FR2556819A1 - AUTOCONTROL FLAME CONTROLLER - Google Patents

Abstract

FLAME CONTROLLER CONNECTED TO THE MAINS VOLTAGE WITHOUT TRANSFORMER, AS WELL AS ITS IONIZATION CIRCUIT5, 46, 47. THE FLAME CONTROLLER CONSISTS OF AN AMPLIFIER2 CIRCUIT, A FLAME RELAY CIRCUIT1, AN INTEGRATOR3 AND A PULSE GENERATOR4. THE IONIZATION CIRCUIT CHARGES A CONDENSER6 WHICH ACTS ON INPUTS19, 20 OF AN AMPLIFIER8. THE OUTPUT OF THE LATTER SIMULTANEOUSLY ACTIVATES TWO SWITCHING ELEMENTS9, 17, ONE OF WHICH IS A SHORT-CIRCUIT SWITCH9 FOR THE SERIAL CONNECTION OF DIODES13, 16, WINDINGS11, 12 OF A CHARGE RELAY10, AND A CHARGING CAPACITOR14 . THE SECOND SWITCHING ELEMENT17 STARTS THE INTEGRATEUR3 WHICH ACTIVATES A SWITCH18 OF THE PULSE GENERATOR4. THIS LAST SHORT CIRCUIT INPUTS 19, 20 OF THE AMPLIFIER8 DURING SHORT INTERVALS. IN THE PRESENCE OF A FLAME46, THE FLAME RELAY10 CAN REMAIN POWERED ONLY IF THE INTERVALS ARE SUCCEEDING IN A DETERMINED TIME RELATION. ANY COMPONENT FAILURE WILL INTERRUPT THIS REPORT AND TRIGGER THE FLAME RELAY 10.

Description

The present invention relates to a self-controlled flame controller for

  flame monitoring of an oil-fired or gas-fired burner, with an ionization flame detector and a connection

  in parallel, which can be connected to an alternating voltage,

  tor with a first capacitor, the charge of which depends

  the ionization current of the flame detector; an influential amplifier

  encontrolled by a control voltage via the first capacitor

  sateur; an output switching element which, as part of a flame relay circuit, is in parallel with the series connection of at least one diode, the partial winding of a flame relay and a charge conductor, while that the series connection of at least one other diode with another partial winding of the relay

  flame and the charge capacitor is connected to the voltage of the

  charging capacitor; and a pulse generator which periodically opens and closes the output switching element in

  presence of an ionization current.

  The performance of heating plants is receiving a lot of attention as a result of the increase in fuel oil and gas prices. Efforts are made to reduce the losses of a boiler room

  when shut down, which leads to a reduction in the boiler room generally

  previously dimensioned and to use burners whose continuous regulation allows to a large extent to adapt the power to the instantaneous thermal need. Such burners do not work in service

  intermittent at a minimum load and are in service

  tinu on the rest of the charging range. The flame controller self-test, previously performed during shutdown or before each

  service to detect any defects in parts, is no longer

  sand in continuous service. Such an installation requires provisions

  flame monitoring units that self-monitor continuously, even in service, to detect faults. Such flame controllers are known, but still represent a significant expense

  for use on small burners or still have insufficient

technical productions.

  Another disadvantage was that the ion electrode

  low cost of conventional oil fired burners

2556819 '

  could not be considered a reliable flame detector

  as a result of its coking. This type of detector will present

  It is becoming increasingly important, even for oil burners, as a result of the increasing use of oil-spraying burners or burners, since blue flames suitable for ionization electrodes are obtained without any soot formation. he

  there is therefore no coking of the electrodes of the detector.

  A known flame detector (German Patent No. 267,787) is suitable for both ionization and UV use. A relatively low resistance service voltage source and error protection voltage limitation is required to maintain a low response time at

  extinction of the flame from the state of service. Ccmte t-

  the fact that the detector voltage required for the function

  UV is higher than the mains voltage, this imposes a

  transformer, expensive and bulky.

  To give up operation with a UV detector and to adopt a transformerless power supply for the known flame controller presents the following risk: a fault causes a growth of the supply voltage, which is not detected in due time. This results in the flame relay circuit an excessive response time to an inadvertent extinguishing of the flame. In the known arrangement, a transformerless direct supply also delivers a lower voltage to the detector by ionization, so that the detector current decreases for the same flame quality and is no longer sufficient to provide the necessary power to the switch.

threshold it feeds.

  The patent application of the Federal Republic of Germany published under No. 24 30 328 describes a flame controller assembly, directly connected to the AC network. However, it requires an extremely high resistance detector power supply circuit, so that a flame of quality but relatively low resistance is not detected as defective. The power supply of the high resistance detector results in an extremely low allowable load by detector line capacitances, and low insulation fault currents already significantly influence the wanted signal. The subject of the invention is a flame controller for a detector

  ionization, which detects any part failure, can function

  directly on the AC transformerless network, and is virtually insensitive to isolation

line.

  According to an essential characteristic of the invention, an inte-

  independent of the state of charge of the first capacitor charged by the ionization current, but influenced by the output state of the amplifier, triggers a pulse generator of a duration

  predetermined by a timer element; a communication device

  tation, actuated by the pulse generator during the pulse duration, bypasses the input of the amplifier and thus simulates a "flame off" signal for the amplifier; and a power supply connected to the alternating voltage of the network delivers two voltages which must have two given values for the

  correct operation of the device.

  Other features and advantages of the invention will be

  better understood using the detailed description below of a

  embodiment and the appended drawing in which: FIG. 1 represents the simplified diagram of a flame controller, divided into basic functional groups, and

  FIG. 2 represents the electrical diagram of an exemplary embodiment

tion.

  The same components are designated by the same references on

  the two figures described below.

  The flame controller according to the invention is constituted, as shown in FIG. 1, by a flame relay circuit 1, an amplifier circuit 2, an integrator 3, a pulse generator 4 and an ionization flame detector, called flame detector below. The flame detector 5 constitutes with the first capacitor 6 of the amplifier circuit 2 a parallel connection, to which another capacitor 7 applies the alternating voltage pre-exposed directly to the network. The amplifier circuit 2 comprises

  furthermore, an amplifier 8 which the voltage influences through the

  of the capacitor 6 and whose output acts on a switching element of the flame relay circuit 1 and on the integrator 3

described later.

  The circuit 1 is constituted by a flame relay 10 with two windings 11 and 12, the first of which, connected in series with a first diode 13, a charge capacitor 14 and a load resistor 15, is connected to a voltage source The second winding 12 is also connected in series with the charge capacitor 14, a second diode 16 of reverse polarity to that of the first diode 13, and the load resistor 15, and consequently connected to it. also at the same source of tension

  keep on going. The switching element 9 is connected in parallel with the branch

  series charging capacitor, two windings

  11, 12 and diodes 13 or 16, and makes it possible to shunt one of these cir-

  cooked, as described below.

  The output of the amplifier 8 also influences a second element.

  switching element 17, triggering the integrator 3. The latter constitutes a timer element and actuates the pulse generator

  after a time independent of the state of charge of the first conden-

  6 when the power supply connected to the alternating voltage

  network is working properly. This phenomenon is also

  described below with the help of Figure 2. The triggering of the genera-

  4 is indicated in FIG. 1 by a switch 18. A switching device 21 is assigned to the pulse generator 4, bypasses the two inputs 19 and 20 of the amplifier 8 and,

  in the closed state of the latter, simulates a "flame off" signal.

  A timer element not shown in Figure 1 sets the pulse duration. The operating principle is described using FIG.

  before addressing the details of the assembly according to figure 2.

  The switching element 9 remains closed at rest, in the absence of any flame. The voltage applied to the circuit 1 of the flame relay appears across the load resistor 15 and the flame relay 10 is not powered. The voltage applied to the integrator 3 is zero and the switching device 21 is open. As long as no flame is present, the applied alternating voltage can not charge the capacitor 6. No voltage is applied to the inputs 19 and 20 of the amplifier. At the appearance of the flame, the capacitor 6 is charged and the amplifier 8 causes an opening of the switching element 9 and a closing of the switching element 17. The capacitor 14 is charged by the winding 11 to resistance high and the diode 13, the charging current can not engage the flame relay 10. The switching element 17 has

  simultaneously integrator integrator 3 which, when a predetermined threshold

  In this case, the pulse generator 18 operates via the switch 18, which actuates the switching device 21 and bypasses the inputs 19, 20 of the amplifier. The capacitor 6 does not substantially discharge on a limiting resistor 22 during this short-circuiting, but the amplifier 8 actuates the switching elements 9 and 17. The flame relay 10 is consequently connected in the circuit 1, the charge capacitor 14 discharging on the winding 12 low resistance and the diode 16, to a residual voltage. The duration of this discharge is fixed by the duration of the pulses of the generator 4 and chosen

  so that the flame relay 10 does not trip. The com element

  mutant 17 simultaneously reset the integrator 3 and reconnects it at the end of the duration of the pulse of the generator 4, after which the same cycle is repeated, the flame relay 10 still being powered by the winding 11 thanks to the holding current. When

  the flame goes out, the capacitor 6 is no longer charging, the amplifier

  indicator 8 keeps the switching element 9 closed and the flame relay 10 triggers when the holding current falls below

  of its value. In the presence of a flame, only the permanent cycle

  keeps the flame relay energized. So that a component fault can not cause an unwanted connection of the flame relay 10 even in the case of a power supply connected directly to the AC voltage of the network, said block is dimensioned so as to deliver two voltages of determined value and polariti opposite for proper operation of the flame controller. Only in this condition, the aforementioned cycle proceeds without disturbance and allows, in the presence of a flame, a

  permanent connection of the flame relay.

  Further details are described below using Figure 2.

  The power supply is constituted by the series connection of a resistor 23, a capacitor 24, a diode 25 and a first accumulator capacitor 26, connected to an alternating voltage of the network UN, between a conductor of Ph phase and a neutral N. The series connection of the diode 25 and the accumulator capacitor 26 is also in parallel with another series connection of a first Zener diode 27, of opposite polarity to that of the diode 25, and a second accumulator capacitor 28, the latter being further shunted by a second Zener diode 29 limiting its

charging voltage.

  The first supply voltage U1 is taken at the terminals of the first storage capacitor 26, that is to say between the neutral N and a conductor 30, and the second supply voltage U2 at the terminals of the second storage capacitor 28, c that is to say,

  between a connection 31 and the neutral N. The two power supply voltages

  U1 and U2 are of opposite polarity relative to the neutral N The flame relay circuit 1, comprising the components 16 according to Figure 1, is connected between the conductor 30 and the neutral N, the resistor 15 terminating in the conductor 30. L 'element

  switch 9 of FIG. 1 is replaced by the collector circuit

emitter of a transistor 32.

  The series connection of a load resistor 33 and an integration capacitor 34 is further connected to the first one.

  supply voltage U1, which is applied at the base of the transis-

  tor 32 by the integration capacitor 34, a resistor 35 and the collector-emitter circuit of another transistor 36. The voltage U1 on the conductor 30 also influences, via a resistor 37, the base of the transistor 36 which is further connected to the neutral N by the drain-source circuit of a transistor TEC 38. In the absence of a flame more negative than the potential of the neutral N, the gate of the transistor TEC 38 is biased by a resistor 39 connected between the gate and the connection 31. The gate is further connected to the neutral N by the collector-emitter circuit

of a bipolar transistor 40.

  It follows from the foregoing that the first voltage U1 feeds both the flame relay circuit 10 to 16, the amplifier circuit

  2 consists of the transistors 32, 36, 38, and the capacitor

  integrator 34, i.e. integrator 3.

  The base of the transistor 40 constitutes an input 19 and the neutral N the other input 20 of the amplifier 8 according to FIG.

  taken from the first capacitor 6 acts between these two inputs.

  The capacitor 6 is connected to the alternating voltage of the network UN in series with a resistor 42 and the capacitor 7, the ionization circuit being connected by a resistor 43 to the point of connection of the capacitor 7 and the resistor 42, and consequently in parallel with the first capacitor 6. The ionization circuit is constituted by a detector 5 immersed in a flame 46 and a burner nozzle 47, connected to the ground or the neutral N.

  The assembly further comprises a capacitor generating

  pulses 48, connected on one side to the connection point between the resistor

  15 and the two diodes 13 and 16. Its second terminal is connected to a socket 49 of a voltage divider, constituted by a resistor 50 and a diode 51 biased in the forward direction, and connected between the conductor 30 and the neutral N. The pulse generator capacitor 48 forms with the resistor 50 a timer element which influences a switching device constituted by a transistor 53, whose emitter-collector circuit is connected between the base of the transistor 40 and the neutral N. transistor 53 is controlled by the timer element 48, 50 and influenced by the second supply voltage U2. To do this, the plug 49 is connected by a resistor 52 to the base of the transistor 53.

  corresponds to the switching device 21 of FIG.

  The base of another transistor 54 is connected by a resistance to the connection between the integration capacitor 34 and the two resistors 33 and 35. The emitter of the transistor 54 is furthermore

  connected by a Zener diode 56 to the conductor 30, while its collar

  The reader is connected by a resistor 57 to the connection 31 and consequently to the second supply voltage U2. A link is further established between the collector of the transistor 54 and the base of the transistor 53 by a diode 58 which, viewed from the collector of the transistor 54, is polarized in the opposite direction. The operation of the flame controller described in Figure 2

is the next.

  After the application of the AC voltage of the UN network, the

  two capacitors 26 and 28 are charged by their corresponding diodes.

  25 or 27 and deliver the two supply voltages U1 and U2, polarized in the opposite direction to the neutral N and called

voltages U1 and U2 below.

  No flame is originally present by hypothesis.

  The voltage U2, whose value is limited by the Zener diode 29, acts on the switching device influenced by the pulse generator described hereinafter, that is to say on the basis of the transistor 33 and therefore also on the input 38a of the amplifier, constituted by the gate of the transistor TEC 38. Thanks to the use of a transistor TEC, the negative voltage applied by the resistor 39 allows its blocking. A positive voltage is then applied by the resistor 37 to the base of the downstream transistor 36, which is in the on state, as well as the transistor 32 therefore. In the absence of a signal

  the series connection of the diode 13 or 16, the winding

  11 or 12 and charge capacitor 14 is thus short-circuited in the circuit of the flame relay. Any current flow in the

  flame relay 10 is thus prohibited.

  The voltage U1 is essentially applied to the pins of the resistor 15 as a result of the on state of the transistor 32, so that a relatively low value of the U1 is established due to this load. Since the transistors 32 and 36 are in the on state and the resistor 35 is relatively small, the base of the transistor 54 is substantially at the potential of the neutral N and the transistor 54 is in the on state. The Zener voltage of the diode 56 is exceeded and the voltage U1 produces another current in 30, 56, 54, 57, 29, the maximum voltage drop occurring across the resistor 57. The potential of the cathode of the diode

  58 becomes positive, which precludes an influence of this cir-

baked on the transistor 53.

  The voltage U1 furthermore produces, by the resistor 50, a charge of the pulse generator capacitor 48, the charge voltage of which is limited by the diode 51. The voltage drop across the diode 51, i.e. the voltage on the plug 49, maintains by the resistor 52 the base of the transistor 53 sufficiently positive for

that the latter remains blocked.

  The assembly stops in the state previously described until after ignition of the flame 46, the capacitor 6 is charged and its voltage U3 controls, through the resistor 41, the bipolar transistor 40 The latter thus transmits the flame signal to the input 38a of the amplifier. The base-emitter diode of transistor 40 simultaneously limits a further increase in

  control voltage U3. This is important because a

  if the U3 value is too high when extinguishing the flame, the response time will be too long; a self-check is also ensured,

  because an interruption of the base-emitter diode is detected. It pro-

  would indicate a triggering of the flame relay 10, the pulse cycle

  sional described below can no longer be continued.

  The control of the transistor 40 makes the input 38a of the amplifier

  the gate of the TEC 38 transistor more positive and

  switches the drain-source circuit of the latter into the on state.

  Transistors 36 and 32 then lock. The charge of the source of U1 by the resistor 35 is suppressed. The pulse generator capacitor 48 is charged by the resistor 15, which is much smaller than the resistor 50, and by the diode 51. The integration capacitor 34 is likewise charged by the resistor 33. A charging current flows. furthermore in the diode 13 and the winding 11, to the charge capacitor 14, but is not sufficient to switch on the flame relay 10. The base of the transistor 50 has a value

  more and more positive with the growth of the conden-

  integration timer 34, until the Zener voltage across the diode 56 is exceeded downward, which causes the blocking of the transistor 54. The time flowing until this block is fixed by the integration capacitor 34. Positive voltage

  across the resistor 57 is suppressed by blocking the tran-

  sistor 54. The diode 58 controls the transistor 53. Now being conductive, it constitutes a short-circuiting switch of the base-emitter circuit of the transistor 40, which it connects the base to the neutral for a time determined by a pulse generator described below. Transistors 40 and 38 are blocked as a result,

  while transistors 32 and 36 become conductive. The neck-

  The charging current 14 discharges at 12, 16, 32 and its current produces the engagement of the flame relay 10, while the integration capacitor 34 discharges very rapidly on 35, 36, 32.

  charge reversal of the pulse generator capacitor 48 com-

  simultaneously with the passage of the transistors 32 and 36 in the on state, the right electrode in Figure 2 of said capacitor 48 being connected to the potential of the neutral N by the transistor 32. The load produced by the passage of the transistor 32 to the The on-state reduces the supply voltage U1. This causes the transistor 54 to be blocked by a reaction effect, a desired flip-flop effect of its switching action, and a sufficiently long residence time.

the blocked state.

  The charge transfer of the pulse generator capacitor

  48 lasts much longer than the very fast discharge of

  integration denser 34, so that the positive voltage on the base of the transistor 54 decreases rapidly and becomes more negative than the voltage on the transmitter. The transistor 54 becomes again

  conductor and the voltage U1 acts again by the resistor 57.

  The connection between the resistor 57 and the diode 58 is now brought to the positive potential, which however can not influence the blocking of the diode 58 of the transistor 53. This point is important because the blocking time of the transistor 54 is lower than the desired duration of the pulses of the generator. The transfer of charge of the pulse generator capacitor 48, which is relatively slow for this purpose, is effected by the resistor 50, the potential of the voltage on the plug 49 being at first negative and then varying. to values

  positive, with transmission at the base of transistor 53 by resistance

  FIG. 52. Transistor 53 is thus turned on by the blocking of transistor 54, but remains conductive beyond the short blocking pulse of transistor 54, until the charge reversal of the generating capacitor pulses 48 again reaches a potential of about 0.5 V on the socket 49. The transistor

53 then hangs again.

  The capacitor 6 has hardly discharged on the resistor 41 during the conduction time of the transistor 53. After blocking of the latter, only the value of the control voltage U3 taken across the capacitor 6 is therefore decisive on the basis of of transistor 40, although transistor 53 and control voltage U3 act on said base as previously described. In other words, and assuming that flame 46 is still present, transistors 40 and 38 return to the conducting state, while

  transistors 36 and 32 are locked again.

  The charging current of the capacitor 14 flowing in the coil is

  11 now maintains the powered flame relay 10. The in-

seems cycle then repeats itself.

  The flame relay 10 can therefore remain powered only

  if the transistor 40 is periodically locked for a short time.

  Any disturbance of the described cycle, taking place in a time

  determined, triggers the flame relay 10. The power supply unit

  The supply voltage, delivering the two power supply voltages U1 and U2, is also monitored in this way, since any failure of one of its components disturbs this cycle and therefore causes a tripping of

flame relay 10.

  In the flame controller described, and contrary to the prior art, the transistor 32 connected in parallel with the circuit of the flame relay was chosen to be in the on state in the absence of any flame signal. . The extinction response time is thus no longer determined by the large charging time constant of the capacitor 14, but by its low discharge time constant. It is therefore possible to choose a supply voltage source U1 of high resistance and therefore

  to work with a relatively flat load curve of

  This will increase the repetition time.

  cycle, that is to say to choose more than 1 second.

  This again results in a more favorable ratio of the charging time to the discharge time of the capacitor 6 charged by the ionisation circuit, that is to say a lower energy consumption in this circuit, and consequently a lower charge of the detector signal and increased sensitivity. The extinction response time is now essentially determined by the discharge time constant,

  resulting from the resistor 41 and the capacitor 6.

  The flame controller described is self-controlled continuously, since any short-circuit or interruption of a semiconductor or a

  capacitor, as well as any interruption of a resistor

  any disturbance of the cycle necessary for the circuit of the

  flame relay, so that the latter is no longer powered.

  To achieve this behavior, it is essential that the triggering

  The discharge of the charge capacitor 14 is not derived from the voltage across the capacitor, but from the independent capacitor 34. It is also essential that the

  duration of the discharge does not depend on the voltage across the con-

  charge generator 14, but of the separate timer circuit of the con-

  pulse generator densifier 48 and its resistor 50.

  The described arrangement allows economical production and, despite the transformerless power supply, ensures that a component fault can not cause an inadmissible increase in the voltage of the transformer.

  service or extension resulting from the response time to the ex-

  extinction. Of course, various modifications may be made by those skilled in the art to the principle and to the devices which have just been described solely by way of nonlimiting examples, without

depart from the scope of the invention.

Claims (8)

claims   1. Self-controlled flame controller for monitoring the flame of an oil-fired or gas-fired burner, with an ionization flame detector and a parallel connection, which can be connected to an alternating voltage, of the flame detector with a first capacitor, the charge of which depends on the ionization current of the flame detector; an amplifier influenced by a control voltage through the first capacitor; an output switching element which, as part of a flame relay circuit, is in parallel with the series connection of at least   a diode, the partial winding of a flame relay and a conden-   the series connection of at least one other diode with another partial winding of the flame relay and the charge capacitor is connected to the supply voltage of the charge capacitor; and a pulse generator which periodically opens and closes the output switching element in the presence   an ionization current, said flame controller being characterized   in that an integrator (3; 34), independent of the state of charge of the first capacitor (6) charged by the ionization current, but   influenced by the output state of the amplifier (8; 36; 38), triggering   releasing a pulse generator (4; 54) of predetermined duration by a timer element (50,48); in that a switching device (21; 53), actuated by the generator t; 54) during the pulse duration, bypasses the input (19, 20; 38a) of the amplifier and thereby simulates a "flame off" signal for the amplifier (8; 38; 36); and in that a power supply, connected to the alternating voltage   of the network (UN), delivers two voltages (U1, U2) which have two values   Iurs determined for the correct operation of the device.   2. flame controller according to claim 1, characterized in that the two voltages (U1, U2) of determined value have a reverse polarity; the first voltage (U1) supplies the flame relay circuit (1, 10 ... 16), the amplifier circuit (2; 36; 38), the pulse generator (4; 54; 48; 50) and the integrator (3; 34); and the second supply voltage (U2) acts on a switching device (21; 53) influenced by the pulse generator   (4; 54, 48, 50) and therefore on the input (19, 20; 38a) of the amplifier fier.   Flame controller according to one of claims 1 or 2,   characterized in that the power supply is constituted by the series connection, connected to the alternating voltage of the network (UN), a resistor (23), a second capacitor (24), a diode ( 25) and a first accumulator capacitor (26); the series connection of the diode (25) and the first accumulator capacitor (26) is furthermore in parallel with the series connection of a first Zener diode (27), of opposite polarity to that of the diode (25), and a second accumulator capacitor (28), which is further shunted by a second Zener diode (29) limiting its charging voltage; and the first supply voltage (U1) is taken at the terminals of the first storage capacitor (26) and the second supply voltage (U2), of opposite polarity, at the terminals of the   second accumulator capacitor (28).   Flame damper according to claim 3, characterized in that the parallel connection of the flame detector (5) and its ionisation circuit (5, 46, 47) with the first capacitor (6) supplying the control voltage (U3) is connected to the AC mains voltage (UN) by at least one resistor (42, 43) in   series with another capacitor (7).   Flame controller according to any of the claims   to 4, characterized in that the output switching element (9; 32), in parallel with the series connection of a diode (13 or 16), a winding (11 or 12) and the charge capacitor (14) in the circuit (1) of the flame relay, is a transistor (32) passing in the absence of a flame signal.   Flame controller according to claim 5, characterized in that   the transmission of the flame signal at the input (38a) of the amplifier   It is provided by a bipolar transistor (40) whose collector-emitter circuit is connected between the input (38a) of the amplifier and the neutral (N), and on the basis of which both the switching device ( 21; 53) actuated by the pulse generator (4; 54) and the control voltage (U3) taken from the   terminals of the first capacitor (6), the base-emitter circuit of the tran-   bipolar sistor (40) simultaneously limiting the control voltage (u3). 7. Flasher controller according to claim 6, characterized in that the switching device (21) is constituted by another transistor (53) whose emitter-collector circuit is connected between the base of the bipolar transistor (40) and the neutral (N) and whose base controlled by the timer element (48, 50) is influenced by   the second supply voltage (U2).   8. flame controller according to claim 7, characterized in that the input (38a) of the amplifier (32, 36) is constituted by the gate of a transistor (38), connected by the second voltage negative power supply (U2).