|Notes practice on resistances||Variable resistors||Characteristics of the potentiometers|
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|Classification of resistances|
|Identification of resistances|
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ELECTRONIC COMPONENTS 1 “2nd PART”
2. 3. - IDENTIFICATION OF RESISTANCES
The replacement of a resistance in an electronic circuit requires the taking into account of the following parameters : the face value, the tolerance, and, nominal power output.Inscriptions :
These data are expressed by means of figures, of letters or rings of color according to a well defined code.
When the ohmic value is indicated in figures, one can find for example the inscriptions following :
0,301W ± 1 % - 1 W ± 1 % - 1 W
19,6W ± 1 %
2.10W ± 1 %
It is noticed that the indication of power can be omitted and that one can find a point in the place of the comma.
Sometimes for the ohmic values lower than 1 000 W, the symbol “W” is replaced by the letter “R”; if this value is decimal, the letter “R” can take the place of the comma or the point as in the examples following :
100 R ± 10 % = 100 W, tolerance ± 10 %
R499 ± 1 % = 0,499 W, tolerance ± 1 %
19R6 ± 1 % = 19,6 W, tolerance ± 1 %
For the resistances produced by the same manufacturer and of value higher than 1.000 W, the letter “R” is replaced by the letter “K” (multiplying : x 1.000) :
8K2 = 8,200 kW = 8 200 W
2K15 = 2,15 kW = 2 150 W
Lastly, the inscription can be shortened as follows :
1,5M /10 / 1 = 1,5 MW ± 10 %, 1 W
Code colors :
If dimensions of agglomerated resistances and with layer do not allow the inscription of the parameters in entirety one has recourse to the system of the rings of color laid out around the cylindrical body of resistances, from one of the two terminals.
With a method, the reading of the parameters is always possible whatever the position of resistance on the assembly and is more durable in time.
The international codes are two :the code with four colors
the code with five colors
The first code with four colors is deferred on figure 11 ; as one can see it, each ring of color, according to the occupied position, has a particular significance.
The reading begins starting from the band nearest to the one from the ends from resistance; the colors of the first two rings indicate the first two significant figures of the ohmic value, the color of the third ring gives the ratio of intensification and finally the fourth the tolerance.
The possible absence of the fourth ring implies a value with tolerance of ± 20 % and if the first ring is broader than the others then they are wire-wound and either agglomerated resistors (see figure 7-a).
Some examples of reading of ohmic values are deferred below :
We obtain for this example a face value of 1 200 W ± 5 % like illustrated figure 11.
In this case the face value is of 56 000 W ± 10 %
The third ring in black indicates that there is no zero, therefore the value is of 12 W ± 1 %.
The resistive value corresponds to 1 000 000 W = 1 MW ; the absence of the fourth ring indicates a tolerance of ± 20 %.
Let us take this time two examples concerning the coding of resistances lower than 10 W :
The face value of a resistance having the rings of color above will be of: 91 / 100 = 0,91 W tolerance ± 5 %
In this case, the nominal ohmic value is 51 / 10 = 5,1 W tolerance = ± 5 %
The code with five colors is deferred in figure 12 below ; it is used when the ohmic value is made up of three significant figures ; the face value is included/understood in the first three rings and the two last correspond to the same parameters as those of the code with four colors.
Examples of reading :
The resistive value is of 1 050 W ± 2 %
In this case the face value is 864 / 100 = 8,64 W ± 0,1 %.
The value of resistance is of 943 W ± 1 %
Agglomerated resistances and to layer have ohmic values standardized for each tolerance. The tables of figures 13 and 14 show the ohmic values of the resistances usually used with the respective tolerances of ± 10 % and ± 5 %.
The maximum power that a resistance can dissipate is expressed in figures (example 1 W, 2 W, etc…) or in the form of a code registered on the body of the element, in the contrary case there is not any indication.
For agglomerated resistances, the maximum power being able to be dissipated is according to its dimensions (see figure 2-c). For the other types of resistance, when this differentiation is not obvious, it is necessary to refer to the techniques of manufacture adopted by the various manufacturers and to consult the catalog of the latter.
To choose the power which a resistance will be able to dissipate, in case of doubt at the time of a replacement, one proceeds in experiments by measuring the tension present at the terminals of resistance when the circuit is fed; by dividing the square of this tension read (V2 = V x V) by the value “R” of the resistive element, one obtains the power which must be dissipated by resistance.
Let us take an example : that is to say a resistance of 1 000 W and a tension applied at the boundaries of 36 Volts ; the power “P” to dissipate will be thus :
This resistance will be able to dissipate a power of 1,296 W ; one will choose the standardized value which is just higher than this computed value ; one finds for this example 2 Watts.
Note : It is always necessary to choose a resistance of power higher than that calculated because the component “does not work” not in extreme cases of its possibilities, which ensures a larger longevity to him.
2. 4. - NOTES PRACTICE ON RESISTANCES
It can arrive, for the technician or that who experienced an assembly, not to have at its disposal a resistance of given value; it is possible to overcome the difficulty by having recourse to association series or parallel of two or several resistances.
By carrying out a connection series of resistances, their ohmic values are added and one can obtain a value not standardized by two or several values which are it.
For example, the value 1 150 W can be obtained by a resistance of 680 W in series with another of 470 W or a resistance of 1 000 W in series with one of 150 W or three resistances : one of 150 W and two of 500 W.
While carrying out, on the contrary, a parallel connection one obtains an ohmic value lower than smallest of the values of resistance of the connection.
In this case, if two resistances R1 and R2 are in parallel, equivalent resistance will be :
For example : R1 = 120 W and R2 = 240 W
To know which value of resistance R2, one must put in parallel with R1 to obtain an equivalent resistance Req well defined, it is necessary to apply the following formula :
Let us suppose that one wants for example, to obtain a Req resistance of 60 W and that we have at our disposal a R1 resistance of 100 W ; the value of R2 resistance in parallel with R1 will be :
In a connection series or parallel of two or several resistances, the sum of the powers which can be dissipated by these last is not always equal to the power being able to be dissipated by substituted resistance. Indeed, so that it is it, it is necessary that all resistances of substitution have the same ohmic value and the same nominal nominal output.
Example : A resistance of 1 500 W - 2 W can be obtained by connecting in series two resistances of 750 W - 1 W or in parallel two resistances of 3 000 W - 1 W.
If this time one carried out 1 500 W - 2 W by two resistances in series of 1 000 W - 1 W and one of 500 W - 1 W, one would realize that the resistance of 1 000 W would not support the dissipated power (here 1,33 Watt).
There is a connection which can be useful in many cases, because he combines two to two in series and parallel four resistances of the same ohmic value and of the same power (figure 15).
The equivalent ohmic value of this combination is the same one as that of a resistance whereas the power is some quadrupled.
The noise of a resistance is taken into account in the assemblies amplifier high fidelity (HI-FI) and in general in those where the signal to be amplified is of low level.
The maximum tension applied at the boundaries of a resistance is to be considered in the assemblies bringing into play impulse signals of a certain amplitude and in the dividing bridges high-tension.
In circuits H.F. (high frequency), it is necessary to avoid the use of resistors wire-wound because of inductance parasitizes high, while employing, preferably, of agglomerated resistances or to layer.
The stability and the temperature coefficient have an interest only for the apparatuses of professional class such as for example the measuring instruments of laboratory, medical and military, for which a reliable operation is necessary.
3. - VARIABLE RESISTORS
The variable resistors are consisted a resistive element on which a contact called moves cursor which is ordered by the user ; thus, the ohmic value of the element varies according to the position of the cursor.
When the ends of the resistive element and the cursor are connected on three distinct external terminals, there is a potentiometer; so on the other hand the cursor is connected to the one of the side contacts and presents only two external terminals, one has a rheostat or a variable resistor.
3. 1. - POTENTIOMETERS
The technique of manufacture of the potentiometers is very variable but according to the type of resistive elements, it is possible to make the following distinctions :
On the figure 16-a the basic structure of a potentiometer to the graphite is illustrated in which the resistive element consists of a layer of graphite deposited on a circular bakelite tape.
The figure 16-b represents a wire-wound potentiometer in which resistance consists of a resistive wire rolled up on a suitable support.
On the resistive element slips a mobile contact (cursor) on which is mechanically fixed a control shaft isolated from the cursor.
While turning this axis, one varies the ohmic value between the terminal connected to the cursor and each side terminal fixed at the ends of resistance (figure 17-a).
A rotation in time direction of the axis involves an increase in resistance (Ra) between the cursor (C) and end A, while resistance Rb between the cursor and the end B decreases. A rotation in opposite direction involves contrary effects (Rb increases, Ra decreases).
Having a different structure and an appearance, the potentiometer illustrated in figure 18, called “SLIDER” or “rectilinear potentiometer”, is characterized by a right configuration of the resistive element. The cursor moves in translation, ordered by an external lever; it is consisted a spring in the shape of “U” which slips in electrical contact on the resistive element and a conducting plate parallel and isolated from this one. This plate is connected on the external central terminal which is in fact the cursor.
3. 2. - DATA CHARACTERISTIC OF THE POTENTIOMETERS
The potentiometers are characterized by their ohmic value, the pace of the variation of this resistivity, the tolerance and the power being able to be dissipated.
The ohmic value indicates the total resistivity (Ra + Rb) between the extreme terminals (A and B on the figure 17-a).
The pace of the ohmic variation is given according to the position of the cursor and the ohmic value present between this last and one of the terminals of the resistive element ; it can be linear, logarithmic curve or pseudo-logarithmic curve (figure 19 and 20).
The tolerance and the dissipated power have the same significance as for fixed resistances.
The face values can be expressed in W (ohm), kW (kiloohm) or MW (megohm). Certain manufacturers indicate all the values in MW, even the low values, without preceding the comma by zero.
Example : .01 MW means 0,01 MW = 10 kW
Often in the place of the comma, the letters R, K and M are used and replace the multiplying coefficients which are worth 1 respectively, 1 000, 1 000 000 (see chapter on the identification of fixed resistances).
The face value of the potentiometer is clearly indicated on the cap at the same time as a code which indicates the type of variation of it.
As all the manufacturers do not follow the same standards, the adopted codes are different.
3. 2. 1. - POTENTIOMETERS WITH GRAPHITE
They are available under various standardized ohmic values.
For those which have a linear progression, the ohmic values are generally included/understood of 100 W to 10 MW whereas those which have a progression logarithmic curve have values between 2 000 W and 1 MW.
The table of the figure 21-a gives the standardized ohmic values of the potentiometers by dividing them in two categories : on the first line the practical values are gathered more running. Certain manufacturers provide however, for important orders of the values of the series indicated in the table of the figure 21-b.
The “rotary” potentiometers with graphite can simple (figure 22-a) or mechanically be coupled “double” (figure 22-b) ; these two types are sometimes provided with a unipolar or bipolar switch.
The double potentiometers can have a single order which positions the two cursors at the same time, or of the separate controls acting each one on the cursor of two resistances. On the figure 22-c, a simple potentiometer is equipped with a bipolar switch which is actuated while drawing or by pushing the control shaft. The double potentiometer, illustrated in the figure 22-d is equipped with two coaxial separate controls for the adjustment independent of two resistances and a unipolar switch which is closed before one of the coaxial orders does not arrive in race end (on only one side).
The axes of the simple or double potentiometers with single order can be various lengths; there are not adopted universal standards: they are sometimes out of plastic, easily adaptable to the wanted length, and sometimes with a metal stem differently machined for the fixing of the control knob (figure 23-a).
On the figure 23-b, one can observe some types of coaxial axes for double potentiometers with separate control.
These potentiometers are fixed mechanically on the frame of the apparatus with a star washer, a nut and a counter-nut (figure 24).
The printed circuits which equip all the electronic instruments today, led the manufacturers to carry out various types of potentiometers with terminals adapted to the new requirements while preserving their characteristics.
On figure 25 are given two types of potentiometers of which one with terminals parallel with the axis of rotation and the other with terminals perpendicular to this same axis, thus placed to be welded onto the printed circuit.
There are many miniaturized potentiometers, whose some specimens are visible in figure 26. Being given their reduced dimensions, these components are rather fragile and in the major part of the cases, they are used like adjustable devices.
These adjustable potentiometers also called “trimmers” are often stripped of control shaft and have in the place a slit or a central screw which allows the adjustment with a screwdriver.
The trimmers of precision have a micrometric adjustable tangent positioning the cursor in a ratio of 1 / 1 to 30 / 1 (figure 27). They are used especially for applications of the professional type ; the resistive element is made up either by a wire rolled up on an insulating support, or by a layer of graphite.
The adjustable potentiometers with graphite can dissipate a power ranging between 0,1 W and 1 W approximately; those which have a higher power are carried out with resistive wire.
3. 2. 2. - WIRE-WOUND POTENTIOMETERS
The choice in the field of the potentiometers with resistive element with layer of graphite is very vast as in that of the wire-wound potentiometers where the resistive element consists of a wire rolled up on a determined support on which a cursor moves.
According to the section, the quantity of wire and the type of rolling up, one obtains potentiometers with different characteristics.
With potentiometers with layer of graphite, one cannot carry out very ohmic low values as with the wire-wound potentiometers from which the resistive beach extends from the ohm to 200 kW (seldom beyond).
The dissipation of power does not exceed 2 W for the potentiometers with layer of graphite whereas it usually reaches between 2 W and 5 W for the wire-wound potentiometers.
For special uses, one finds wire-wound potentiometers, called this time rheostats, which dissipate from 50 to 100 Watts.
Like those out of graphite, the wire-wound potentiometers can be simple or double, with single or separated order, with or without switch; their appearance resembles that of the potentiometers with the graphite put besides their dimensions which are proportional to the power being able to be dissipated.
Figure 28 illustrates some specimens of obstruction means and usually used in the electronics industry; the resistive element and the machine parts are contained in a phenolic resin case with a metal bottom of closing.
We have just seen fixed resistances and the variables which are the most used components and simplest to examine in electronics.
In the next lesson, we will look at another component as useful attentively as resistance and often associated with the latter in the electronic assemblies : the condenser.
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