Created it, 05/10/15
Update it, 06/01/19
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SEMICONDUCTORS 8 “15th PART”
During this lesson we will study three new types of semiconductors, especially used in the field of commutation. It is of the THYRISTOR, the DIAC and the TRIAC.
It is a switch almost perfect, at the same time rectifying one-way and amplifying.
As for the DIAC, it is about a bidirectional device, becoming conducting when the tension applied exceeds a certain threshold.
Finally the TRIAC, of the same family that the thyristor, differs from this last by the fact that it is bidirectional.
After having seen these three new elements, we will finish this last lesson of the basic course OF FUNDAMENTAL ELECTRONICS, by the examination of the new the electronic lessons digital ones.
1 - THE
THYRISTOR
The thyristor is a diode particular to silicon, having a CONTROL CIRCUIT. This element can pass from the state of PROHIBITION to the state of CONDUCTION but as soon as this one is reached, the control electrode does not have any more the possibility of ordering the flow of the current.
The structure of this component is rather complex. Indeed, a THYRISTOR includes/understands THREE JUNCTIONS, made up of two zones N and two zones P (figure 1).
The lowest zone P, constitutes the ANODE of the diode, whereas CATHODE is formed by the highest zone N.
The zone P being under cathode, constitutes the Trigger and thus comprises the contact necessary, for the connection with the external circuit.
The whole of these various zones, formed by the processes described in the preceding lessons, is reinforced by two tungsten discs, as one can see it figure 2.
One of these discs carries a braided cable of connection, corresponding to CATHODE. This unit is locked up in a hermetic case, whose base ends in a threading corresponding to the ANODE.
This one makes it possible moreover to fix a metal plate, being used as radiator, in order to dissipate heat being able to damage the component.
Let us see the operation of the THYRISTOR now.
Let us consider the figure 3-a to this end, representing the thyristor of figure 1, in another form.
Let us suppose that one cuts the two central blocks, in order to be able to break up the thyristor into two parts. Let us connect those between them, by means of connection, as on the figure 3-b.
One of the two parts thus obtained is made of a block P, being between two blocks N, constituting a transistor of type N.P.N. (TR1 on the figure 3-b).
The other part is formed of a block of silicon N, being between two blocks P, constituting a transistor of type P.N.P. (TR2 on the figure 3-b).
By representing these two transistors and by connecting them as on the figure 3-b, one obtains the representation of the figure 3-c.
Each transistor has its base connected directly with the collector of the other and the unit comprises three connections of exit (A, C and P), corresponding to the anode, cathode and the trigger (one says also the DOOR).
Let us feed the circuit by means of two batteries, assembled as on the figure 4-a.
Under these conditions, the transmitter of TR2 (corresponding to the ANODE of the thyristor), is positive compared to the transmitter of TR1 (corresponding to the CATHODE of this thyristor). Although this component has its positive anode compared to its cathode, no current can circulate when the switch (I) is open. Indeed, the basic current of TR1 being null, the collector current is also null.
The base of TR1 being connected directly on the basis of TR2, which is true for the first transistor is also valid for the second.
By closing switch I, the junction base-transmitter of TR1 is polarized in the direct direction, insofar as the base is positive compared to the transmitter. Consequently, this junction is crossed by a direct current IB, in the direction indicated by the arrows (figure 4-b). This current determines the passage of a collector current IC in TR1, and also crosses the junction transmitter-bases TR2.
Since current IC, crosses the junction transmitter-bases TR2, it determines the passage of a new current I'C in this last transistor. This current traverses the circuit in the direction indicated by the arrows (figure 4-c), i.e. it crosses the junction base-transmitter of TR1, in the same direction as current IB (figure 4-b).
The current which crosses the junction base-transmitter of TR1 thus increased, the current I'C being added to IB. It results from it that current IC of TR1 also increases, and, while crossing the junction TR2, product in its turn transmitter-bases, an increase in IC.
The current crossing the junction base-transmitter of TR1 thus rises, just as current IC and than the current I'C, and so on.
One includes/understands thus how, by the action of each transistor one on the other, the current which passes between point A and the point C, i.e. between the anode and cathode increases up to a limiting value, only given by resistance R, being in series in the circuit.
When this limiting value is reached, one can open switch I, as shown in the figure 4-c. The transistors are indeed from now on able to be maintained one and the other in a state of conduction.
One thus sees that the THYRISTOR CAN PASS FROM THE STATE OF PROHIBITION A THE STATE OF CONDUCTION, by applying a short moment a suitable current to the circuit of TRIGGER.
The fact that the current continues to circulate after the opening of switch I, means that the TRIGGER cannot influence the value of this one any more. As soon as possible to give the thyristor to the state of prohibition, it is enough to apply a negative tension to the anode.
To include/understand what arrives in this case, it is necessary to refer to the structure of the thyristor and to examine polarization, when this one is in a state of conduction (figure 5).
In this case, the three junctions are polarized in the direct direction and near each one of them, there are a great number of holes or free electrons.
By applying the negative tension to the anode, one stops the current crossing the diode and one notes the circulation of a reverse current, due to the fact that the free loads are far away from the junctions located by G1 and G3 on figure 5.
After displacement of these loads, the reverse current ceases and the junctions G1 and G3 are polarized in opposite direction. The thyristor however is not in condition of prohibition, because there still remains a considerable number of holes and free electrons, near G2. These last free loads are eliminated reciprocally by recombination insofar as the junctions G1 and G3 are polarized in opposite direction.
When this recombination is finished, one can apply a positive tension to the anode, without giving the thyristor in a state of conduction. At this time, the trigger thus took again the possibility of controlling the thyristor.
The time which passes between the moment when cease the passage of the current and the moment when one can D-apply a positive tension to the anode, without the thyristor returning to conduction, is known as TIME OF RETURN A THE STATE OF PROHIBITION ; it normally lies between 10 µ seconds and 15 µ seconds.
It should be specified that on the figure 4-a, to simplify the explanation, one supposed that when the THYRISTOR is with prohibition, it is not crossed by a current, but that is not rigorously exact.
Actually, each of the two transistors composing the THYRISTOR is crossed by a residual current, as one saw in the preceding lessons relating to the transistors. This current circulates, even when the basic circuit is open.
Thus, the residual current of the two transistors representing the thyristor, circulates in the direction indicated figure 4 for the collector currents, but being given its small intensity, it is not sufficient to carry the diode, in normal condition of operation, with the state of conduction.
However the presence of this residual current, makes that the diode can pass from prohibition to conduction, even if the current of trigger is null.
One can check this fact while applying to the anode of the thyristor, a continuous tension of suitable value. This tension gives to the carriers constituting the residual current, a sufficient energy to release from other carriers in greater number, those, in their releasing turn of other loads and so on.
There is then the EFFECT AVALANCHE. Consequently, the current increases very quickly and the thyristor passes thus from the state of prohibition to that of conduction.
It is wise to insist on this phenomenon, having a certain influence, at the time of the measurement of the characteristics.
Will determine those, allowing to know how the anode current (Ia) varies according to the anode voltage (Va), for various values of the current of trigger (IP), one has recourse to the circuit represented figure 6.
On this figure, one can see the graphic symbol of the thyristor, similar to that of a diode, with moreover, on the side of cathode, an electrode corresponding to the DOOR.
By means of P1, one can vary the anode voltage (Va), indicated by the measuring instrument V, whereas the apparatus (I) indicates the values of the current, corresponding to the various tensions.
As for P2, it is used to regulate the tension applied between the door and cathode, i.e., in practice, to proportion the current of the circuit of DOOR.
When this current has a zero value, while varying the anode voltage, one can determine the characteristic relative to IP = 0 V.
The pace of this one is shown figure 7. It is seen that, when the anode voltage passes from a zero value to a positive tension (+Va), the anode current, consisted the residual current, increases initially gradually because of the EFFECT AVALANCHE.
This current reaches thus the POINT OF COMMUTATION, corresponding to a sufficient value, to carry the thyristor of the state of prohibition to the state of conduction.
One speaks then about CURRENT OF COMMUTATION.
As soon as the thyristor passed to the state of conduction, it is necessary to reduce the anode voltage to prevent that the anode current takes excessive values.
It is seen indeed that the characteristic is almost vertical.
One can deduce from it that it is enough to anodic low tensions, to obtain high anode currents.
The thyristor remains with the state of conduction, even if the anode voltage falls to rather low values, provided that one does not go down below a value, known as VALUE OF BEHAVIOR.
Below this one, the thyristor returns to the state of prohibition. In addition, when the anode voltage increases towards negative values (- Va), the characteristic takes a form very similar to that of a normal diode, polarized in opposite direction (figure 7).
On the figure 8-a, one can see on the contrary, the modification of the anodic characteristic, when the current of TRIGGER, takes values higher than zero.
One can note the reduction in the anode voltage, according to which the current of commutation is located.
If the current of trigger is much higher than zero, the characteristic takes the illustrated form appears 8-b. This pace is very similar to that of the characteristic of a junction P.N.
For the use of a THYRISTOR, it is also necessary to know the control characteristic, i.e. the characteristic showing how the current of trigger Ip varies, when the Vp tension, applied between the trigger and cathode, is modified.
One can find the values of these sizes, by means of the circuit of figure 9.
By deferring on a diagram the values of the tension and current thus determined, one can trace the control characteristic of the thyristor considered.
By remaking this layout with another thyristor of the same type, one would find a characteristic which could be very different. This fact is due to the inevitable differences in construction which one meets in these components.
For this reason, the control characteristics of the thyristors, provided by the manufacturers, include/understand two curves, delimiting a zone in which the characteristic can be, for a type of thyristor given.
It should be remembered that by increasing the tension of the Vp trigger, one reaches a value, in correspondence of which the current of trigger Ip, proves to be sufficient to cause the conduction of the thyristor.
Because of the differences in construction, these values vary from a thyristor to another for the same type of component.
The hatched surface of the figure 10-b, indicates the possible points of commutation. It is delimited by the Vpmin values. and Ipmin.
This surface thus represents the zone, in which commutation is possible, but not some, whereas the higher zone indicates the values where commutation is certain, in all the cases.
It still should be noted that all the values included/understood in the higher zone cannot be adopted for the ordering of a thyristor. Indeed, for some of these values the power dissipated in the junction trigger-cathode, would exceed the possibilities of the thyristor. Consequently, the maximum power dissipable without risk, is indicated by the curve in dotted line.
The control characteristic, in its final aspect, is represented on the figure 10-c.
1. 1.
- PRINCIPLE
OF STARTING BY THE TRIGGER
The starting of the thyristor by its trigger or carries, is the system of starting more running.
The thyristor is assembled on the circuit, in order to be polarized in the direct direction (see figure 11).
One applies a POSITIVE IMPULSE to trigger (IG).
Transistor TR1
thus receives IG like basic current. This
fact its collector current passes IG 
1,
(where 1
= profit while running of TR1). This current is in its turn injected
into the base of TR2, which outputs then a
current IG 1
2
(where 2
= profit while running of TR2).
This same current IG
1
2
of collector of TR2 is reinjected on the
basis of TR1.
Two cases must then be considered.
1°) the product 1
2
is smaller than 1 : THE
DEVICE DOES NOT START.
2°) the product 1
2
is close to the unit : the process of amplification
appears and the thyristor passes in a conducting state.
These two conditions (1
2
< 1 and 1
2
close relation of 1) characterize the state of the thyristor
according to the current.
The profit
of a transistor to silicon indeed generally grows with the current (more exactly
the profit while running grows with the current one of transmitter).
With a weak current of trigger,
the product 1
2 is lower than 1. The
thyristor remains blocked.
With a current of trigger of
higher value, i.e. with an impulse of sufficient order, the currents of
transmitters are high enough for 12
gives a value tending towards the unit, i.e. 12
-------> 1.
As soon as starting is carried out, positive reaction (the collector current of each transistor being applied on the basis of other transistor) made lead TR1 and TR2 to saturation. These two components are maintained in this state, even if the control signal disappears.
1. 2.
- OTHER
POSSIBILITIES OF STARTING
As we have just said it the property essential of a transistor with silicon is to have a profit of current, growing with the current one of transmitter IE. So all the causes likely to cause an increase in current IE, make it possible to start starting.
One can thus act :
1°) ON THE TENSION : If the tension cathode-anode increases, it arrives one moment when the LEAKAGE CURRENT is sufficient to start a fast increase in IE, therefore to cause starting.
2°) THE SLOPE OF THE TENSION : Junction PN has a certain CAPACITY. Thus, by increasing the tension anode-cathode abruptly, one charges this capacity and one obtains a current of :
i = (C
V) /
t
C = value of capacity of the junction
V (delta V) = variation of the tension
t (delta t) = lasted of the variation
When the current (i) reached a certain value, starting occurs.
3°) THE TEMPERATURE : the opposite leakage current of a transistor to silicon, doubles all roughly the 14° C (when the temperature grows).
There still, when the leakage current is sufficient, the thyristor starts.
We quoted these possibilities only as information, because in the large majority of the cases, one causes the RELEASE OF the THYRISTOR by injecting an IMPULSE of order on the TRIGGER, i.e. by using the TRANSISTOR EFFECT.
1. 3. - TENSION OF REVERSAL
As we have just said it in the preceding paragraph, it is possible to start a thyristor, while acting on TENSION CATHODE-ANODE.
The value of the tension for which the thyristor starts, is called tension of reversal. The value of this tension depends however on the control signal, possibly applied to the trigger. Figure 12 highlights this relation.
When the current of trigger IG is null (on figure 12, IG1 = 0), the tension anode-cathode, must reach the tension of reversal so that the thyristor starts. On the other hand with an increasing current of trigger, the tension of reversal falls to values much lower.
In extreme cases, the thyristor behaves like a diode (for IG5, on figure 12). That means that if the current of trigger is rather strong, a small tension of anode is enough to cause release.
Also, to prevent erratic startings, one fear of assembling a resistance enters the trigger and cathode.
Very often besides the manufacturers integrate by diffusion, this resistance in thyristor (technical SHORTED EMITTER).
It causes to require a more intense current of trigger, for the starting of the thyristor, but consequently, its behavior in a blocked state improves.
II
- USE
OF THE THYRISTOR
The thyristors are used more and more in the current control circuits.
The research carried out in this field allowed the realization of thyristors able to pass an intensity about several hundreds of amps, with an opposite tension of peak of 1200 Volts.
Such thyristors however are reserved for fine the quite special ones. In the field running, one finds especially :
The applications of the thyristors are very vast and more particularly in industrial electronics.
One also finds them in certain apparatuses electric household appliances, where they can in addition to one specific function, to replace a transformer.
Figure 13 illustrates a current application : THE VARIABLE SPEED TRANSMISSION.
Catch directly on the sector, the tension takes the sinusoidal form, represented figure 14-a.
While inserting the circuit of figure 13, the tension to the maximum is nothing any more but one half-sinusoid. In this case, the thyristor behaves like a diode (figure 14-b).
Thus, while inserting the device between the catch sector and the engine, the speed of the engine M decreases ; one uses indeed that the positive halfs-wave to feed it.
To decrease speed further, one operates the P1 potentiometer. The tension which supplies the engine then takes the form indicated figure 14-c. It is noticed that there remains only part of the positive half-wave.
Continuously to act on P1, one can be able not to have any more but one small part of the positive half-wave (figure 14-d) or even nothing any more the whole, i.e. complete suppression of the positive half-wave.
Should be noticed an essential fact : The REDUCTION SPEED is carried out without REDUCING the TENSION APPLIED TO the ENGINE (except at very low speed) but only while ACTING over the TIME OF CONDUCTION. That means that the engine preserves practically all its POWER, whatever its mode.
Let us return to the diagram of figure 13.
In a circuit of this type, the tension of ordering of the trigger is obtained by a dividing bridge R1, R2 and R3, connected between the anode of the thyristor and the P1 potentiometer.
When the tension applied to the anode of the thyristor increases positively, that applied to the trigger also increases, since it is transmitted by D1. In this manner, one thus reaches the value necessary to start the conduction of the thyristor. This value is reached in different times, according to the position of the cursor of the potentiometer.
When the cursor is in A, the tension of release is reached little time after the beginning of the positive half-wave (illustrated case appears 14-b).
By moving the cursor of P1 towards B, one decreases the tension of order by the introduction of an additional resistance ; the conduction of the thyristor does not occur whereas a certain time after the beginning of the positive half-wave (illustrated case appears 14-c).
According to what was known as, one could think that it is impossible to leave the THYRISTOR in a state of conduction after time t3 (figure 14), but it is seen that this is possible (figure 14-d).
One arrives at this result, thanks to the C2 condenser, which determines a dephasing between the tension of the sector present on the anode of the thyristor and the tension of trigger.
The two tensions do not vary together, the second being LATE on the first. Indeed, the tension of trigger reaches its positive maximum, whereas the tension of anode of the thyristor already started to decrease.
The time of conduction between t4 and t5 is obviously very short, which wants to say that the speed of the engine is then very low.
At the end of any positive half-wave, the tension sector is cancelled, STOPPING the conduction of the thyristor.
During the negative halfs-wave, the D1 diodes and D2 are polarized in direction reverses and the system remains blocked.
Conduction begins again with the following positive half-wave, in the diodes initially, the thyristor then, when the tension of release is reached.
When the supply voltage of the engine has the pace of the figure 14-d (almost impulse), there are parasites in the radio frequencies, being able to disturb the radio reception. To eliminate this disadvantage, one placed a C1 condenser reducing these disorders mainly.
This assembly is given here only like example of application of the thyristors, which one finds in very many circuits in INDUSTRIAL ELECTRONICS.
III
- TRIACS
The TRIAC is a semiconductor device with three electrodes (anode 1, anode 2, trigger) being able to pass from the state blocked to the state of conduction in its two directions of polarization. In other words, it is about a component of the same family as the thyristor, but which is BIDIRECTIONAL (the thyristor being one-way).
The TRIAC can be compared besides with two thyristors in parallel, gone up head-digs (figure 15).
One can consider the TRIAC, like a STRUCTURE P1 N1 P2 N2 of thyristor, in which A1 is cathode (connected to N2), A2 the anode (connected to P1) G, the trigger (connected to P2), but with moreover :
The structure P2 N1 P1 N4, thus constitutes a second thyristor, assembled in opposite parallel with the thyristor P1 N1 P2 N2.
The characteristic tension-current is symmetrical (figure 16 below).
This device can pass from a state blocked to a conducting state in the two directions of polarization (quadrant 1 and 3) and pass by again in a state blocked by inversion of tension or reduction in the current below the value of the current of maintenance IH.
In the absence of signal on the trigger, the device can be regarded as two polarized rectifiers in opposite direction. No current circulates in the triac, therefore in the load (except very light running of escape).
It is thus admitted that the TRIAC behaves like an open switch. However, according to polarization one can have the following states :
1°) If A2 is with a positive potential of 1,5 volt compared to A1, a positive or negative tension of suitable value, applied to the trigger, causes starting : The TRIAC starts to lead.
2°) If A2 is with a negative potential of 1,5 volt compared to A1, a positive or negative tension of suitable value, applied to the trigger, causes starting : The TRIAC starts to lead.
3°) If the going current of A2 with A1 or A1 with A2 is established, the TRIAC IS LOCKED and the tension of trigger can be removed, whether it is POSITIVE or NEGATIVE : The TRIAC remains in a state of conduction.
4°) When the current in the TRIAC is established (in a direction or the other), it is necessary to block it, to reduce the intensity of this current, with a value close to zero.
The intensity minimum for which the TRIAC remains conducting is called minimal intensity of maintenance (IH).
The condition above (running near to zero), exists obviously each time the alternating voltage of the network, passes by zero, i.e. with each half cycle.
As we have just seen it, the TRIAC can be started by a POSITIVE or NEGATIVE impulse on the TRIGGER, some that is the polarity of A2 compared to A1. However, there is PREFERENTIAL SENS, illustrated figure 17.
When release took place, resistance interns triac is low ; so the voltage drop between A2 and A1 has an also low value (about 1,2 volt). That means that the power dissipated to no purpose in the TRIAC is very low compared to the power of the load.
Still let us mention that a TRIAC can support without disadvantage of the short overloads, rather intense. Thus, a TRIAC of 6 amps for example can support during some alternations, a current of about 100 amps.
This characteristic is very interesting, because with the starting of an engine for example, the required instantaneous intensity, is much more important than the intensity of operation of the engine in normal circumstances.
III
- 1 - STARTING
OF THE TRIAC
By applying a V1 tension to A1, V2 with A2 and VG with the TRIGGER, by taking V1 like reference, i.e. V1 = 0, one can define the four quadrants of polarization of figure 17.
How does starting in the 4 possible cases occur ?
a) STARTING QUADRANT 1 (+ +)
In this case we have V2 > V1.
One applies a positive impulse to the trigger between G and A1 (+ on G).
By calling T the thyristor P1 N1 P2 N2 having A2 like anode and A1 like cathode (see figure 15) and T' it thyristor P2 N1 P1 N4 with A1 like anode and A2 like cathode, we have:
T is under direct tension; the positive current of trigger causes the starting of T like a normal transistor.
b) STARTING QUADRANT 3 (- -)
In this case we have V2 < V1
One applies a negative impulse to the trigger between G and A1 (- on G).
The current of trigger IG enters by A1, crosses the diode P2 N3 in the direct direction and thus involves the depression of the barrier of potential P1 N1. Indeed, the diode P2 N3 is crossed by HOLES of P2 towards N3 and by electrons of N3 towards P2.
These electrons diffuse through P2 until the junction P2 N1, which direct them in N1 (action of the junction on the minority carriers which reach it) ; it results from it a reduction from the barrier of potential P2 N1 and consequently an increase in the current of the HOLES of P2 towards N1
These holes are absorbed by the junction P1 N1 whose reverse current increases, with for effect to start T'.
c) STARTING QUADRANT 2 (+ -)
In this case we have V2 < V1
One applies a negative impulse to the trigger between G and A1 (- on G).
The current of release circulates of P2 towards N3 and starts T', like previously.
d) STARTING QUADRANT 4 (- +)
In this case we have V2 < V1
One applies a positive impulse to the trigger between G and A1 (+ on G).
The process of release can be compared with that of the first quadrant, therefore T conducting.
Actually the phenomenon is more complex, because for a study detailed and in addition hypothetical, it would be advisable to consider the zone of conduction N3 P2 N1 P1, from where he arises that in the fourth quadrant, the sensitivity to release is more reduced than in the other cases.
The two most used methods of starting are those described in a) and b), i.e. that of the first and the 3rd quadrant. Indeed, when A2 and G have the same polarity, the current of trigger necessary to cause starting is much weaker than when these polarities are opposite.
PREFERENTIAL SENS of starting, while referring on figure 17 is thus that where we have :
A2 + VG + and A2 - VG -
III
- 2 - CHARACTERISTICS
OF THE TRIACS
The most complete works treating thyristors and triacs being American (what explains the increasingly frequent use of Anglo-Saxon terms in the French texts and confirms that the technical evolution has an unquestionable influence on the language), it is good to give the significance of the symbols used to give the characteristics of these components (figure 18).
Extracted from a document SILEC (manufacturer of semiconductors), here how the essential characteristics of a TRIAC (standard TTAL 220) arise.
*** VALUES OF USE ***
ITeff 200 A = effective Current in a busy state.
ITRM 600 A = maximum Current of point located in a busy state.
ITSM 1600 A with 50 Hz = Current of maximum point accidental.
VDWM 200 V = maximum Tension of peak in a blocked state.
VRSM 300 V = Tension reverses of accidental point
*** CHARACTERISTIC OF TRIGGER ***
IFGM 5 A = Forward current of point.
VFGM 10 V = Tension direct of point
VRGM 5 V = Tension reverses of point
PG 10 W = Power trigger (of English POWER SPOILS).
Among the other important characteristics that it is necessary to quote, let us mention :
VBO = maximum Tension which the component can support while remaining maintained in a blocked state. If this tension is exceeded the triac starts.
dv / dt = maximum Growth rate of the tension of anode which can be supported by the device, without risk of starting.
di / dt = maximum Growth rate of the current of anode which can be supported by the device without involving its destruction.
Using these indications, one can thus supplement the diagram of figure 16 by the following indications (figure 19).
IV
- DIACS
The DIAC is solid-state component, used TO START the thyristors and the triacs.
It is a SYMMETRICAL element, therefore a BIDIRECTIONAL component, becoming conducting when the tension exceeds a certain threshold (tension of reversal).
Its structure is very simple, since it is about a double diffusion of impurities of the TYPE OPPOSED to that of the substrate (monocrystal).
The symbol, or rather the symbols of the DIAC is represented figure 20.
The symbol of the figure 20-b is contestable because it is practically identical to that of the TRIAC.
The symbol of the figure 20-c is simplest and most easily makes it possible to include/understand the operation of this component.
Let us see on this subject the behavior of the DIAC. For this purpose, we defer to the diagram of figure 21.
Let us regulate the potential P with its MAXIMUM VALUE. The tension applied to the DIAC and read on the voltmeter V is very weak ; the current measured by ammeter A is also very weak and corresponds to very light running of escape.
Let us operate P to increase the value of the tension V.
The current increases but very slightly, as one can see it figure 22 (IBR +), then abruptly, for a certain value of well defined tension, the current increases in an intense way and the dynamic resistance of the DIAC BECOMES NEGATIVE. That means, that this resistance while being of value relatively low, increases according to the current.
Figure 22 illustrates what has just been known as : (part on the right of axis I).
So now, in one second experiment one reverses the primary battery, one notes that the same phenomenon occurs, but opposite direction.
One obtains thus, a practically symmetrical curve, as one can see it figure 22.
Taking into account what has just been known as, one can draw the two following conclusions :
1°) the DIAC is not a RECTIFIER.
2°) It is not possible to have between its terminals a tension higher than VBR + and VBR -, without risk of destruction of the component.
IV - 1 - USE OF THE DIAC
The DIAC is used in partnership with the TRIAC, for the ordering of this last.
One can for example carry out a GRADATEUR of LIGHT. In this type of assembly the THYRISTOR is appropriate badly, because since it leads only in one direction, a flutter of the bulb is perceptible, especially with the low light intensities.
In this case indeed, the time of conduction compared to complete alternation is very short. Thus, between each period of conduction, the filament of the bulb cools and it results a reduction from it from the emitted light.
When the thyristor leads again, the filament again emits a more intense light, which a reduction and so on succeeds.
The TRIAC being CONDUCTIVE in the two directions of the alternating voltage, avoids this flutter.
The standard assembly of a GRADATEUR of LIGHT is given figure 23.
Note on this subject that this same assembly can, without any modification, to be used as VARIABLE SPEED TRANSMISSION.
How does this circuit function?
The right part of the diagram includes/understands a device of DEPHASING, consisted a condenser C and a P. potentiometer.
Part of the current provided by the sector passes through these two elements. This current generates at the boundaries of P, a tension which is in phase with this one.
This tension is given by the law of ohm :
Up = RI
At the boundaries of the condenser, a tension also occurs, but it is OUT OF PHASE, from p / 2 (90°) compared to the current.
The value of this tension is given by the formula :
Uc = 1 / (C x w) x I
The vectorial representation is that of figure 24.
The vector U, nap geometrical of Uc and Up is out of phase compared to current I. It is obvious that this dephasing (angle j), depends primarily on the two vectors Uc and Up, therefore of C and P.
However P is a potentiometer; it is thus enough to operate the cursor to modify the DEPHASING of U compared to I. But U is the terminal voltage of the resistance-capacity unit, therefore the tension of the sector. The DEPHASING of Uc compared to U, thus varies also according to the adjustment of P.
In conclusion :
At the boundaries of C one has a tension whose phase varies with the value of P.
Figure 25 represents the tension U (immutable) of the tension Uc, whose dephasing in time varies according to the value of R.
In general, the DIAC starts for a tension close to 30 Volts, which comes down to saying that VBR of figure 22 = 30 Volts.
If P and C were judiciously selected, the maximum value of Uc can be equal to 30 Volts. That means that each time there is a tension of 30 Volts at the boundaries of Uc, which is presented at times t1, t2, etc, there will be release of the DIAC, and consequently TRIAC.
Figure 26 represents the form of the tension delivered by the TRIAC, i.e. the TIME OF CONDUCTION of this element, according to three adjustments different of P.
1°) the electric bulb is fed almost normally. The time of conduction of the TRIAC is almost equal to t.
LONG TIME OF CONDUCTION (left hatched).
2°) the electric bulb is fed only half of time during each half cycle.
AVERAGE TIME OF CONDUCTION.
3°) the electric bulb is fed only during one fraction of time during each half cycle.
TIME OF CONDUCTION RUNS.
Note well that in the VARIATORS or GRADATEURS with THYRISTORS or TRIACS, one acts over the TIME OF CONDUCTION and not on the value of the tension.
IV - 2 - OTHER DEVICES OF RELEASE
The DIAC is not the only one currently composing not used for the order of the TRIACS.
It is indeed necessary to mention :
1°) THE UNILATERAL SWITCH (KNOWN).
This component, only intended for the release of the THYRISTORS is consisted a miniature THYRISTOR and a ZENER diode.
Figure 27 represents the symbol of this component, its equivalent circuit and its characteristic curve.
It is seen that roughly, this unit functions as a DIAC which would be Unidirectionnel.
This switch with the advantage of starting with FIXED VOLTAGE, given by the ZENER DIODE.
2°) THE BILATERAL SWITCH (SBS).
This component, derived from KNOWN preceding, is formed by two switches BILATERAL, is assembled head-digs.
It functions thus in the two directions and so is especially used for the order of the TRIACS.
Figure 28 gives the essential information on this element.
3°) DIODE SHOCKLEY
Diode SHOCKLEY, so called DIODE WITH FOUR LAYERS or DIODE THYRISTOR is component BIPOLAR ONE-WAY.
Of type PN PN, this diode is comparable with a THYRISTOR which would comprise only the ANODE and CATHODE.
Figure 29 represents the structure, the symbol and the curve characteristic of this component.
4°) THE QUADRAC.
The QUADRAC is not strictly speaking a device of release, because it is made of a TRIAC, containing in the same case a DIAC (figure 30).
It is thus there only about one composed element, of which the goal is to simplify the circuits.
We thus finish our concepts of electronic fundamental which, we hope for, will help you with better seizing the other parts of the new theoretical and practical lessons the electronic DIGITAL ones with for goal to explain the digital circuits concerning the computers including the integrated circuits at the very least, to increase your knowledge in this field.
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