ) cause an increased leakage current: one of the two carries the full mains voltage to earth or protective conductor, a reactive current of 0 , 5 mA (medical devices) or 3.5 mA (other devices of protection class I) or 5 mA (industrial assemblies) flow to earth.
If several devices with a line filter are operated on one residual current circuit breaker , the leakage currents may add up to a value above its non-tripping residual current (0.5 * rated residual current ) and it trips. This differential current is particularly high when switching on at the maximum voltage of the mains AC voltage, which is why FI circuit breakers are offered with delayed response behavior.
Housing potential
If the protective conductor of a protection class I device is interrupted, its housing becomes live against earth potential. This is due to the capacitive voltage divider that the Y capacitors form, so it is approx. 230 V / 2 = 115 volts. The currents are not life-threatening, but are one reason why medical devices may only have a lower leakage current (0.5 mA). Their line filters are therefore dimensioned differently; their Y capacitors are smaller, the choke inductances must be correspondingly larger.
Furthermore, this voltage can be "dragged" via asymmetrical signal connections. The return conductor (ground) of (audio) signal connections is usually connected to the extra-low voltage ground and also to the housing of the devices for shielding . For example, if an LF amplifier ( protection class III , usually ungrounded housing) is connected to a personal computer (protection class I, earthed housing), there is a likelihood that the leakage current will flow into the signal input when it is inserted. The voltage can damage the electronics when plugged in. Therefore, such plug connections should only be made when at least one of the power plugs of the devices is unplugged. The same applies to digital data connections, which is why the ground contact with USB plugs is leading.
Ground loops
If a grounded device (for example a computer) is connected via an unbalanced signal connection to a likewise grounded device, for example a radio receiver with a connected grounded antenna, there is often a hum loop through the ground line of the signal cable and the grounding , which cannot be remedied with line filters is.