Capacitors and Capacitance
A Brief History of Capacitance
Reversible Electrolytic Capacitors
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Metallized Film Capacitors
Whilst the ALCAP audio quality range of Reversible Electrolytic Capacitors is ideally suited to most loudspeaker crossover applications, the materials employed in manufacture impose certain limits on their specification and performance: 150V is the highest voltage formed foil available, and the lowest DF which can be achieved in production is approximately 3% at 1KHz. Physical parameters also limit the lowest capacitance which may be reliably produced to 0.6µF.
For very high power applications, where voltages higher than 150V must be handled, or where a lower DF is required, a film capacitor should be considered. Although film capacitors are unavailable in the larger capacitance values of the electrolytics, they can be produced to much smaller values.
Materials and construction are quite different from those used in the electrolytic - the material differs also in being very much more expensive. Instead of a 'Swiss Roll' like winding of formed aluminium and insulating paper, a plastic base is employed, coated with a metallic film typically 4 to 6 microns thick. The two most common bases are polyester and polypropylene, supplied in strip form, and metallized according to the parameters required in the finished component, but leaving a blank margin on one side of the strip. The winding process is not dissimilar to that of the electrolytic: a tight, even wind at uniform tension results in a better capacitor. The ends of the wound component are metal sprayed and leadout wires welded on.
The peculiarly stringent demands placed by today's loudspeaker designs upon crossover networks may demand capacitors with specifications other than those listed on the following pages. If you require non-standard components, Expotus will be pleased to quote for items manufactured to your exact specifications.
The metallized film capacitor is specified by four principal parameters:
2. Working Voltage
Measured in picofarads (pF), nanofarads (nF) or microfarads (µF) according to size, which can be very small - down to 100pF, for example, but the maximum value doesn't normally exceed 10µF. Values greater than that are very special and very large. Choice of capacitance is determined by the circuit configuration.
2. Working Voltage
This varies widely for film capacitors but, for use in loudspeaker crossovers, practical values range from 100V to 400V DC. The material employed has an inherently higher voltage capability than the aluminium foil used in electrolytic capacitors.
These film capacitors are not suitable for AC mains applications. A range of class 'X2' and 'Y' capacitors is available for 'across the mains' applications.
The working voltage should be determined by examining the configuration of the particular capacitor in circuit and the loads applied to it in all conditions of use and abuse. Consideration of the loudspeaker's overall power rating is usually a safe guide but may result in some components being over-specified. See also Matching Capacitor Type to Application.
This is the extent to which the actual capacitance is allowed to vary from its nominal value. It is invariably expressed in the form of equal positive and negative limits - e.g. ±10% means that the capacitance can vary up to 10% above, or 10% below the nominal value. Thus a 10µF capacitor with a ±10% tolerance could legitimately vary from 9µF to 11µF and still remain within tolerance. Please see Definition of Tolerance for a full explanation and definition of tolerance as applied to electronic components.
It is possible to obtain much closer tolerances with film capacitors than with electrolytics: ±1% is not uncommon. As a result of the automatic process used for production of metallized polyester capacitors, there may often be a wider 'spread' of values within the specified tolerance than is achieved by ALCAP electrolytics, so for critical applications it is advisable to select a closer tolerance component.
The circuit designer must determine the tolerance which should be specified for each component by examining performance variations of the entire circuit when using components at extremes of the tolerance band. It should be borne in mind that different components within the circuit may interact when more than one is beyond its nominal value. It is quite possible that a circuit may require ± 5% tolerance on some components while ±10% is adequate for others to maintain sufficiently consistent behaviour.
The four basic types of metallized film capacitor are:
a) Metallized Polyester Radial PER
b) Metallized Polyester Axial Flat PEF
c) Metallized Polyester Axial PE
d) Metallized Polypropylene Axial MPP
a), b) & c) Metallized Polyester
If a film capacitor is required, be it for reasons of capacitance, voltage capability or DF, there is nothing to choose between the performance of the first three types PER, PEF, PE - merely convenience and available space. All three offer a DF approximately 1/10 that of the electrolytic, albeit at a much higher price. They do not always appear to share the same acoustic qualities as the ALCAP electrolytic.
d) Metallized Polypropylene
The Metallized Polypropylene Capacitor MPP may be preferable to polyester if acoustic qualities are paramount. These also offer greater accuracy within a production batch than do polyesters, and are available in similar values to the ALCAP electrolytics, ranging from 1µF to 200µF.
Standard tolerance on this type is ± 5% (tighter tolerances are available) and working voltage 250V DC (400V is available). Instead of the usual wrap and fill manufacturing technique, these are clamped into a sleeve which is then hermetically sealed with epoxy resin at each end, which also helps to keep the winding tight. Shape may be either cylindrical, as wound, or flattened, but only with axial lead out wires in each case.
A special low resistance end spray is employed, and the leadout wires are made of a very high purity copper. Solid silver leadout wires are also available to order.
Since the equivalent series resistance of a Metallized Polypropylene Capacitor is extremely low, its current handling capability is exceptionally high - higher than is likely ever to be encountered in a loudspeaker crossover circuit.
Radial or axial. For full details please see individual specification sheets.
The radial type is wound with a hollow core and then pressed after winding to flatten it. A radial type may be dipped in epoxy, or sealed into a case of plastic or other material.
The axial type may also be supplied in a flattened form if required. This can make it easier to glue down on a hard wired circuit, or to fit into a narrow space. The usual finish for an axial capacitor is 'wrap and fill' which is a wrapping of insulating tape with the ends epoxy filled.
As MPP types are very large, the leadout wires are not literally 'axial' - in other words they emanate not from the centre of the cylindrically wound component, but from a point nearer the circumference of the capacitor in line with each other.
Standard Specifications for Metallized Film Capacitors common to all types:
Dissipation Factor: <0.001 @ 100Hz @ 20ºC
Insulation Resistance: >50,000 MW.µF
Proof Voltage Test: 480V DC for 5 minutes
Pulse Performance: 10V / µsec
Temperature Range: -35ºC to +55ºC
The capacitance of Metallized Film Capacitors may be expected to vary with temperature approximately in accordance with the following graph: