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Created it, 05/10/15
Update it, 05/12/28
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NOTE :
Into the preceding theories, we almost introduced all the electric quantities which one meets in electronics with their respective measuring units. We must see now how these sizes are measured, i.e. how, in practice, one can know their value.
To this end, let us specify before very that, for the electronics specialist, it is primarily interesting to measure the current, the tension and resistance. Indeed, when one knows the values taken by these three sizes in various points of the circuit, one can already have a precise idea of his operation and thus identify one of his possible failures.
In this lesson, we will see the apparatuses and the circuits most usually used to measure the currents, the tensions and resistances.
1. - MEASUREMENT
OF THE CURRENT
We of the measurement of the current occupy initially because, as we will see it thereafter, measurements of the tension and resistance are done via this measurement.
In one of the preceding theories, we said to you that one could measure the electrical current by exploiting one of the effects which it produces while crossing a driver; generally, one uses the magnetic effect of the current, i.e. the property which it has to produce a magnetic field around the drivers that it crosses.
In practice, the current to measure master key in a reel and thus produces a magnetic field whose tension fields, indicated on the figure 1-a, have the direction determined by the direction of flow of the current, in accordance with the law of the corkscrew.
We observe that these tension fields leave the right end of the reel and return by its left end after having crossed external space, in whom they progress like the tension fields of a permanent magnet (figure 1-b).
With regard to external space, the reel thus behaves like a permanent magnet, and we can thus regard as North Pole and South Pole the ends of the reel by which the tension fields leave and enter.
For these poles, one can also apply the general law which we know already, according to which the of the same poles name are pushed back, while the poles of contrary name attract each other; one can note it, for example, thanks to the device of figure 2, which, with particular modifications, is also used to measure the currents.
As one sees it on this figure, one placed between the poles of a U-shaped magnet, a reel rolled up on a small light frame provided with two pivots which are based on special supports, so that the reel is free to turn around a vertical axis.
Let us suppose that rolling up is traversed by a current which circulates in a direction such as it produces directed tension fields as one sees it on the figure 2-a ; the North Pole and the South Pole of the reel are in the positions indicated by this figure.
Under these conditions, one notes that the North Pole of the reel is pushed back by the North Pole of the magnet and is attracted by its South Pole ; in the same way, the South Pole of the reel is pushed back by the South Pole of the magnet and is attracted by its North Pole, in agreement with the law about which we spoke higher.
In consequence of these repulsions and of these attractions between the reel and magnetic poles, this one turns in the direction indicated by the arrow of the figure 2-a, until it stops in the position of the figure 2-b, thus carrying its North Pole close to the South Pole to the magnet and its South Pole close to the North Pole of this magnet.
It was as noted as the forces of repulsion and of attraction which are exerted between the poles are all the more important as the current which traverses the reel is larger. This fact made it possible to use the device of figure 2 for the measurement of the current.
For that, the device is modified as on the figure 3-a and one thus produces a measuring instrument of the current, as called moving-coil galvanometer because of rotation as its reel can achieve, rotation which is used to move opposite a dial graduated a needle fixed at this reel.
Inside the reel, one places a small soft iron cylinder which is fixed, i.e. which does not turn with the reel, while the ends of the magnetic poles are shaped in order to to the minimum reduce the air-gap (or the space occupied by the air) in which the reel moves during its rotation.
The moving-coil galvanometer can also be produced in a different way although the principle remains identical. Instead of being in the horseshoe shape, the magnet can of cylindrical and inserted form inside the mobile reel like be represented on the figure 3-b. In this case, a soft iron ring placed outside the reel closes again the magnetic circuit.
With the adopted processes, one obtains for the two types of galvanometers an important magnetic field and one can thus measure currents of very reduced intensity, because the forces due to this important field are sufficient to determine the rotation of the reel, even if the reduced current which traverses it creates a very weak field. Moreover, the tension fields of the field produced by the magnet are laid out uniformly around the reel, which makes it possible to obtain a dial with equidistant graduations, as one sees it on figure 3.
Let us note finally that around the pivots of the reel, there are two springs with spiral whose end is fixed at the pivots and the other with the supports ; these springs are laid out in order to be able to block the rotation of the reel and this is why they are called antagonistic springs.
Us to account for the function carried out by the antagonistic springs, let us see the operation of the instrument.
When no current traverses the reel, the springs maintain it in a position such as its needle is opposite the left end of the dial, where the zero are registered; this zero indicate that the current is null. Under these conditions, one says that the needle is in the home position.
When a current traverses the reel, it turns as we saw previously and its needle moves towards the line while moving away from the home position, as one sees it on the figure 3-a. The rotation of the reel then operates the springs fixed at the pivots by making them tighten itself, which gives rise to forces which block this rotation, because they seek to push back the reel and its needle towards the left, i.e. towards the home position.
As the reel turns, the springs are tightened more and more and consequently the forces which block rotation increase until they are equal to those which determine this rotation. Thus, the reel stops because there is balance between the forces due to the current, which tend to make it turn towards the line, and the forces due to the springs, which tend to bring back it towards the left.
Let us suppose, for example, that the reel and its needle stopped in the position indicated figure 3-a ; if one now amplifies the current which traverses the reel, this one recovers to turn towards the line because the forces which determine rotation in this direction increase as at the same time as the current and balance is thus broken. In addition, because of the new rotation, the springs are still tightened and the forces which are due for them increase in their turn, until they balance again the forces due to the current; when that occurs, the reel stops in a new position, more distant than the preceding one from the home position.
We see all in all that to each value of the current a well defined position taken by the needle corresponds. To measure the currents with this galvanometer, it is thus enough to know the values of the current which correspond to the various positions of the needle. For that, one can make traverse the reel of the galvanometer by various currents of known value, that is to say initially by a current of 1 mA, then 2 mA, then of 3 mA, etc… ; one marks then these values on the dial opposite the position taken by the needle for each current.
When these values are registered on the dial, the galvanometer can be useful for measurement of a current which one does not know the value : indeed, one can now read this value directly on the dial according to the position in which the needle stops when the galvanometer is crossed by the current to measure.
For example, the numbers deferred on the graduated scale of the figure 3-b indicate the value of the current in milliamperes, as one sees it on the inscription “mA”, symbol of the milliampere, than one finds under the dial.
If we thus make cross this galvanometer by a current of unknown value and if we see his needle stopping opposite number 1, as on the figure 3-a, we can say that the current has a value of 1 mA. Since this instrument is all in all used to measure the milliamperes, it is called a milliammeter.
All the numbers registered on the dial of the galvanometer, most important is that which is at the right end, i.e. at the end of scale, because it indicates the gauge of the galvanometer, i.e. the maximum current which it can measure.
The galvanometers of figure 3 have a gauge of 10 my bus, as we see it on this figure, the number registered at the end of the dial is precisely 10.
Each galvanometer is characterized by its gauge, which is a data which one must remember when a determined galvanometer is used, to avoid making it cross by a current higher than the maximum current than it can measure. Indeed, if that occurred, the galvanometer could worsen all the more seriously as the current which it cross-piece is higher than its gauge.
Let us suppose, for example, that the galvanometer of the figure 3-a is crossed by error by a current of 20 mA. In this case, the forces which determine the rotation of the reel towards the line are multiplied by two compared to those which are necessary to carry the needle at the end of scale. Consequently, the needle exceeds the final position very quickly and can be deformed by the violent shock against the thrust which, as one sees it on figure 3, is placed shortly after the end of scale, to stop the needle.
Another data characteristic of a galvanometer with mobile reel is its internal resistance, i.e. the resistance which the current to measure meeting when it passes in this galvanometer ; this resistance is due to the driver rolled up which constitutes the mobile reel and can lie between a few tens and a few hundred ohms, according to the type of galvanometer.
In certain cases, internal resistance can have an influence to the measure; we will be able to note it by examining the insertion of a measuring instrument of the current, i.e. the manner of connecting it to a circuit to measure the current which circulates there.
Since the current to be measured must cross the galvanometer, it is obvious that this one must be connected in series with the circuit in which this current circulates.
Let us suppose, for example, that one wants to measure the current which circulates in the circuit of the figure 4-a ; since this circuit is fed by a tension of 20 V and that it has a resistance of 2 kW, the current which circulates there is 20 / 2 = 10 mA .
We point out to you that, like one expressed resistance in kilo-ohms, rather than in ohms, the current is expressed in milliamperes instead of being it in amps. Thus the operations are simpler, because they can be made with numbers which do not comprise too many of zeros ; if on the contrary, one had used as measuring unit the ohm and the amp, one should have made division 20 / 2 000 = 0,01 Amp = 10 mA.
Since the current has an intensity of 10 mA, we can measure it with a milliammeter of a gauge of 10 mA, like those of figure 3.
On the figure 4-b, one sees the insertion of this galvanometer represented by a small circle inside whose the inscription “my” recalls that it is about the milliammeter.
Under the graphic symbol of the galvanometer, gauge 10 is indicated mA ; the internal resistance (r) of the milliammeter is also indicated, it is of 500 W, i.e. of 0,5 kW.
Since the milliammeter is connected in series, its internal resistance of 0,5 kW is added to that of 2 kW of resistance R and, after the insertion of the milliammeter, the circuit thus has a total resistance of 0,5 + 2 = 2,5 kW.
Under these conditions, the current which traverses the circuit is 20 / 2,5 = 8 mA. We thus see that the insertion of the milliammeter disturbs the operation of the circuit, while making pass from 10 mA to 8 mA.
The milliammeter will indicate a current of 8 mA, while in reality the current which traverses the circuit, when the instrument is not inserted, is 10 mA.
We cannot thus know with exactitude the intensity of the current, because the internal resistance of the galvanometer increases the resistance of the circuit and consequently decreases the current ; to obtain a more exact measurement, we must thus use a galvanometer which has a resistance interns much weaker.
For example, if
one inserts in the circuit a milliammeter whose internal resistance is only of 10
W, i.e. of 0,01
kW, as on the figure 4-c, the
resistance of the circuit will pass from 2
kW to 2,01
kW
and the current will be thus 20 / 2,01
9,95 mA.
This value differs only from 0,05 mA of the actual value of the current which is 10 mA, and in this case, measurement is sufficiently precise for the practical studies.
These examples thus enable us to conclude that a measuring instrument of the current provides all the more exact indications as its internal resistance is lower.
Let us observe finally that, for the insertion of a measuring instrument of a current, it is necessary to take account of the direction of circulation in which the current must traverse the mobile reel.
We saw indeed that the direction of the tension fields, and thus the poles which are created at the ends of the reel, depend on the direction of flow of the current ; if it circulated in the contrary direction of that in which it should circulate, the tension fields would be also directed in contrary direction ; at an end of the reel, one would thus have the South Pole instead of the North Pole and conversely.
In consequence of this inversion of the poles, the direction in which the reel would turn would be also reversed and the needle would thus move on the left home position, with the place that it is on the right. It would thus leave the limits of the scale and it would not be possible to read the value of the current.
To prevent that the galvanometer is not inserted thus, the manufacturer distinguishes the two ends from the reel by marking them of the signs + and -, to indicate that the current must traverse the reel of the positive end towards the negative end, in agreement with the conventional direction according to which the current is directed the positive one towards the negative one.
These signs are also indicated beside the galvanometer on the diagrams of figure 4 ; one can thus see that the current which crosses the milliammeter is always directed the positive one towards the negative one.
The fact that the displacement of the needle depends on the direction in which circulates the current in the mobile reel has as a consequence that an instrument of this type cannot be used to measure the alternating currents. Indeed, these currents would traverse the mobile reel of its positive end towards its negative end during a half-period and in contrary direction during the following half-period ; with each cycle of the current, the needle should initially move on a side of the home position then other.
Let us take the example of the AC current of the sector of frequency 50 Hz. Since it achieves 50 cycles a second, the needle should repeat 50 times a second this displacement around the home position. Actually, the needle cannot achieve such fast displacements and it remains in the home position thus indicating a null current.
Until now, we examined a milliammeter with a gauge of 10 mA but very often one must measure currents of larger intensity, reaching a few hundred milliamperes. One can measure these currents with the same galvanometer as that which we have just seen, by increasing its gauge.
We for that with the circuit of the figure 5-a refer, where the battery of 100 V makes circulate in the resistance of 5 kW a current of 100 / 5 = 20 mA and suppose that one wants to measure this current with a galvanometer which precisely has a gauge of 10 mA.
Since the gauge of the galvanometer (10 mA) is equal to half of the current to measure (20 mA), it is necessary that there is only half of the current which crosses the galvanometer not to overload it.
One thus connects in parallel to the galvanometer a resistance, called resistance shunt or more simply a shunt, so that other half of the current, which should not traverse the instrument, can pass there.
For that, it is necessary that the shunt has a resistance equal to the internal resistance of the instrument ; since in the case of the figure 5-a, the milliammeter has an internal resistance (r) of 10 W, one adopted thus for Rs resistance of the shunt the value of 10 W. Thus, the current (I) of 20 mA, arrived at point A, is divided into two equal currents of 10 mA each one ; one traverses the shunt, while the other crosses the galvanometer, which thus indicates the value of 10 mA.
The galvanometer which has a shunt connected at its ends thus measures half of the current which circulates in the circuit ; one can thus know the value of it while multiplying by two the value read on the dial.
In this particular case, the shunt is used to multiply by two the gauge of the milliammeter and makes it possible to know the value of the currents until a maximum of 20 mA ; but one can also triple, quadruple, etc, the gauge of a galvanometer, by choosing the value of the shunt correctly.
On the figure 5-b, one sees for example, how one can quintuple the gauge of the galvanometer examined higher to be able to measure currents until a maximum of 50 mA.
In this case, so that a maximum current of 10 mA master key still in the galvanometer, one needs that the shunt is crossed by a current of 40 mA, four times more intense. This is obtained by giving to the shunt a value four times smaller than that of the internal resistance of the galvanometer, i.e. a value of 2,5 W (10 / 4 = 2,5 W). Thus, the galvanometer measures only one fifth of the current which circulates in the circuit and one obtains his value while multiplying by five the value read on the dial.
Generally, let us call Ig the maximum current of the galvanometer and the Ic current maximum which one wants to measure, therefore gauge. Rs resistance of the shunt is traversed by current Ic - Ig : one with the relation Rs (Ic - Ig) = r x Ig is :
Rs = Ig / (Ic - Ig) x r
This formula thus allows the calculation of the resistance of the shunt starting from resistance interns r of the galvanometer, its maximum current Ig and gauge Ic desired.
The possibility of easily increasing the gauge of a galvanometer thanks to the shunts makes it possible to use not only the milliammeters but also the microammeters, i.e. the instruments whose gauge is about the microamperes. The use of instruments with a current of very low end of scale is useful in electronics for the measurement of the tensions, as we will see it.
2. - MEASUREMENT OF THE TENSION
One can measure the electric tension with the same galvanometer with mobile reel as that which was used for measurement of the current.
For us to give an account of it, let us see the figure 6-a on which one again deferred the circuit of the figure 4-c, with the only difference that resistance R, instead of having the value of 2 kW, i.e. of 2 000 W, with the value of 1 990 W. In this way, by taking account of resistance interns milliammeter, which is 10mA, the circuit has a resistance on the whole of : 1 990 + 10 = 2 000W, is 2 kW ; it thus circulates there a current of 10 mA which is exactly that indicated by the galvanometer.
Let us note now that the galvanometer indicates the current of 10 mA when the pile applies a tension of 20 V to the instrument and resistance in series, i.e. to the two elements ranging between the points indicated by A and B on the figure 6-a. Indeed, according to the law of OHM, the tension of 20 V of the pile must be equal to the product of the current of 10 mA which traverses the instrument by the “resistance” placed in series (the resistance of 2 kW presented on the whole by these two elements). One has well then: 10 x 2 = 20 V.
One thus includes/understands why, even if we do not know the value of the tension of the pile, we could always determine it by multiplying the current indicated by the galvanometer by the resistance of 2 kW.
Let us suppose for example that one replaces the pile of 20 V by another pile having an unknown tension and that, under these conditions, the milliammeter indicates a current of 6 mA ; by multiplying this current by the resistance of 2 kW (6 x 2 = 12 V), we find that the new pile provides a power of 12 V.
The unit formed by the galvanometer and the resistance put in series can thus be used for to measure the tension applied between ends A and B of these two elements.
As we saw, to know the value of this tension, it is necessary to multiply the current indicated by the galvanometer by the resistance ranging between the points A and B ; one can however avoid making this multiplication by writing directly on the instrument the values of the tension, as on the figure 6-b.
Above the scale of the instrument which, as we saw on figure 3, is used to read the milliamperes, one made one second scale. Each value of the tension is registered there opposite the value of the current indicated by the galvanometer when it is applied between the points A and B.
For example, the value of 20 V is registered opposite the value of 10 mA bus the galvanometer indicates this current when this tension is applied between A and B.
Since the galvanometer indicates the tension in volts directly, the milliammeter with resistance in series is called voltmeter.
In the diagrams, one does not represent in general the voltmeter by the symbol of the milliammeter with a resistance in series, but by that indicated on the figure 6-c, i.e. by a small circle in which is registered the lette V, symbol of the volt. Beside the graphic symbol of the voltmeter, one indicates the gauge and internal resistance.
The gauge of the voltmeter indicates the maximum tension which one can measure: the voltmeter of figure 6 has a gauge of 20 V bus with this tension, the needle of the milliammeter is at the end of the scale.
With regard to internal resistance, one indicates for the voltmeter the total resistance obtained by making the sum of the internal resistance of the milliammeter and the resistance placed in series: the voltmeter of figure 6 thus has an internal resistance of 2 kW.
The resistance which one connects in series to the milliammeter to be able to use it as voltmeter is called additional resistance. Its value determines the gauge of the voltmeter.
Let us suppose, for example, that one wants to use the milliammeter examined until now carrying out a voltmeter of gauge 50 V.
That means that power of end of scale of 10 mA must be on in the milliammeter when a tension of 50 V is measured; according to the law of OHM, the voltmeter must have an internal resistance of 50 / 10 = 5 kW, i.e. of 5 000 W.
Since the milliammeter has an internal resistance of 10 W, it is necessary to use an additional resistance of 5 000 - 10 = 4 990 W.
We thus point out that the value to be given to additional resistance is obtained by dividing the maximum tension that one wants to measure by the current of end of scale of the galvanometer and by withdrawing result obtained the internal resistance of the instrument.
Let us note finally that, on the figure 6-c the ends of the voltmeter are indicated by the signs + and - because the current which passes in this one thanks to the tension that one measurement must be directed of positive towards the negative one so that the needle moves towards the line.
Obviously, with this type of voltmeter one cannot measure alternating voltages because they would make circulate the current not only of positive towards the negative one but also of negative towards the positive one.
Since in electronics, it is interesting to also measure alternating voltages, one transforms them into continuous tensions using diodes.
A voltmeter can be used to measure the tension not only at the ends of a pile but also between two points of a circuit between which there is a potential difference.
For example, the tension of 20 V which exist at the ends of the resistance of 2 kW connected between the points A and B in the circuit of the figure 7-a, can also be measured with the voltmeter of figure 6, which precisely has a gauge of 20 V.
As one sees it on the figure 7-b, the voltmeter is connected in parallel on resistance, i.e. between the points A and B between which the tension is that one wants to measure.
The difference between this type of insertion and that which is used to measure the current is obvious: to measure the current which crosses resistance, the milliammeter must be connected in series, while to measure the tension between the ends of the same resistance, the voltmeter must be connected in parallel.
When one connects in parallel on the resistance of 2 kW the voltmeter whose internal resistance is also 2 kW, the conductance between points A and B of the figure 7-b is doubled. Resistance between these two points is thus reduced half, i.e. of 1 kW. By adding this resistance to that of 4 kw of the resistance connected between the points B and C, one sees that after the insertion of the voltmeter, the circuit on the whole has a resistance of 5 kW.
Under these conditions, a current of 60 / 5 = 12 mA traverses the circuit whereas before the insertion of the voltmeter the current was 10 mA, as one sees it on the figure 7-a ; that means that the insertion of the voltmeter disturbs the operation of the circuit while varying the current which circulates there, because part of this current must cross the voltmeter.
Since the voltmeter has an internal resistance equal to that of the resistance on which it is connected, the current of 12 mA is divided into two equal currents of 6 mA each one, of which one traverses resistance and the other the voltmeter.
At the ends of the resistance of 2 kW in which the power of 6 mA, one thus obtains a tension of 6 x 2 = 12 V, which are indicated by the voltmeter. That is confirmed by the figure 6-b which shows that the instrument indicates a tension of 12 V when it is crossed by a current of 6 mA like that precisely occurs in this case.
The voltmeter thus indicates a tension of 12 V, lower of 8 V than that which one has between A and B when the voltmeter is not inserted in the circuit (figure 7-a).
As one said, this error in measurement is due to the current which must cross the voltmeter, because if the instrument did not absorb a current, it would not make vary that which circulates in the circuit before its insertion.
To limit the error in measurement, it is thus necessary to reduce the current which crosses the voltmeter ; one can obtain that by using a milliammeter with a current of end of scale of low intensity. This is why in electronics, one uses milliammeters and microammeters, as we already announced it to you.
The voltmeter of the figure 7-b disturbs much the operation of the circuit because it is consisted a milliammeter of a current of end of scale of 10 mA, equal to that which circulates in the circuit before its insertion.
If one used, on the contrary, a voltmeter consisted a milliammeter with a current of end of scale of 1 mA, i.e. equal to the tenth of that which circulated normally in the circuit, its insertion would disturb much less operation and would make of it measurement righter.
To run the current of 1 mA in this instrument when a tension of 20 V is measured, one needs that the voltmeter has an internal resistance of 20 / 1 = 20 kW, ten times higher than that of the voltmeter of the figure 7-b.
A voltmeter thus disturbs as much less circuit to which it is connected and provides thus all the more right measurements as its internal resistance is larger ; but this resistance also depends on the gauge of the voltmeter, as we saw previously.
To be able to compare two voltmeters independently of their gauge, one indicates the internal resistance which they have for each volt.
This data which indicates the sensitivity of the voltmeters can be easily given by dividing resistance interns by the gauge : one expresses the sensitivity of a voltmeter in ohms per volt (W / V).
The voltmeter of the figure 7-b having an internal resistance of 2 kW, i.e. 2 000 W is a gauge of 20 V, has a sensitivity of :
2 000 / 20 = 100 W / V
On the contrary, the voltmeter having an internal resistance of 20 kW, i.e. 20 000 W and a gauge of 20 V, has a sensitivity of 20 000 / 20 = 1 000 W / V. It thus ten times higher than the preceding one.
We can thus conclude that a voltmeter provides all the more right indications as its sensitivity is larger.
As it is important to introduce less possible disturbance into the circuits not to distort measurements, one does not use voltmeters of a sensitivity lower than 1.000 W/V in electronics and one very often has recourse to voltmeters having a sensitivity of 5 000 W / V with 10 000 W / V and even with 20 000 W / V, for which one uses the microammeters.
3. - MEASUREMENT OF RESISTANCE
A galvanometer with mobile reel can be also used to measure electric resistance, by means of a circuit of the type of that represented on the figure 8-a. As we will see it, the galvanometer indicates on a special dial the value of resistances which are connected to ends A and B of the circuit.
The elements of this circuit are selected so that the needle of the instrument is at the end of the scale when one connects, a simple driver of very low resistance (figure 8-b) between ends A and B, i.e. when these ends are in short-circuit.
By using a pile of 4,5 V and a galvanometer of gauge 1 mA to make circulate this current and determine the displacement of the needle at the end of the scale, it is necessary that the resistance of the circuit, given by the tension of the pile divided by the current, is 4,5 kW (4,5 / 1 = 4,5). Since it is supposed that the instrument has an internal resistance of 0,1 kW, one connected to him in series a resistance of 4,4 kW.
If we want that this galvanometer indicates the resistance which exists between the points A and B, it is thus necessary to register number zero at the end of the scale, as one sees it on the right part of the figure 8-b, since the needle is precisely in this position when these points are in short-circuit and that there is thus between them a resistance equal to zero ohm.
If on the contrary, between the points A and B one inserts a resistance of 4,5 kW, as on the figure 8-c, one doubles the resistance of the circuit and consequently, one divides by two the current ; the needle thus arrives at half of the dial, i.e. in the middle of the scale, as indicated in the straight lines part of the figure 8-c.
We must thus register number 4 500 with this position, it indicates the value in ohms of the “resistance” connected between the points A and B.
If no resistance is connected between A and B, as on the figure 8-a, the needle of the instrument remains in its home position (indicated in the left part of the figure 8-a) because no current circulates in the circuit. One can thus retain that, under these conditions, there is between A and B an infinitely large resistance ; therefore one puts the sign Ą opposite the home position of the needle ; this sign indicates an infinitely large value.
If one inserts between A and B of other resistances of known value, one can mark this value on the dial opposite the graduation on which the needle stops ; thus, one will be able to read directly on the dial the value of the resistances connected between A and B.
Thus, one produced an ohmmeter, i.e. a device which makes it possible to measure the value of resistances in ohms.
One sees on figure 8 that the dial of the ohmmeter has its zero at the right end, unlike the dials of the milliammeter and the voltmeter which have to them zero at the left end, as one sees it on the figure 6-b.
The type of ohmmeter thus described presents a disadvantage. Indeed, progressively with its employment, the pile wears and provides a power lower than 4,5 V ; consequently, the current which circulates in the circuit decreases and switches does not arrive more at the end of scale when, between A and B, there is a resistance of zero value.
Under these conditions, the needle does not indicate any more zero value marked at the end of the scale, although measured resistance has a value equalizes to zero. One notes a similar error on all the other points of the dial.
To eliminate these errors of measurement, it should be made so that the needle can arrive at the end of the scale even when the tension of the pile decreases ; this is why the circuit is modified (figure 9).
The value of resistance connected in series to the instrument was reduced to 3,9 kW so that the total resistance of the circuit is 4 kW and that the current of 1 mA can thus circulate there, even when the tension of the pile goes down to 4 V (4 / 4 = 1).
But when the pile is new, a current higher than 1 mA circulates in the galvanometer and the needle moves then further the end from the scale. One cures it by connecting in parallel on the galvanometer an adjustable resistance, as one sees it on figure 9. Thus, only part of the current crosses the apparatus.
Since resistance is adjustable, one can regulate his value so that the current in the instrument keeps the value of 1 mA when the tension of the pile decreases by 4,5 to 4 V.
For using the ohmmeter, it is thus necessary to proceed to its zero setting, which consists in putting ends A and B in short-circuit and regulating adjustable resistance in order to carry the needle of the instrument on the zero of the dial of the ohmmeter. Consequently, one can remove the contact between A and B and put between these points the resistance which one wants to measure the value; this one will be indicated with exactitude by this instrument.
We observe finally that, since the resistance of the circuit was reduced to 4 kW, the needle is in the middle of the scale when the value of the resistance placed between A and B is 4 kW, as one sees it on figure 9, and either of 4,5 kW as indicated on the figure 8-c.
In this case, one must thus register in the middle of the scale number 4.000 (as on figure 9) because this number indicates in ohms the value of the resistance placed between A and B.
We thus point out that the resistance marked in the middle of the scale of an ohmmeter also indicates the resistance of the circuit of the ohmmeter itself.
Thus, we saw that measurements of current, tension and resistance can be carried out by using only one galvanometer with mobile reel inserted in an adapted circuit.
For measurements in electronics, one uses the universal controllers who include/understand, in addition to the galvanometer, the shunts and additional resistances to increase the gauge in measurements of current and tension, as well as the pile and adjustable resistance necessary for the ohmmeter.
These elements can be connected to the galvanometer to carry out a circuit adapted to the type of measurement which one must make; in this case, the galvanometer is provided with several scales which make it possible to measure currents, tensions or resistances in a broad range of values.
To finish this lesson, we reveal to you an electric diagram of the universal controller and who will allow you to include/understand well in order to know to read on this one and now, you must know all the components of this last including its operation already explained higher.
Compared to the known partial diagrams, that of figure 10 fact of appearing the presence of the fuse of 1 A placed in series with the casing “commun run”. This fuse protects the unit from the controller.
It also appears the C10 condenser of 0,1 µF, placed in series with the casing “BF”, which has as a role to eliminate any component continues possibly superimposed on the signal low frequency controlled. Casing BF makes it possible to measure the decibel i.e. the loudness in source for example of the loudspeakers.
The electric characteristics of the galvanometer are not unknown for you, its current of end of scale is 40 µA and its internal resistance of 1 200 W.
The galvanometer should not thus be crossed by a current higher than 40 µA.
If following an error of connection the galvanometer were crossed by a current much higher than the 40 µA, it would result from this from serious damage. It is thus necessary to protect the galvanometer against the overcurrents by means of two silicon diodes. As shows it figures 10 and 11, the diodes are connected head-digs at the boundaries of the galvanometer.
To include/understand how these components fulfill their role, it should be remembered that the full conduction of a silicon diode requires a potential difference between anode and its cathode from at least 0,6 to 0,7 V. This tension constitutes the threshold of conduction of the diode; below this value, the diode is blocked and the current which it cross-piece can be regarded as negligible.
Therefore, by connecting two silicon diodes with the galvanometer according to the principle of figure 11, one prevents that the terminal voltage of the galvanometer (between points A and B) exceeds 0,6 V.
If between A and B a tension higher than 0,6 V appears and si the current (I) circulates of A towards B (figure 11-a), the D4 diode enters in conduction ; so on the other hand, the current circulates of B towards A (figure 11-b) led the D3 diode.
In one as in the other case, the diode which leads comprises like a resistance shunt of low value and limit the maximum value of the current which circulates in the mobile reel, thus avoiding an unquestionable deterioration to him.
For values of intensity lower or equal to that of end of scale of the galvanometer (40 µA), the terminal voltage of this last is largely below the threshold of conduction of the diodes and their presence does not have any influence on the operation of the galvanometer nor on the result of measurement (figure 11-c).
We defer you the practical diagram of the universal controller (figure 12) so that you can realize to the level of the practice. This last varies between 500,00 to 1500,00 frank according to the precision of the components thus of its sensitivity. Between in addition to, there are digital apparatuses with posting, more precise than the galvanometer but more expensive.
This diagram below shows all the assembled components and the various connections carried out between them. On this last also the pile of the ohmmeter appears.
Using the practical diagram, it is possible to carry out an ultimate checking of the assembly.
In the next one, we will continue the continuation of the lessons of semiconductors entitled “Semiconductor N°6” about the transistors. (See Summary electronics fundamental, paragraphs 14, 14.1 and 14.2 for a possible revision if necessary).
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