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Line-Filter Capacitors . A special class of capacitor is used in power-line (mains)
filters. Line filtering is a very tough application for a capacitor. Power lines are subject to a variety of disturbances that induce transients and surges. These vary from a few hundred or a few
thousand volts (motors and triac controllers) to many thousands of volts (lightning). The filter capacitor has to endure these transients for years without failure, or at worst, failing without exploding, catching
fire, or subjecting the user to the risk of shock.
Line-filter capacitors are used for a number of purposes. The first is to keep potentially disruptive or damaging line transients and EMI out
of susceptible equipment.
The third use of line-filter capacitors is in a sort of snubber called a "spark quencher". A spark
quencher is a potted series R-C network, where the capacitor is typically from 0.01 to 2 uF, and the resistor
is from 10 to 500 ohms. The resistor lowers the "Q" to suppress L-C ringing. People who make the capacitors will usually make the spark quenchers as well. Spark quenchers are used to protect switches,
relays, triacs, and scrs from transients generated from switching inductive loads, and to suppress RF generated by contact arcing. Ideally, both the the R and the C should be correctly sized for each application;
as the load current goes up, the capacitor size goes up and the resistor value falls. The resistor type is not always stated, but some people use carbon composite resistors for their good peak current handling
capability. Not every manufacturer makes a broad selection of R-C combinations however. Spark quenchers are aimed at relatively low current and low voltage applications, no more than 10 A and 250 Volts AC or
DC. For more information on spark quenchers and contact protection, try these sites:
Some consumer electronics uses the mains as a cheap (if dirty) antenna. A small but very reliable
capacitor must be used to pass RF, but block line frequency and protect the user. They are also used to isolate the antenna from high-voltage DC in the transmitter.
Epoxy-filled metallized paper, metallized polyester film, polypropylene, and ceramic (typically Z5U) are
the common dielectrics for line-filter capacitors although film-foil polyester, and paper-polyester, have also
been used at one time or another. The film dielectrics are used both with and without oil impregnation. The
metallization can be aluminum, but zinc and other metals may be used for better self-healing. Why the variety
of materials? Some manufacturers (see Rifa) argue that epoxy-paper is safer and more reliable because of its
better self-healing properties and voidless construction (which helps prevent corona). Also, epoxy-paper
canīt be made to burn. However, the conventional plastic films, with their higher dielectric strengths, result in
a smaller part where space is critical. Epoxy-paper capacitors can be 30-100% larger in volume of their
plastic-film equivalents, and they are not found in the higher values. This has helped to make polyester and
polypropylene the more popular dielectrics. Epoxy-paper's other electrical properties (temperature drift, dissipation factor) are poor so you won't see this dielectric in other applications.
Line-filter capacitors come in two general classes (as defined by the International Electrotechnical
Commission (IEC), the international standards organization for electrical and electronics matters) and must be
approved by whichever safety agency holds sway where the equipment will be sold. In fact, you can always recognize a line-filter capacitor by the "EN132400" printed on it, plus agency acronyms (UL, VDE, CSA,
etc.). Older capacitors would have the symbols for SEMKO, DEMKO, and others, but these no longer exist as governmental agencies and are, if they exist at all, commercial enterprises carrying out testing.
The IEC only produces standards, not legally-binding regulations. Many IEC standards are adopted in
Europe by CENELEC as European Standards (ENs), which may effectively be legally binding if they can be used to demonstrate compliance with a Directive, such as the Low Voltage Directive. Most are adopted
unchanged, but some ENs have Common Modifications, which apply all over the CENELEC area, and Special National Conditions, which apply only in the country concerned.
Class X is for applications where failure could not lead to electric shock (hot to neutral). Class X1
capacitors are intended to operate safely even in the presence of spikes on the mains supply of up to 4 kV (installation category 3 or overvoltage category 3 according to IEC60664), which are normally industrial
supplies, but some standards call up class X1 capacitors if they are connected directly to the mains supply
upstream of the equipment fuse, irrespective of the type of mains supply. Class X2 capacitors are intended to
operate safely even in the presence of spikes on the mains supply of up to 2.5 kV (installation category 2 or overvoltage category 2 according to IEC60060), which are normally residential, commercial and light
industrial supplies. X capacitors can be found from 0.001 uF to at least 10 uF and are only made in film.
Class Y capacitors are for applications where failure could lead to electric shock if the ground
connection were lost. This includes hot/neutral to ground, and antenna isolation capacitors. Because Y
capacitors shunt current to ground, leakage-current limitations limit their size to a maximum of about 4700 pF
in many commercial and industrial applications (but refer to the relevant standard for definite information) and about 470 pF in medical applications.
Because most applications require one X and two Y capacitors, multi-capacitor devices are available. Also available are a variety of metal or plastic housing
styles, mounting options (pc mount, pigtail) and 4-wire connections. At least one manufacturer (WIMA) has X2 and Y2 capacitors in SMD, with more expected to follow.
Classes X1, X2, and Y were originally defined by the IEC in IEC 60384-14. CENELEC has
adopted EN 132400 (technically equivalent to, but structurally different from IEC 384-14 2nd edition), which
now defines seven classes of line-filter capacitors. Class X1 capacitors are impulse tested to 4 kV (higher for
capacitors over 1.0 uF). Class X2 capacitors are impulse tested to 2.5 kV (higher for capacitors over 1.0
uF). Class Y1 capacitors are impulse tested to 8 kV, and Class Y2 are impulse tested to 5 kV. Classes
X3, Y3, and Y4 are for lower-voltage capacitors, none of which are presently called up in safety standards.
Other impulse tests also apply. These include a 1000 hour endurance test during which the capacitor is subjected to a continuous overvoltage condition, plus periodic 1000 VAC spikes, and a flammability test
during which the capacitor is hit with a series of transients while under rated voltage. Capacitors conforming to IEC60384-14 normally also conform to EN132400, and vice versa, and should be accepted in all
European countries. Line-filter capacitor voltage ratings are based on what the agency considers the "nominal" line voltage
in its jurisdiction to be. A part can have one voltage rating in the US and Canada (125 or 250 VAC), a different rating in Europe (perhaps 275 or 300 VAC), and the manufacturor might recommend its use at a
higher voltage yet (a 250 VAC part on a 260 VAC measured line). This can make for a little confusion in application, and it may be best to ask the manufacturor for details.
These capacitors are also rated according to min/max temperature and allowed average annual
humidity exposure. IEC 68-1 calls for an X/Y/Z marking system where X is the low-temperature limit, Y is
the high-temperature limit, and Z is the number of days tested in a damp heat test (+40C at 93% RH). For example, 40/100/56 would mean the capacitor was rated from -40 to +100C and tested for 56 days in the
damp heat test. This is printed on the capacitor. UL (and CSA) has no blanket policy to cover all line-filter capacitors, it ranges from moderately tough
for some equipment to nonexistent for others. The capacitors may be affectively tested when the equipment they are in is tested however. The IEEE has standard IEEE C62.41 (formerly IEEE 587), for industrial
equipment where downtime is not an option, even from the affects of lightning strikes. Transients to 8 kV are
assumed. This would include uninteruptable power supplies and some computer equipment, all of which
would have line-filter capacitors. They differ in spirit from other agency's tests in that while others will pass a
capacitor if it fails certain tests safely, the IEEE requires capacitors to survive its very tough tests. However, the capacitors may need the protection of varistors.
See also UL 1414 (some entertainment and communications equipment, similar to CSA C22.2-1), and UL 1283 (potted line-filter modules, similar to CSA C22.2-8). A "typical" basic filter using line-filter capacitors: C1=0.1 uF (the X capacitor)
Some equipment requires an inductor or ferrite on the ground as a defence against incoming common-mode noise. A few filter modules do have ground inductors.
Some examples of complete filter modules:
Companies that make line filters include: http://www.eichhoffelectronics.com/ http://www.elect-spec.com/index.htm http://www.schurterinc.com/wwwsc/con_pg06.asp http://www.pricewheeler.com/index.htm Also See:
Pictures on this page courtesy of
Eichhoff Electronics. |
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Larger ones are available however. Y caps are available from 1000 pF to 0.1 uF and are made in both film and ceramic. 
