|Manufacture of the fixed capacitors||Condensers with the mica and the silver plated mica||Electrolytique capacitors|
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ELECTRONIC COMPONENTS 2 “2nd part”
In a preceding lesson, we examined the simplest component which exists : resistance. We will see another component as essential now as resistance in the electronic circuits : the condenser.
1. - CONDENSERS
They are formed by two plates conducting in glance, called reinforcements and separated by an insulator which is named dielectric. For a type of dielectric given, plus the surface of the reinforcements is large or the distance which separates them small, larger east the capacity of the condenser.
The condensers can fixed or variable and be indexed according to the dielectric one as the table of figure 1 shows it. According to the type of dielectric, one defines the electric and mechanical characteristics (dimensions and provision of the terminals) according to which one will make a correct use of the condenser.
1. 1. - ELECTRIC CHARACTERISTICS OF THE CONDENSERS
The electric characteristics most important to know to use the condensers as well as possible are given below :
The relations between these submultiples are indicated in the table of figure 2.
By observing this table, it appears obvious first of all that the nanofarad (nF) is equivalent to the kilopicofarad (kpF) ; indeed for these two units, one uses the same multipliers.
The first column indicates the measuring units of the condensers which one wants to convert into another unit while referring to the corresponding column and while multiplying by the coefficient of the same line.
Let us see some examples for better interpreting the table of figure 2 and clearing up above:
0,47 µF x 1 000 = 470 nF = 470 kpF ;
0,47 µF x 1 000 000 = 470 000 pF ;
22 nF x 0,001 = 0,022 µF ;
33 nF x 1 000 = 33 000 pF ;
8 200 pF x 0,001 = 8,2 nF ;
0,0027 µF x 1 000 000 = 2 700 pF ;
0,0027 µF x 1 000 = 2,7 nF = 2,7 kpF ;
1 500 pF x 0,000 001 = 0,0015 µF.
The tension of test indicated by VP is in general 2,5 times larger than the tension of service VN. Therefore, for a condenser with VP of 1 500 V, it is possible to apply a tension from service maximum VN of 1 500 / 2,5 = 600 Volts.
When the coefficient is negative, the expressed number is preceded by the sign “-” or the letter N (negative) ; when it is positive, this number is preceded by the sign “+” or the lette P (positive). Initials NPO indicate that the temperature coefficient is null.
For example, for a condenser of value 500 nF with 25° C having a temperature coefficient of - 75 ppm/° C, the capacity decreases by 37,5 PF to each degree of increase in the temperature:
This coefficient must be smallest possible in order to minimize the variations of the capacity according to the temperature.
We recall on this subject which the temperature inside an apparatus can be quite higher than that ambient because of the calorific energy dissipated by other components belonging to the same circuit as the condenser.
On the other hand, in certain cases, the temperature coefficient must have a value well defined to compensate for the effect of the variations of other components of the same circuit.
It is thus seen that in a real condenser, the angle of dephasing is lower than 90° whereas for an ideal condenser, the loss angle is null because dephasing between V and I is of 90° precisely. One often uses the factor of loss by calculating the tangent , indicated in %, to express the quality of a condenser : more the percentage is weak, better is the component one.
2. - TECHNIQUE OF MANUFACTURE OF THE FIXED CAPACITORS
The techniques of manufacture employed by the manufacturers of condensers to meet the various requirements imposed by their use, are very numerous. One can affirm that the condensers are the components which are carried out with the largest variety of forms and dimensions. One finds condensers with a cylindrical body, in the shape of disc, plate, drop … ; for each form, the capacity report/ratio - volume can be very different according to manufacturers' and no standard governs that.
We will now examine the most common types of condenser by taking for guide the dielectric one used.
2. 1. - PAPER CAPACITORS AND A ALUMINUM
These condensers are consisted the winding of two very fine sheets of aluminum separated by several paraffin or oil paper sheets impregnated. The two very pure aluminum foils (99,99%), to avoid oxidation during manufacture, constitute the reinforcements while the insulator intercalated between them forms the dielectric one (figure 3-a, below).
The capacity of the condenser is all the more large as the surface of the reinforcements in glance is large and that the distance which separates them, weak. The maximum tension applicable to the reinforcements depends on the thickness on dielectric and its insulating properties.
Rolling up obtained can be locked up hermetically in an envelope of glass, plastic, galvanized brass … ; two terminals welded with the reinforcements ensure the external connection.
This type of condenser presents a serious disadvantage because its reinforcements, being rolled up on themselves, involve the appearance of an inductance in series with the capacity of the condenser for the high frequencies.
To cure it, one generally proceeds by metallizing the section exceeding of each reinforcement, by cathodic evaporation with copper. The output are welded onto this metallization. Noninductive condensers are obtained ; the contact is perfect and the small overall dimensions.
The values of capacity generally lie between 500 pF and 0,5 µF with some exceptions for certain particular types. The values of nominal voltage oscillate between 125 VN and 1 000 VN.
In current employment, these condensers were replaced by models with plastic film, of more reduced size.
2. 2. - CONDENSERS A PLASTIC FILM
These condensers are similar to the precedents, but their dielectric is consisted a very fine plastic film (figure 3-c). The dielectric employees are polystyrene (styroflex), polyester (mylar) and polycarbonate.
In order to reduce the volume of the condensers, one thought of also metallizing plastic film.
A margin is reserved on one on the sides of two films in order to be able to metallize the section and to adopt the principle of manufacture retained for the paper capacitors and with aluminum. In this way, one obtains a compact condenser, of low inductance, similar to that considering previously but with a value of capacity two to four times larger, with equal volume.
External protection can be ensured according to the type of condenser by a resin coating moulded under pressure or by metal cases filled of wax or oil.
Sometimes, the body of the component is marked by a ring (figure 3-b) so that, in the assemblies bringing into play raised tensions, the terminal nearest to this mark is with a potential playing the part of shielding for the condenser (it is often the potential low).
The range of their capacity lies between ten picofarads and ten microfarads ; stability is good beyond even of (85° C) and the values of nominal voltage spread out of 25 Volts to more than 2 000 V.
The condensers with plastic film are largely used because they are of a reduced cost and show good electric characteristics ; on figure 4, is illustrated the external aspect which they can take to meet the many requirements of manufacture of the electronic instruments with printed circuit. The denomination depends on the technique of manufacture and can vary from one manufacturer to another.
2. 3. - CONDENSERS WITH THE MICA AND THE SILVER PLATED MICA
The mica was used like dielectric since the first years of manufacture of the condensers ; the first types were carried out by alternating mica sheets and very fine aluminum or copper sheets so as to form a stacking which was then compressed then impregnated of an insulating material. A reinforcement of the condenser, welded with the one of the terminals, is consisted of the odd metal sheets connected between them; the other braces formed of the even sheets is welded on the other terminal (figure 5-a).
In the new version, the same structure is carried out by depositing a very light layer of money on the mica sheets. These last are connected electrically to two metallized faces to which the terminals are welded. The unit is then coated by a moulded insulating resin which confers on the condenser a rigid structure (figure 5-b).
These condensers are characterized by a raised stability, a very low temperature coefficient and they are particularly adapted to professional uses in circuits H.F of measuring instruments. The range of the capacities extends from a few picofarads to a few hundreds of nanofarads for tensions of service of 300 V to more than 2 500 V.
In the general public sector, the condensers with the mica yielded the place to the polystyrene condensers which are not also stable and operation in a range of more limited temperature ; n the other hand they present an obstruction more reduced and are especially more economic.
2. 4. - CERAMIC CONDENSERS
One mixes and one finely crushes magnesium silicate, alumina and corundum to which one adds barium, titanium oxides or strontium. The powder obtained is dried, filtered then moulded under pressure with clay or a binder organic and cooked at a temperature higher than 1.000° C.
The parts obtained are enamelled with the electric furnace to remove their porosity. The reinforcements are obtained by money metallization on the two faces.
Protection is normally assured by a coat of enamelled paint cooked with the furnace. The condensers of decoupling can be coated vacuum with a protective wax to improve their insulation.
According to the aspect of coating, one distinguishes, inter alia, the tubular types, with plates, with discs, pinup (figure 6).
In the tubular ceramic condensers, the terminal connected to the external reinforcement is sometimes located by a point or a ring or laid out in withdrawal of the end of the body of the component.
The variety of the external formats and the pace of the terminals (their spacing and their length) are due to the various requirements imposed by the assemblies and the weldings which are carried out by automatic operations that one finds on the chains of great series.
Dimensions are related on the capacity and the tension of service of these condensers; however, it sometimes happens to have very different capacities (1 pF, 1 nF) for same dimensions because the permittivities are very varied to carry out ceramics.
To recognize the values, it is thus necessary to refer to the markings adopted by the manufacturer.
One of the data characteristic of the ceramic condensers is the temperature coefficient which strongly acts on the value of the capacity of some of these components whose employment is proscribed in the pointed electronic assemblies. There are nevertheless ceramics with very low temperature coefficient, even no one; one also arrives at very narrow tolerances and the most current use of these condensers is decoupling H.F and U.H.F because of their low parasitic inductance.
Among the ceramic condensers of special use, used in H.F and U.H.F., there are the types “BY-PASS” which are used for the wire decoupling crossing a frame or a shielding. The colleret which represents the external reinforcement is welded with the frame ; the other reinforcement is connected on two axial terminals which leave at each end the component (figure 7-a) or on a terminal in form eyelet (figure 7-b).
2. 5. - ELECTROLYTIQUE CAPACITORS
The aluminum electrolytique capacitors belong to the category of the rolled up fixed capacitors.
They are different from the other types (paper, plastic film…) by the fact that a reinforcement (anode) consists of an aluminum foil smooth or engraved on which a very thin layer of alumina by a chemical process was deposited. The dielectric one is formed here by alumina and the second reinforcement is consisted the electrolyte retained in porous paper called sometimes “blotting paper”. The connection with the electrolyte is carried out by means of a second aluminum foil, called cathode on which is fixed an output. The other braces (anode) also has an output which it will be necessary imperatively to connect to a potential larger than that of cathode (figure 8).
The alumina oxide has a high dielectric rigidity and can be formed in extremely fine layers, so that one obtains a high value of capacity per unit of volume of the condenser. That involves that the electrolytique capacitors have a capacity higher than all the other types for dimensions and equal tensions of service.
One obtains electrolytique capacitors having capacities of about 1 µF with more than 10 000 µF with tensions of service going from approximately 3 to 500 V. the tolerance on the face values is rather broad and can reach until + 100%.
As we already specified, these condensers have the characteristic to be polarized and their terminals are located by the signs (+) and (-). When they miss, it is necessary to notice the terminal which is connected to the aluminum case (cathode : -) or if it has a throttling to indicate that the terminal nearest is the anode (+).
An electrolytique capacitor is used for filtering or decoupling. One applies on his terminals a continuous tension and an alternating voltage superimposed (50 Hz, 100 Hz or B.F.). The percentage of the alternating voltage compared to the tension continues should not exceed 15% for the tensions of service higher than 50 V. Moreover, the sum of the continuous tension and of the alternating voltage of peak does not have to exceed the nominal tension of service of the condenser.
One realizes, however, of the electrolytique capacitors for alternating current obtained by winding together two anodes formed instead of an anode and of a cathode.
The diagram carried out is that of two polarized condensers, opposed and assembled in series (figure 9). The separators are doubled (four thicknesses of paper between each electrode). All equal things, the value of the capacity obtained is half of that of a normal condenser.
The nonpolarized electrolytique capacitors can function with continuous or alternative sizes.
Some electrolytique capacitors of cylindrical form particularly used for the filtering of rectified electric quantities are illustrated appears 10-a. The terminals are axial and the aluminum case can be presented with or without flexible pavement insulating.
The table of the figure 10-b gives an idea of their dimensions according to their value of capacity and most current tensions of service ; in another manufacturer, these values can change appreciably.
Appear 10-b. - Dimensions of obstruction of the electrolytique capacitors of the figure 10-a proportional to the values of capacity and most current tension of service.
There are also condensers with two positive terminals and negative; they are used whenever the place available is too small. They are in fact two condensers enclosed in a cylindrical envelope equipped with three terminals.
Figure 11 represents this type of double condenser, one with terminals in galvanized copper wire and the other with terminals in strips.
The screw condensers illustrated on figure 12 are used to resist the shocks and the vibrations. The reinforcements and the electrolyte are locked up hermetically in a metal case from which the connecting terminals leave.
Two screw electrolytique capacitors which differ between them only by the system from exit from the terminals are illustrated on the figures 12-a and 12-b; for the type illustrated on the figure 12-a, the negative terminal misses sometimes; in this case, electric connection is obtained by fixing the condenser by means of a nut on the metal frame of the apparatus which is the mass (0 V) electric circuit.
On the figure 12-c, one finds a double condenser but which can be screwed. The remark concerning the negative terminal of the figure 12-a applies to that of the figure 12-c.
Similar to the latter, the cylindrical condensers out of aluminum with mounting feet in the shape of pin for printed circuits, while using as limits negative commune, for the multiple capacities (figure 13).
Figure 13. - Electrolytique capacitors with capacity multiples with mounting feet out of pin for printed circuit.
In the apparatuses of reduced size, when particularly compact assemblies are necessary and the tensions concerned low, one uses miniature condensers as on the figure 14-a.
The appearance of the most current condensers of this type with their dimensions according to the capacitive value and of the tension of service is illustrated there in a table appears 14-b (below).
The terminals can be axial for the horizontal assemblies (see figure 14-a in top on the left) or vertical (figure 14-b in centered top) or axial with exit on the same side for exclusively vertical fixings on printed circuit (figure 14-c on the right).
According to the manufacturer, various benchmark systems are adopted as one can see it on figure 15.
Of more recent production, the electrolytique capacitors with tantalum show characteristics definitely more advantageous than those out of aluminum.
First of all, the permittivity of tantalum oxide is approximately double of that of aluminum oxide; moreover, the film of tantalum oxide is much finer and present qualities of higher stability.
The characteristics show that one can use the condensers with tantalum at maximum temperatures of 125° C, whereas the electrolytique capacitors with aluminum are usable only until 85° C.
Dimensions of a condenser to tantalum are definitely smaller than those of its equivalent to aluminum for the same capacity and the same tension of service. The tolerance undergoes the same law and one reaches ± 5% with tantalum, unrealizable security with an aluminum electrolytique capacitor. The principle is the same one as that of the electrolytique capacitors with anode out of aluminum ; the difference is that here, the anode is with tantalum.
The dielectric one is a tantalum oxide film whose relative permittivity goes from 11 to 26, which makes it possible to produce subminiature condensers and of a great reliability.
Tantalum is a refractory metal treated by metallurgy of the powders. This tantalum powder is in a hurry in bars subjected to the first vacuum sintering. Then, they are forged cold to be less porous. Again, they are sintered vacuum towards 2.900° C to reach a density of approximately 16,5. The ingots can be rolled and stretched cold until obtaining sheets of 12 µm thickness and wire 0,1 mm in diameter.
Wound model :
It is the transposition with the tantalum of the model to aluminum. Tantalum is rolled in bands of 12 µm thickness of which each one is formed by electrolysis under variable tension. The band is covered by an oxide coating (Ta2 O5) of which the thickness is approximately 10-7 cm per volt ; the tension of service is limited to 150 V. the tantalum band does not need to be engraved, because it is naturally porous, its useful surface area is the double of real surface.
Winding is placed in a plated copper or silver case. The impregnation is carried out vacuum with an electrolyte with resistance raised containing glycol, of boric acid, sulfate of lithium sodium or chloride. The wire of exit out of tantalum, are prolonged by galvanized wire of nickel, electrically welded (figure 16).
Model with sintered massive anode :
The anode is consisted a tantalum powder pastille in a hurry and sintered (also called “pellet”). The pastille being porous, one obtains an active surface of 1 m2 per cm3 of volume.
One forms the anode by means of a very fluid electrolyte and of low resistance. It is the sulphuric acid which is appropriate best. Cathode is formed by a silver case which ensures a good contact with the electrolyte and is not attacked by him. The cathodic exit is carried out by a wire of galvanized copper welded with the case. The anodic exit is consisted a tantalum wire pressed against the pastille. It is welded electrically with a galvanized nickel wire which leaves the condenser. The silver case is sealed by a thermohardening resin or is protected by a sleeve metal or insulating according to case's (figures 17-a, 17-b and 17-c).
Model with solid electrolyte :
It is a condenser with sintered massive anode in which the liquid electrolyte is replaced by solid manganese dioxide. Such a condenser is more robust, it can be stored for a long period without deterioration.
The anode is obtained and formed like previously. Then, it is covered by a layer of dioxide with manganese obtained by pyrolysis with an aqueous solution with nitrate with manganese which penetrates in all the pores of the anode. It is then coated with colloidal carbon and is silver plated by chemical reduction. The connection of anode is obtained like previously. The unit is introduced into a silver plated metal case. It is essential that the contact of cathode is perfect. The case is closed by a synthetic resin stopper. Before storage, one carries out an ageing, as remainder for all the electrolytique capacitors. Not containing liquid, no congelation of the electrolyte at the low temperatures is to be feared (figures 18-a, 18-b and 18-c).
Figure 19 illustrates the appearance of the most current condensers with tantalum.
End of this technology and we will continue the continuation of identification of the condensers.
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