Capacitors and Capacitance
A Brief History of Capacitance
Reversible Electrolytic Capacitors
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Metallized Film Capacitors
Polarized versus Reversible
Electrolytic capacitors were originally developed to improve the capacitance/size ratio of the individual components and so to reduce equipment size and cost.
In their construction, normal electrolytic capacitors bear a superficial resemblance to rechargeable batteries - although their physical properties are very different. Like batteries, however, they will only accept and hold a charge if it is correctly polarized. 'Polarized' or 'polar' electrolytic capacitors of this type are widely used throughout the world's electronics industries.
The voltage applied to a loudspeaker alternates either side of zero, so components used in a loudspeaker signal path must be capable of safely handling both positive and negative voltage pulses. Hence the need for 'non-polar' ('bi-polar' / 'reversible') electrolytic capacitors.
Capacitors and Loudspeakers
The performance of a loudspeaker depends not only on skilled design but also on the quality of its components, standard of assembly and consistency in production. Amongst the myriad acoustic and mechanical details which contribute to the perceived sound, it is easy to overlook the importance of choosing the best crossover elements, yet there can be substantial degradation of sound quality if inappropriate or poorly specified components are used.
Thus the choice of capacitor becomes one of the most critical decisions in the overall design of a loudspeaker. For superior sound quality, capacitors should pass the intended signal with the minimum loss or change in character. To maintain product consistency, the crossover must conform to a tightly controlled specification. These factors demand both high quality, high efficiency components and minimum variation between components of the same batch and between one production run and another.
The capacitor parameters important to loudspeaker performance are often of little significance in normal electronic circuits, and the production of these specialised capacitors, in turn, demands totally different manufacturing techniques from those used for ordinary polar capacitors which are produced in their millions by automated machinery.
The ALCAP Capacitor Company was founded over two decades ago to manufacture reversible electrolytic capacitors expressly designed for loudspeaker crossover applications. ALCAP collaborates closely with Expotus Components Limited to offer high quality components manufactured and delivered to your exact requirements.
The manufacture of electrolytic capacitors demands a particular expertise in controlling the chemistry of the electrolyte mix and ALCAP's experience in this area is second to none. Close monitoring of the properties of the capacitor foil, as the rolls of bulk material are consumed, characterises ALCAP's attention to detail at all stages of manufacture. The result: capacitors of outstanding quality and unsurpassed consistency.
A loudspeaker which has been voiced using ALCAP capacitors will remain faithful to the designer's intended sound character throughout its production life.
Incoming batches of formed aluminium foil are inspected and tested for gain; they are then slit to the required widths and cut to length. Considerable experience is required here since the gain of the foil varies between each end and the middle of the roll and adjustments must be made during the process to maintain consistency.
Different foils are selected for different types and values of capacitor. Higher gain foils are used where size and/or cost of the finished capacitor is more important than efficiency, while lower gain foils are used to achieve higher efficiencies and/or working voltages. Using a low gain foil confers the additional benefits of greater consistency, lower equivalent series resistance giving greater efficiency, and increased ripple current handling.
Many of the more critical aspects of manufacture are unsuited to fully automated machinery and, in order to maintain full control over each process, they are carried out by highly skilled workers on manually operated machines. The precise chemistry of the electrolyte is to some extent dependent upon the performance of the foil used and must, therefore, be kept under constant close scrutiny.
Manufacture is carried out according to a system known as Statistical Process Control (SPC) whereby production is halted at regular intervals to log certain critical parameters which are plotted on an X/R chart. A number of readings are taken, and their average and 'range' (difference between smallest and largest) used to derive Upper and Lower Control Limits. Should subsequent readings stray beyond these limits, production will not recommence until the cause or causes have been traced and remedial action taken and documented.
Following encapsulation of the wound components, they are put through a 'forming' process, during which their rated voltage is applied for a pre-determined time.
Every capacitor is then checked for basic parameters and packed, ready for despatch, together with its accompanying statistical process control documentation.
Gain. In the context of electrolytic capacitors the gain of the foil used during manufacture is taken as an indication of the capacitance achievable from a given area of foil. A high gain foil will result in a larger capacitance than will the same area of a low gain foil, but the greater surface area presented by a low gain foil allows a higher current to pass - reducing the effective series resistance of the capacitor (less heating effect), and improving the DF and ripple current specifications. The use of a larger area of low gain foil also results in improved accuracy and consistency in production (tolerance) since small changes in a large area will have less effect on capacitance than the same changes in the correspondingly smaller area of a high gain foil.
The reversible electrolytic capacitor is specified by four principal parameters:
2. Working Voltage
Measured in microfarads (µF, mF or MFD): the standard production values range from 0.6 µF to 800 µF, according to type. Choice of capacitance is determined by the circuit configuration.
2. Working Voltage
Although the reversible electrolytic is intended to handle AC voltages, the working voltage is usually quoted in volts DC, in effect to be regarded as the maximum allowable peak AC voltage.
The DC voltage is valid within the specified temperature range (normally - 40°C to + 85°C, but see individual specification sheets). The capacitor may fail in service if its temperature exceeds the maximum specified. DC voltages in excess of its working voltage or excessive AC ripple current may cause excessive internal heating, resulting in failure. It follows that the capacitor will have a longer working life if operated in a cool environment and within its rated voltage.
ALCAP reversible electrolytic capacitors are specified at voltages between 50V and 150V with standard values of 50, 63, 70, 90, 100 & 150V, although some of these are only available to special order. The permutations of capacitance and voltage vary according to type.
In the UK, the working voltage is normally specified as 50V, in Europe it is often 63V. High power applications may demand 100 or 150V. 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.
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.
In practice, one does not expect to receive a batch of components whose capacitance is equally divided right across the tolerance band. Rather, the expectation is of a Normal Distribution, with a mean value very close to the nominal. Just such a distribution, tightly centred on the specified value, is ALCAP's aim during manufacture.
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 ±10% tolerance on some components while ± 20% is adequate for others to maintain sufficiently consistent behaviour.
Having determined capacitance, voltage and tolerance, the final consideration is dissipation factor (DF), which is a measure of the capacitor's efficiency, expressed as a percentage and usually measured at a signal frequency of 1KHz. DF is governed by the construction of the capacitor; lower dissipation is achieved by the use of a low gain and/or high voltage foil, but must be offset against increased size and cost.
An alternative way of expressing dissipation factor is to use 'Q' which is the reciprocal of DF. This means the higher the Q, the lower the losses See ESR & DF.
The three basic types of ALCAP capacitor are:
a) Standard STD
b) Low Loss LL
c) High Voltage HV
This type has a 50V DC rating, and is suited to many less critical hi-fi loudspeaker applications. DF is 10% maximum at 1KHz. Typical values will be around 7%, and will remain close throughout a batch, although some variation must be expected from batch to batch.
Note: The STD range is also available in a miniature can size suitable where space is restricted (e.g. automotive applications). A higher gain foil is used, with a correspondingly higher DF. Usually manufactured only to special order.
b) Low Loss
These are much larger than the STD type boasting a lower DF - maximum 5% at 1KHz - and a higher current handling capability.
They are available in the same capacitance values as the STD range - up to 33µF where physical size becomes prohibitive. ( DF is rarely critical much above a capacitance of 10µF.)
An LL type is recommended where high efficiency is paramount, or where a higher ripple current is anticipated. A series feed to a high power and/or low efficiency tweeter would be a typical application for such a component.
Note: A further range of LLA capacitors is available with an elevated working voltage of 90V, combining the highest practicable working voltage and lowest DF - specified as 5% maximum at 1KHz, typically 3 to 4% with little variation between batches. Available to special order in capacitances up to 80µF and featuring very high ripple current ratings.
c) High Voltage
The 100V and 150V types are designed for higher power hi-fi and PA applications. The large amount of foil required in the 150V types provides an even greater reduction in DF over the 50V and 100V types.
Most users prefer the standard axial leadout format which is convenient for both PCB connection and hard wiring, but radial (RAD) types are also available if required.
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.
1. Do not apply a voltage greater than the rated DC voltage
Capacitors used at voltages below their rating will have a prolonged working life. Remember a given AC voltage is equivalent to a much higher DC voltage. See AC and DC Voltage Relationships
2. Do not allow excessive ripple current to pass through the Capacitor
Overheating and failure may occur if ripple current exceeds the specified limit. Ripple current may be limited by increasing the circuit resistance in series with the Capacitor. High ripple current types are available if required. See Ripple Current
3. Do not use ALCAP Reversible Capacitors in circuits where they might be subject to frequent charging and discharging, or in motor start applications
ALCAP Reversible Capacitors have been specifically developed for audio related applications.
4. Do not allow working temperature to exceed the specified temperature range
The characteristics of Electrolytic Capacitors vary with temperature but, within the specified temperature range, such changes are temporary i.e. characteristics return to normal as temperature does. Failure may occur if the specified temperature range is exceeded. A longer working life will be assured by use at room temperature.
5. Check Capacitor performance at its operating frequency range
The capacitance is usually specified at a frequency of 1KHz but will decrease as frequency increases.
6. Do not use force when forming leadout wires or when fitting a Capacitor to a printed circuit board
Applying excessive force to the Capacitor leadout wires may weaken the internal stitched joint to the foil, increasing the effective series resistance, or it may even break the joint completely.
7. Do not overheat Capacitors during soldering
This could adversely affect the Capacitor's characteristics and the insulation sleeve. When soldering all the components to a printed circuit board, or flow soldering, care must be taken to ensure that the soldering temperature is not too high and that dipping time is not too long.
8. Do not allow cleaning solvents to contact the Capacitor when cleaning circuit boards after soldering
Some cleaning solvents can cause damage - if in doubt, please check with Expotus.
9. Re-form old or unused Capacitors before putting them into service
Long periods of storage have virtually no effect on a Capacitor's resistance and dissipation factor, but there is a tendency for leakage current to increase and for voltage capability to decrease. This may be remedied by 're-forming' the Capacitor before use by applying the rated DC voltage for 24 hours.
Leakage current, as applied to electrolytic capacitors, is a DC parameter which is therefore insignificant in terms of measured performance in loudspeaker crossover circuits. It may, however, contribute to the audible performance of a loudspeaker system. Generally a lower leakage current is preferred to a higher value.
Maximum allowable leakage current is determined by the formula:
I = 0.03 x C x V + 5µA
I = Current (µA) C = Capacitance (µF) V = Rated working voltage (DC V)
Capacitors must be pre-conditioned - at least 24 hours and not more than 48 hours before testing - by applying the rated DC voltage for 15 minutes.
Leakage current may then be measured by applying the rated DC voltage to the Capacitor connected in series with a 1KW resistor for 5 minutes, then read the leakage current in Amps.
Measure Capacitor parameters at room temperature (25°C ± 1°C).
Conduct test in a suitable chamber with circulating air. Apply the rated DC voltage for 500 hours at + 85°C ± 2°C, reversing polarity every 24 hours throughout the test.
Remove Capacitor from chamber and allow to cool to normal room temperature (25°C ± 1°C).
Capacitance shall be within ± 20% of initial measured value.
DF shall not have increased to more than 150% of initial measurement.
Leakage current shall be the same as initial measurement.
A capacitor's Equivalent Series Resistance and Dissipation Factor are related by the formulae:
ESR = equivalent series resistance (ohms) DF = dissipation factor (%)
f = frequency (Hz) C = capacitance (µF)
Alcap Reversible Electrolytic Capacitors intended for use in loudspeakers are designed to achieve minimum ESR at a working frequency of 10KHz, rising both below and above that frequency. In some circumstances, the internal construction can be modified to hold the minimum ESR up to a signal frequency of 100KHz for special requirements.
Dissipation Factor always rises with frequency, but actual values depend upon size, working voltage and internal construction of the capacitor, so for each component the circuit designer should optimise DF in conjunction with considerations of cost, size and working voltage.