Fixed resistances Technique of manufacture of fixed resistances Return to the synopsis To contact the author Low of page

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ELECTRONIC COMPONENTS  1      “1st PART”

An electric or electronic circuit is carried out by a whole of parts of various species having various and generally known functions under the name of “components”.

These components are made of resistances, condensers, inductances, transformers, tubes electron, semiconductor, integrated circuits, materials of connection and commutation.

The technician must know these components in order to use them under the best conditions.

1. - CLASSIFICATION OF RESISTANCES

Resistances are the components most used in the circuits ; they have a resistance to the passage of the current.

One finds of them many types, different by their electric structure, their form, their characteristics according to the technique of adopted manufacture and the employment for which they are intended.

One can classify resistances according to the model of figure 1.

2. - FIXED RESISTANCES

These resistances have a given value and arise under three types : agglomerate, with layer and wound.

It is according to this model that depend the characteristics necessary to the realization on an assembly.

2. 1. - TECHNICAL DATA OF FIXED RESISTANCES

The electrical resistance is the face value in ohm (W symbol) at the ambient temperature of 25°C. The high values are expressed with multiples of the ohm.

the kilo-ohm (kW) = 1 000 W

the megohm (MW) = 1 000 000 W

The tolerance is a percentage, in more and less around the face value, that the supplier commits himself respecting for all the delivered parts. It gets along for new coins before use, because thereafter the variations can be more important after a prolonged operation.

More the tolerance is tight, better is the precision; if we take a resistance of face value 120 W with a tolerance of ± 10%, we have a variation of ± 12 W (120 x ± 10 / 100 = ± 12).

Resistance can then present an actual value ranging between a minimum of 108 W (120 - 12 = 108) and a maximum of 132 W (120 + 12 = 132). If this time the tolerance is only 2%, we have this time a variation of ± 2,4 W (120 x ± 2 / 100 = ± 2,4) ; the actual value lies between a minimum of 117,6 W (120 - 2,4 = 117,6) and a maximum of 122,4 W (120 + 2,4 = 122,4).

The nominal power output is the power which resistance in calm air can dissipate to the normal atmospheric pressure and for an ambient temperature generally of 20 or 25°C. It is given so that under these conditions no point of resistance exceeds the temperature fixed for this type of manufacture.

If the ambient temperature is higher, it is necessary to decrease the acceptable power, in order to remain within the limits of temperature of resistance. Curves of reduction are established for each model.

The temperature coefficient is the quotient of the relative variation of resistance by the variation in temperature. It is expressed partly per million per degree centigrade (ppm /°C). For current resistances, the temperature coefficient lies between 50 ppm /°C and 100 ppm /°C.

For example, a resistance of 2 000 W to 20°C having a temperature coefficient of 80 ppm /°C increases its value of 0,16 W for each rise in the temperature of 1°C :

2 000 x 80 / 1 000 000 = 0,16 W /°C

In ambient conditions at the temperature of 60°C, resistance takes a value of 2 006,4 W (0,16 W/°C x 40° C = 6,4 W). For the negative temperature coefficients, the value of resistance decreases when the temperature increases.

** The maximum tension at the boundaries. The terminal voltage of a resistance is given by the formula :

U in volts, P in Watts (it is the nominal nominal output of resistance), R in ohms.

Without overloading resistance in power, one obtains very high tensions at the boundaries, for the strong ohmic values. However, in consequence of the dielectric rigidity of the matters used, one should not exceed a tension, generally weaker, fixed by the numbers. It is thus the terminal voltage which limits the acceptable power at this time.

** Stability. It is said that a resistance is stable, when after a long use, its value remains close to that which it had in the beginning. This variation of value depends on the type and the technology of manufacture. The wire-wound resistors or to metal film are very stable, agglomerated resistances are it less.

** The coefficient of tension is the measurement of the variation of the ohmic value of resistance according to the tension on its terminals. It expresses as a percentage variation per volt. The coefficient of tension is negligible for the wire-wound resistors and the resistors film of metal. It is appreciable for agglomerated resistances (0,02 % V).

** The tension of noise. All resistances produce at the boundaries a parasitic tension produced by the thermal agitation of the molecules. This tension is very weak for the wire-wound resistors and with metal film, it is more important for agglomerated resistances, because in addition a tension of noise caused by the passage of the current in a heterogeneous matter.

It is measured in microvolts per volts applied at the boundaries (µV / V). It limits the possibility of amplification of the apparatuses, bus on a certain level, it becomes awkward.

This noise is especially composed of low frequencies lower than 10 kHz thus audible and it increases with the ohmic value of resistance.

2. 2. - TECHNIQUE OF MANUFACTURE OF FIXED RESISTANCES

Agglomerated resistances are consisted a mixture of carbon, insulating matter and binder. The body of these resistances in which the two terminals (wire of connection) are established is protected by an envelope from insulating matter (figure 2-a).

The percentage of carbon makes it possible to determine the value of resistance for dimensions given. The more there is carbon and the less resistance to a high ohmic value.

A technique of more recent manufacture consists in depositing the resistive mixture carbon and binding in the form of layer around a small tube of special glass in which the two terminals in contact with resistive material are threaded. Resistance thus formed is coated in a thermohardening insulating plastic resin (figure 2-b).

The terminals introduced in-depth into the drinking glass holder allow a better thermal conduction from where a better dissipation of heat.

The standardized values of resistances agglomerated available in the trade extend from 1 W to 22 MW with tolerances of ± 5 %, ± 10 % and ± 20 %. One finds the powers following : 1 / 8 W, 1 / 4 W, 1 / 2 W, 1 W, 2 W, 3 W and 4 W whose dimensions vary proportionally (figure 2-c).

The power and the ohmic value of a resistance must appear in light beside the electric symbol of this one.

However, certain manufacturers reserve the right, in the design of the diagrams, to adopt graphic symbols which will be interpreted if need be, according to provided indications'.

Particularly adapted for everyday usages, agglomerated resistances whose cost price is weak, are very much used in the electronic circuits; they have a good mechanical resistance, a low temperature coefficient in normal ambient conditions (lower than 60° C) and one good behavior at the high frequencies (negligible parasitic inductance). On the other hand, from their heterogeneous resistive element, they can have a too high tension of noise and an insufficient stability for certain uses.

The resistors film called also resistances to film are produced by various techniques; one of them consists in making deposit by cracking (deposit by chemical process at high temperature) on a central insulating matter support (ceramic) a fine layer of carbon or oxide of tin or of another resistive material.

The ohmic value depends on the thickness of the layer deposited and the substance used. However, it was observed that the very thin layers, required by the strong values, were fragile and unstable. Thus, a layer of 1 µm gives a temperature coefficient of - 180 ppm/°C, while a layer of 1 / 1 000 µm has a temperature coefficient of - 1800 ppm/°C, that is to say ten times higher.

This is why one prefers to choose a layer a little thicker and to increase his length by tracing a spiral on the tube (figure 3-a).

It is obvious that the finer the step of the net is and the narrower the resistive band will be long and; its resistivity also increases.

The spiralage is carried out on a special lathe by a set with diamonds grinding stone. The turn stops automatically when the desired ohmic value is obtained thanks to a bridge of measurement. The precision lies between ± 2 % and ± 5 %.

Another manufacturing method, adapted better to obtain low values of resistances, consists in depositing on a support in the shape of tube or plate of resistive metallic materials (noble alloys, metals or oxides) by vacuum vaporization and in the form of extremely thin successive layers until obtaining the thickness which gives the desired ohmic value (figure 3-b).

With a metal film deposit on a ceramics support, one carries out resistances of ohmic low values and larger thermal dissipation (of 0,07 W with 4 W for 1 W and of 0,1 W with 100 W for 14 W) ; the absence of spiralage and thus of inductance (coil) allows their use in high frequency.

Whatever the method used, the material deposited ends in two terminals made up of galvanized copper wire, fixed by two cups metal or welded onto the two ends of the resistive body. The unit is protected by a plastic varnish, a synthetic resin coating or a moulded coating which make that outside these resistances resemble current agglomerated resistances (figure 3-c).

The multiple varieties of manufactoring processes of resistances and the types of film adopted by the manufacturers as well as the direct correlation between the power and dimensions make so that resistances show rather different electric characteristics, while having the same appearance.

The resistors film of carbon whose face value extends from 1 W to 22 MW with typical tolerances of ± 5 % or ± 10 %, have a temperature coefficient ranging between 150 and 800 ppm/°C, and, an acceptable power going from 1 / 20 W to 2 W. They are employed for a general use.

They have a great reliability and a good behavior at the high frequencies. Moreover, they present a weak noise and are of a moderate cost. They are in many points comparable with agglomerated resistances.

The table of figure 4 indicates as example, the principal characteristics of the resistors film of carbon from Philips and dimensions compared to the acceptable power at ambient temperature Ta = 70° C.

 Power (Ta = 70° C) W Range of the values of resistances Tolerances ± % Applicable tension max Veff Dimensions D x L in mm 0,2 10 W - 220 kW 5 150 1,6 x 4 0,2 270 W - 1 MW 10 150 1,6 x 4 0,2 10 W - 220 kW 5 150 1,6 x 4 0,2 270 W - 1 MW 10 150 1,6 x 4 0,33 1 W - 1 MW 5 150 2,5 x 6,5 0,33 1,2 MW - 10 MW 10 150 2,5 x 6,5 0,33 1 W - 1 MW 5 150 2,5 x 6,5 0,33 1,2 MW - 10 MW 10 150 2,5 x 6,5 0,5 1 W - 1 MW 5 350 9,7 x 10 0,5 1,2 MW - 10 MW 10 350 9,7 x 10 0,5 1 W - 1 MW 5 350 9,7 x 10 0,5 1,2 MW - 10 MW 10 350 9,7 x 10 0,67 1 W - 1 MW 5 500 5,2 x 16,5 1,15 1 W - 1 MW 5 750 6,8 x 18 2 10 W - 1 MW 5 1 000 9 x 31,7
4. - Electric Characteristics and dimensions of the resistors film of carbon produced by Philips.

Resistances to metal film are used for professional uses, where are required : the precision (tolerances lower or equal to 0,1 %), the temperature coefficient close to ± 25 ppm/° C, a great stability and a weak noise.

Naturally, the electric characteristics can vary from a type of resistance to the other according to specifications' of use.

The table of figure 5 summarizes the technical data and dimensions of the resistors film metal from Philips and constitutes a comparative data with the resistors film of carbon seen previously.

 Power (Ta = 70° C) W Range of the values of resistances Tolerances ± % Applicable tension max Veff Temperature coefficient ± ppm/°C Dimensions D x L mm 0,4 4,99 W - 681 kW 0,5 250 50 * 2,5 x 6,5 0,4 4,99 W - 681 kW 1 250 50 * 2,5 x 6,5 0,4 1 W - 680 kW 2 250 100 2,5 x 6,5 0,4 1 W - 680 kW 5 250 200 2,5 x 6,5 0,5 4,99 W - 1 MW 0,5 350 50 * 3 x10 0,5 4,99 W - 1 MW 1 350 50 * 3 x 10 0,5 5,1 W - 1 MW 2 350 100 3 x 10 0,5 5,1 W - 1 MW 5 350 200 3 x 10 0, 75 4,99 W - 1 MW 1 500 100 5,2 x 16,5 1,6 10 W - 10 kW 5 500 500 3,7 x 10 2,5 10 W - 27 kW 5 500 500 5,2 x 16,7
 * For values £ (lower or equal) 49,9 W : 100 ppm/°C

Fig. 5. - Electric Characteristics and dimensions of the resistors film metal (general use) from Philips.

Resistances to metallic oxide film whose resistive element is consisted tin oxide heated with 800° and pulverized on the support, are characterized by a tolerance of ± 1 %, a level of very low noise, a great stability and a correct operation at the high frequencies; their employment is particularly professional.

Resistances to film “metal-glass” have the resistive element obtained by taking on a support isolating from noble metal powder like money and palladium, mixed with glass powder.

Although these resistances do not have the same precision that those with metal film, they nevertheless have a great stability, a weak noise and especially a great immunity with the atmospheric agents ; they also tolerate high temperatures and strong overloads without damage. With the same method of spiralage adopted for the resistors film of carbon and while moving away the adjacent whorls more, one carries out resistances supporting of high voltages: the electric arc being less solicited between two whorls.

Figure 6 gives some data on these resistances.

 Maximum value of tension Range of the values of resistance Tolerance ± % Power (Ta = 25°C) W Temperature coefficient ± ppm/°C Dimensions D x L mm Vcc 3 500, Veff 2 500 1 MW - 33 MW 1 to 5 0,5 200 3,7 x 10 Vcc 10 000, Veff 7 000 1 MW - 68 MW 1 to 5 1 200 6,8 x 18
6. - Electronic Characteristics and dimensions of resistances out of metal-glass from Philips

The wire-wound resistors are obtained by rolling up a wire of high resistivity (for example: nickel-chromium, constantan, manganin) on a support insulating out of ceramics impregnated, at high temperature, of special resins. The techniques of manufacture adopted for these resistances are also very variable so that one can generalize their form and their dimensions.

On the figure 7-a is represented a type of wire-wound resistor obtained by rolling up a resistive wire around a support rough and insulating, not very sensitive to the high temperatures; the ends of the wire are welded at the boundaries and the unit and protected with a cylindrical moulded plastic coating, tolerating a temperature of 220° C.

This type of resistor wire-wound to thus an aspect similar to that of an ordinary agglomerated resistance, with the difference which the first ring of color is the double of the others. The power of these resistances generally does not exceed 2 Watts and the values spread out of 1 W with 10 W.

The resistor wire-wound illustrated in the figure 7-b differs from the preceding one by the external coating insulator which can tolerate a temperature of 350°C. The ohmic values of these resistances go from 0,1 W to 33 kW with a maximum power of 30 Watts for everyday usages.

In the figure 7-c, this type of wire-wound resistors is particularly adapted to dissipate high powers; they are carried out with a steatite envelope with square section and are sealed with special cement.

In figure 8 some examples of these resistances are illustrated of which some are provided with a steel spring acting as cooling rib. In this way, one evacuates of advantage the calories and one can increase the nominal nominal output dissipated by resistance. This spring also constitutes a body of fixing to the frame and sometimes it forms integral part of resistance (figure 8-b).

There are also some types of resistances equipped with a thermal safety device delayed, consisted a plate with spring welded with tin onto one of the terminals (figure 8-c) ; if the temperature of resistance exceeds a fixed threshold, because of a failure on the circuit, tin melts, releasing the end of the plate which slackens towards outside and removes the electric connection, thus protecting the circuit from later damage. When the breakdown on the circuit is detected, one will be able to fix the plate on the terminal of resistance, simply by welding with tin. (In theory, one should even replace it, because this last with sudden a heating).

Other types of wire-wound resistors are illustrated in figure 9 ; they are specimens equipped with one or several catches intermediate and thus ready to be used like resistive dividing bridges.

They are manufactured with ohmic values and various powers which can reach for these last from 8 to 250 Watts.

Insulation is obtained by a special vitrified enamel. The ceramics support with circular section (sometimes elliptic to reduce the obstruction of it) is porous in order to support cooling.

In the measuring instruments, one uses resistances whose face value is lower than the ohm, the very large precision and the maximum power of a few Watts ; the resistant wire is rolled up on a chuck not very sensitive to heat (figure 10).

The two terminals of the chuck, at which the ends of resistive rolling up are fixed, can be connected to the electronic circuit by welding with tin.

These wire-wound resistors are available on a broad range of values going from some tenth of ohm to 100 kW with tolerances about ± 10 %, of ± 5 % and even of ± 0,5 % for most precise. They are used when those of the agglomerated type of carbon are not appropriate: for a raised dissipation, a narrow tolerance and especially for a great stability of the ohmic value.

The tension of noise of these resistances is generally negligible but one avoids using them in assemblies where the frequency is higher than the audio band because of their high residual inductance.

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Daniel