DE3026787A1 - INTRINSICALLY SAFE FLAME GUARD - Google Patents

Description

Pattntanwälte BipL Ing. Hans-Jürgem ΜβΗ «
Dr! r «r. nat. Thomas Be «** t

I Hns LeyM ο η ο ß 7 Q ¹ ^

i ** -. 3026/87

/ - ~ Π LGZ LANDIS & GYR ZUG AG

C LANDIS & GYR) _CH _ _{6301 ZUGj}

Intrinsically safe flame monitor

PA 2108

130052/0369

30267a?

Intrinsically safe flame guard

The invention relates to an intrinsically safe flame monitor according to the preamble of claim 1. 5

Flame monitors are required for the fully automatic commissioning and monitoring of oil or gas burners. whose job it is to indicate the presence of a flame at all times. These flame monitors must be used together with their Flame sensors are largely intrinsically safe, which means that every possible defect in a component must be triggered by the "Flame extinguished ". This can be done by checking the functionality of the flame sensor before each commissioning of the system. There are also systems with two flame monitors that work independently of one another used and monitored for continuous signaling in the same direction. This means a lot of effort. Besides that Flame monitors with sensors that respond in particular to ultraviolet radiation, henceforth called UV cells, are known in which the radiation to the sensor is covered at least once per second in the same cycle and constant cycle ratio and so the extinguishing ability of the UV cell is monitored within the permitted log-off time. A constant clock ratio however leads to a noticeable loss of sensitivity.

Furthermore, it is known from DE-OS 2 809 993, the voltage influenced by an ionization sensor across a capacitor to use to control an amplifier and this DC voltage with an additional transistor from the To superimpose the alternating signal dictated by the mains frequency. A series connection of two transistors with one flame relay and two Capacitors only keep the flame relay in an excited state if the alternating signal is superimposed on the flame signal and it proves that the amplifier is functional. However, this circuit is not yet intrinsically safe because of their Switching elements are not all monitored: For example

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If one transistor shows a short circuit, this will not be recognized and any further short circuit in one of the other components is then no longer tested and can lead to the pretense of a flame.
5

The invention is based on the object of specifying a flame monitor that provides intrinsically safe flame monitoring with little effort and can also work with both ionization electrodes and a UV cell. 10

The invention is characterized in claim 1.

Two exemplary embodiments are explained in more detail below with reference to the drawings:
15th

They show: FIG. 1 an intrinsically safe flame monitor circuit

with a first UV sensor circuit and FIG. 2 with a similar flame monitor circuit

a second UV sensor circuit. 20th

The shades described below, of which for the time being that is described in more detail according to FIG. 1, consist essentially of the following, according to their functional task

structured circuit parts:
25th

A sensor circuit 1 with pulse generation, which can be operated either with an ionization sensor 2 or with a UV sensor 3 can,

a flame relay circuit 4 and

a circuit 5 (Fig. 1) or 6 (Fig, 2) for alternative use when operating with a UV sensor.

The sensor circuit 1, explained in the following with reference to Fig. 1, consists of a series supplied by an alternating voltage U.

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switching of a flame sensor, be this the ionization sensor or the UV sensor 3, with a first capacitor 7. The alternating voltage U. is a first winding 8 of a power transformer. 9 removed and with a first line 10 to earth and to the two sensors 2 and 3, while a collecting line 11 serves as the second line. The capacitor 7 connects the bus line 11 on the one hand via a protective resistor with the ionization sensor 2 and on the other hand via a Zener diode 13 and a resistor 14 with the UV sensor 3. The further description of FIG. 1 is initially made together with the Use of the ionization sensor 2.

The rectifying effect of a flame creates a DC voltage across the capacitor 7, the polarity of which is on the Side of the bus line 11 is positive.

On the collecting line 11, a second capacitor 15 is located on a positive one with respect to the collecting line 11 and is described further below DC voltage source connected via two charging resistors 16 and 17. The capacitor 15 is a diode connected in parallel, which blocks the mentioned DC voltage. The connection point of the diode 18 with the second capacitor 15 and the charging resistor 16 form an input 19 to the flame relay circuit 4.

The voltage across the first capacitor 7 is mainly composed of two transistors 20 and 21 and one Zener diode 22 formed toggling threshold switch 74 is monitored. The collector-emitter path of the first transistor 20 is connected in series with two resistors 23 and 24 to the first capacitor 7 in parallel, while its base via the Zener diode 22 is connected to the manifold 11. Furthermore, starting from the connection point 25, there is the two resistors 23 and 24, a series connection of the emitter-collector path of the second transistor 21 and a resistor 26 to the negative side of the capacitor 7. The base of the first

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Transistor 20 is also connected to that of the second transistor connected side of the resistor 26, while the base of the second transistor 21 to the connection of the resistor 23 is connected to the first transistor 20. Between the connection point 25 and the input 19 to the flame relay circuit 4 there is also a series connection of a further Zener diode with a resistor 28.

Before going to the mode of operation of the sensor circuit described is received, there is an explanation of the flame relay circuit 4. It is fed from the previous one already mentioned DC voltage source, which consists of a second winding 29 of the mains transformer 9, a diode 30 and a smoothing capacitor 31 and its negative potential is connected to the manifold 11. The DC voltage source feeds the flame relay circuit 4 via a load resistor 32 and a feed line 33. Between the feed line and the collecting line 11 there is a flame relay 34 containing Circuit that is formed from two oppositely polarized current paths that both share a common Have capacitor 35. The first current path, which is the direction of current flow corresponds to the DC voltage source, consists of the series connection of a first diode 36, a first winding 37 of the flame relay 34, a second diode 38 and the capacitor 35. The second current path in turn includes the capacitor 35, a third diode 39, a second winding 40 of the flame relay 34 and a fourth diode 41, all of which are connected to one another Are connected in series.

The flame relay 34 has switching contacts, not shown in the figures, with which it is present in the excited state a flame. Its windings 37 and 40, each assigned to one of the two current paths, are designed in such a way that that only one of the two current paths can attract the flame relay 34, while the other current path only a holding current for the flame relay 34 supplies.

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In this example, only the one in the second rung can 35, 39, 40, 41 flowing discharge current of the capacitor 35 bring the flame relay 34 to attract, during the in the first Current path 36, 37, 38, 35 through the first winding 37 charging current of the capacitor 35, the flame relay 34 only for can keep energized for a limited period of time.

The flame relay circuit 4 also includes a switching device consisting of a flip-flop consisting of three transistors 42, 43, 44, the input 19 of which is the side of the capacitor connected to the resistors 16 and 17 to the direct voltage source 29, 30, 31 15 forms. The output transistor 44 of the flip-flop is connected with its collector-emitter path to the feed line 33 and connected to the bus line 11 and short-circuits the circuit containing the flame relay 34 when it is conductive.

The output transistor 44 is polarized so that it is the supply voltage removes via the first current path 36, 37, 38, 35 and applies it to the load resistor 32, and at the same time the second Current path 35, 39, 40, 41 for discharging the capacitor 35 closes.

The collector-emitter path of the second transistor 43 is parallel to the collector-base path of the output transistor 44 switched. The base of the second transistor 43 is connected to the connection point via a voltage divider made up of two resistors 45 and 46 between the resistors 16 and 17 and thus connected to the DC voltage source 29, 30, 31. The first In the conductive state, transistor 42 connects to its collector-emitter path the connection point between the resistors 45 and 46 of the voltage divider with the bus line 11. Its Entrance 19 forms the basis.

In the described arrangement determines the polarity of the voltage The instantaneous switching state of the three transistors 42, 43, 44 via the second capacitor 15.

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The described works using the ionization sensor 2 Setup as follows:

When the transformer 9 is connected to a mains voltage, the capacitors 15 and 35 are charged. The condenser at the input 19 of the flip-flop 42, 43, 44 is charged within about 25 ms and in this state causes the output transistor 44 blocks. The capacitor 35 in the circuit of the flame relay 34 can therefore be via the diodes 36 and 38 as well charge the first winding 37 of the flame relay 34. The current flowing through the winding 37 is not sufficient for the Flame relay 34 to be excited.

As soon as a flame hits the ionization sensor, the first capacitor 7 charges within about 0.5 seconds to the upper threshold voltage of the threshold switch 74, that is, up to the breakdown voltage above the Zener diode 22 is reached. Then the first transistor 20 becomes conductive and suddenly the second transistor also becomes conductive due to the positive feedback circuit 21. Both activate a first and a second discharge circuit of the first capacitor 7.

The first discharge circuit is used to recharge the second capacitor 15, with a discharge current from the first capacitor 7 via the second capacitor 15, the resistor 28, the Zener diode 27, the resistor 23 and the transistor 20 flows back to the capacitor 7. The forward voltage of the diode 18 is limited the charging voltage of the second capacitor 15. As soon as the capacitor 7 is discharged to the extent that the voltage is above the Zener diode 27 serving as a voltage-dependent switch drops below its breakdown voltage, the charge reversal begins again of the capacitor 15 in its original state.

The second, longer activated discharge circuit of the first capacitor 7 runs through the resistor 24, which mainly determines the remaining discharge duration, the resistor 23 and the

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Transistor 20. As soon as the voltage drop across the resistor, the emitter-base voltage that keeps transistor 21 conductive falls below, both transistors 21 and 20 tilt back due to the positive feedback and make the discharge circuit high-resistance again. The size of the resistor 24 is not critical, it only has to be sufficiently low to the first capacitor 7 to Tilting back of the transistors 20 and 21 to be able to discharge sufficiently

As soon as the transistors 20 and 21 block again and, provided that the flame is still present, charges the capacitor 7 opens again and the game begins again.

The described charge reversal of the second capacitor 15 has the effect that the input transistor 42 to the flame relay circuit 4 blocks. Thus, the second and the output transistor 43 and become conductive, the capacitor 35 discharges through the diodes 39 and 41 and the second winding 40. The flame relay 34 picks up and is now discharged from the discharge current for the following approx. 50 ms kept excited. When the charge reversal of the second capacitor 15 is completed, the voltage source forms at the input 19 29, 30, 31 forced polarity back within about 50 ms, which can be set with the resistor 16. The input transistor 42 becomes conductive again and thus the output transistor 44 is blocked. The first current path charging the capacitor 35 is reactivated and keeps the flame relay 34 energized.

The state of charge of the condensate, Tor 7 generates, as described above, a reloading that takes place in a certain cycle influenced by the flame sensor of the capacitor 15. The constantly changing state of charge of the capacitor 15 in turn affects the flame relay circuit 4 switching of the output transistor in the same cycle 44, whereby the flame relay 34 can be kept energized. To ensure this, the time between two

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Discharges of the capacitor 35 amount to a maximum of about 900 ms, otherwise the holding current for the flame relay 34 is not reached will. Likewise, the discharge time of the capacitor 35 must not significantly less than 50 ms, because otherwise the capacitor is not discharged enough for the subsequent recharge.

To ensure the necessary cycle, it is advantageous if the time of the current flow in the first discharge circuit of the capacitor 7 compared to the time of the current flow in the second discharge circuit is small and their duration of action ratio is at least 1:10. In the present example, the Charge reversal of the capacitor 15, generated by the first discharge circuit within about 2 ms, while the discharge of the capacitor 7 after about 30 ms by falling below the lower Threshold voltage of the threshold switch 74 is stopped.

Since every component of the circuit is based on the creation of the described Charge and discharge cycles are involved, every failure indicated by a fault in this cycle and thus by the flame relay 34 dropping out. This results in the required Your own security.

In the previous description it was assumed that the flame sensor is an ionization sensor. The following relates refer to the use of a UV sensor 3. This consists of a UV cell 47, with which a diode 48 and a protective resistor 49 are connected in series. Because the series connection of the UV sensor 3 with the relatively high-resistance sensor circuit no clear glow discharge of the UV cell 47 guaranteed,

3Q is the capacitor 7 and the capacitor 7 connected in parallel Threshold switch 74 bridged by a shunt resistor 50.

The Zener diode 13 in the supply line to the UV sensor 3 prevents, on the one hand, a discharge of the capacitor 7 via the resistors 13, 14 and 50 during the power breaks and

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light, on the other hand, a flame simulated by interspersed alternating voltage in a possibly very long sensor line. Such stresses by far exceed the Zener stress and are therefore diverted.
5

When a flame is present, the charging current of the sensor circuit 1 controlled by the UV cell 47 is at most in spite of the shunt resistor 50 is even greater than the holding current of the threshold switch 74. In this case, it must be ensured that after Once the first capacitor 7 has been charged, no further current pulses can reach the capacitor 7. That purpose circuit 5 or 6 is used for temporary suppression of the charging current controlled by the UV cell 47 to the first capacitor 7 in the cycle of the transistors 42, 43, 44 of the Flame relay circuit 4 is effective.

In the example of FIG. 1, it is used together with the dashed line Line delimited circuit 5 a device 51 interrupting the radiation to the UV cell 47 for temporary Suppression of the charging current to the first capacitor 7. A fixed diaphragm 52 and a movable one are located in the beam path Flap 53, which can cover the aperture by an electromagnet 54 displaceably. The mentioned parts of the Device 51 are all accommodated in a housing which forms the UV sensor 3. One feed line of the electromagnet is connected to the positive pole of the direct current source 29, 30, 31. In a second supply line to the electromagnet 54, which is connected to the The negative pole, that is to say is connected to the bus line 11, is the collector-emitter path of an output transistor 55 a flip-flop circuit consisting of two transistors 55 and 56. In this is the base-emitter path of the output transistor 55 of the collector-emitter path of the input transistor 56 connected in parallel, with both emitters at the Manifold 11 are connected. The base of the output transistor 55 is together with the collector of the input transistor 56 via a common resistor 57 with the plus

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pol of the direct current source 29, 30, 31 connected. The base of the input transistor 56 is one through the series connection Resistor 58 with a Zener diode 59 and a capacitor connected to the bus line 11. Zwisehen by a tap the capacitor 60 and the Zener diode 59, the capacitor 60 is further provided with a charging resistor 61 and a charging resistor 61 bridging diode 62 between the feed line 33 and the bus line 11 and therefore with the flame relay 34 containing circuit 35 to 41 connected in parallel. The series connection of the capacitor 60 with the charging resistor 61 or the diode 62 is therefore together with the circuit 35 to 41 containing the flame relay 34 from the output transistor 44 can be short-circuited.

The switching state of the toggle switch 55, 56, and thus also the position of the flap 53 in the UV sensor 3, depends on the voltage across the capacitor 60 dependent. In the idle state, that is to say initially without UV radiation, the capacitor 60 charges in about 300 ms to the sum of the Zener voltage of the Zener diode 59 and the voltage drop across the base-emitter path of transistor 56, whereby the input transistor 56 conducts and the output transistor 55 blocks. The aperture 52 is then open minded. A diode connected in parallel with the electromagnet 54 protects the transistor 55 from overvoltages.

As soon as current pulses occur in the UV cell 47, the capacitor 7 begins to charge, for which the pulses of three half-waves of the alternating voltage supply are already sufficient. The pulses do not have to occur together; in the case of a weak radiation source, they can also occur individually within 600 ms and still be evaluated as a signal for reporting a flame, as explained below. It can be seen from this that of the 30 half-waves occurring during 600 ms at a frequency of 50 Hz, only 10 % are sufficient for flame detection. This means a decisive advantage of the flame monitor described, the UV sensor sensitivity of which is therefore very high.

^{PA 2108} 130052/0369

As soon as the voltage across the capacitor 7 in the sensor circuit 1 reaches the trigger voltage of its threshold switch 74, the previously described charge reversal of the capacitor 15 is initiated. Thus, the flip-flop in the flame relay circuit is activated and its output transistor 44 becomes conductive and does two things: firstly, the resulting discharge circuit of the capacitor 35 in a current path through the winding 40 attracts the flame relay 34, and secondly, the capacitor 60 in the Circuit 5. Its toggle switch responds, the electromagnet 54 is excited and the flap 53 interrupts the UV radiation. No further charge is supplied to the capacitor 7 and its discharge is ended after about 30 ms by tilting the threshold switch 74 back. The circuit is designed in such a way that one or two current pulses can still occur from good UV cells 47 immediately after the UV radiation has ceased, without the charge reversal to the capacitor 15 and thus the entire cycle being disrupted. In contrast, the charging and discharging cycle in the flame relay circuit 4 is disturbed if the UV cell 47 nevertheless delivers irregular current pulses during the approx. 300 ms cell cover by the flap 53, a phenomenon that occurs with aging UV cells. Then the flame relay drops out, even if the flame is burning correctly.

In the case of trouble-free operation, the flame relay 34 remains between the discharge of the capacitor 35 within a repetition time attracted approx. 350 and 950 ms. As soon as these times are exceeded or fallen short of, it falls off. Thanks to the one described Circuit 5 is a further advantage of the device -that, despite great sensor sensitivity, every defect in both the Circuit 5 as well as in the UV cell 47 itself at any time, that is to say also very quickly during operation, that is to say within less is displayed more than one second after it occurs. Therefore, the device described is particularly suitable for a plant in continuous operation.

^PA2108 130052/0369

302678?

In contrast, the flame monitor according to FIG. 2 is related to UV monitoring only in systems with intermittent operation intrinsically safe, since there is no repetitive covering of the UV cell 47. The UV sensor 3 can, however, have a simpler structure be designed. The circuit is interrupted apart from what has been said for the UV sensor 3 and with the exception of the one with one Line enclosed circuit 6 exactly the same as in FIG. The circuit 6 is used analogously to the circuit 5 of FIG. 1 the temporary suppression of the further charging current to the capacitor 7, but this is done purely electrically. Movable Parts are therefore omitted.

The circuit 6 consists of one with its emitter-collector path the first capacitor 7 parallel-connected transistor 64, whose collector at the connection between the Zener diode 13 and the resistor 14 are tapped and the emitter of which is connected to the bus line 11. The base-emitter path of the transistor 64, a capacitor 65 is connected in parallel. For this purpose, the capacitor 65 is on on the one hand connected to the bus line 11 and on the other hand via a Zener diode 66 and a resistor 67 to the base of the transistor 64 connected. A diode 73 with oppositely polarized polarity is also connected in parallel to the base-emitter path. In addition, the capacitor 65 is in series with a resistor 68, to which a diode 69 is connected in parallel, and another Resistor 70 is also connected in parallel with the circuit containing flame relay 34. The capacitor 65 can therefore also together with the circuit of the flame relay 34 from the output transistor 44 of the flame relay circuit 4 short- ' getting closed. From the base of the transistor 64 there is a further series connection of a resistor 71 and a diode 72 to the connection of the winding 29 that feeds the bus line 33. The diode 72 is polarized so that the the half-waves acting on the base of the transistor 64 are in phase with the half-waves flowing through the UV cell 47.

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The circuit 6 described works as follows: When ready for operation, that is to say when there is still no flame present, the capacitor 65 is charged via the resistors 68 and 70 from the voltage source 29, 30, 31. The charging voltage is limited by the voltage drop across the Zener diode 66, the resistor 67 and the diode 73. During the negative half-waves at the diode 72, only part of the current impressed in the diode 73 flows through the resistor 71 and the diode, so that the transistor 64 always blocks. After a flame has arisen, the transistor 64 connected in parallel with the sensor circuit 1 does not initially affect the charging of the capacitor 7. The same process takes place as was described for FIG. 1. The capacitor 15 is reloaded and the output transistor 44 on the one hand closes the discharge circuit for the capacitor 35, and the flame relay 34 picks up. On the other hand, the transistor 44 discharges the capacitor 65 via the diode 69 and the resistor 70. As a result, each half-cycle for which the diode 72 is conductive can now generate a base current in the transistor 64, so that it is periodically conductive. These base current half-waves are in phase with the current pulses of the UV cell 47. The UV cell current therefore no longer flows to the capacitor 7, but is diverted directly via the collector-emitter path of the transistor 64. The capacitor 7 can thus be discharged, its threshold switch 74 tilts back and the whole process starts again.

The flame monitor described includes the advantage of only having a single, intrinsically safe amplifier circuit with only to consist of a relay that can be used for both ionization sensors and UV sensors, and also with a By periodically covering intrinsically safe UV sensors, high sensor sensitivities can still be achieved. thanks to The described, permanent change in state of the assemblies results in the intrinsic safety related to all components. Every failure of a component causes the flame relay to drop out.

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Due to the deliberately chosen large cycle ratios of the charging and discharge cycles of the various circuits, and because of them If clock frequencies are far outside the 50 Hz mains frequency, there is a risk of faking a flame AC voltage interference on the amplifier input, which is usually unavoidable in practice, is completely excluded.

The appropriate use of the circuits described with UV sensors is not limited to this type of sensor alone, they could also be adapted accordingly with a photoresistor operate.

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SUMMARY

Flame monitor for optional use with an ionization sensor (2) or a UV sensor (3), which are in series with a first capacitor (7) are connected to an alternating voltage (U). The sensor current charges the capacitor (7). A Threshold switch (74) discharges the capacitor (7) depending on the voltage and transfers some of its charge to a second one Capacitor (15) and reloads it. Its voltage influences the input (19) of a trigger stage (42, 43, 44). Their exit switch (44) short-circuits the supply circuit of a flame relay (34) in a time-limited permissible cycle. At the Short-circuiting, a capacitor (35) discharges via the flame relay (34), which then picks up, the following charging current holds that

'' 5 flame relay (34) energized. The output switch (44) also short-circuits another capacitor (60) and, via a toggle switch (56, 55), influences a flap (53) for temporarily covering the UV radiation. Only the limited radiation enables the permissible switching cycle to occur so that the flame relay (34) remains energized.

(Fig. 1) 25

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Claims (10)

■ · PATENT CLAIMS MJ flame monitor for monitoring flames on oil or gas burners with one connected to AC voltage Series connection of a capacitor (7) and a flame sensor (2, 3), one influenced by the voltage across the capacitor (7) Threshold switch (74) which, when its switching threshold is reached, causes a signal to attract the flame relay (34), with a switching device (42, 43, 44) for periodic blocking of the signal to the flame relay (34) and simultaneous discharge of the capacitor (7) connected in series with the flame sensor (2, 3) and with a flame relay circuit (35 to 41), the the flame relay (34) can only hold in its position indicating a flame by an alternating signal, characterized in that that the voltage on a second capacitor (15), which is connected to a direct current source with at least one charging resistor (16, 17) (29, 30, 31) is connected, the switching state of the switching device (42, 43, 44) determines that the threshold switch (74) for discharging the first capacitor (7) connected in series with the flame sensor (2, 3), a first and a second capacitor Discharge circuit (15, 18, 28, 27, 23, 20 or 24, 23, 20) activated, whereby the first discharge circuit (15, 18, 28, 27, 23, 20) is used to charge the second capacitor (15) and from a parallel connection of the second capacitor (15) with a the magnitude of the reversed voltage across the second capacitor (15) limiting diode (18), a resistor (28) and a there is a voltage-dependent switch (27) which interrupts the charging process, while in the second discharge circuit (24, 23, 20) a resistor (24) determining the remaining discharge duration of the first capacitor (7) is arranged. 2. Flame monitor according to claim 1, characterized in that the time of the current flow in the first discharge circuit (15, 18, 28, 27, 23, 20) is small compared to the time of current flow in the second discharge circuit (24, 23, 20) and whose duration of action ratio is at least 1:10. 130052/0369 3026797 3. Flame monitor according to claim 1, characterized in that ' that depends on the polarity of the voltage on the second capacitor (15) influenced switching device is a flip-flop (42, 43, 44), the input (19) of which with the DC voltage source (29, 30, 31) connected terminal of the capacitor (15) and its output switch (44) in the conductive state the das Short-circuits circuit (34 to 41) containing flame relay (34), while in the state of charge of the capacitor (15) conveyed by the direct current source (29, 30, 31), the output switch (44) is open. 4. Flame monitor according to claim 3, characterized in that the circuit containing the flame relay (34) (34 to 41) of two oppositely polarized, a common capacitor (35) having current paths (36, 37, 38, 35; 35, 39, 40, 41) is formed, of which the first, the current flow direction of the DC voltage source (29, 30, 31) corresponding Current path (36, 37, 38, 35) from the series connection of a first winding (37) of the flame relay (34), at least a diode (36, 38) and the capacitor (35), while in the second current path (35, 39, 40, 41) the capacitor (35), at least one further diode (39, 41) and a second winding (40) of the flame relay (34) are connected in series and that only one of the two current paths can attract the flame relay (34), while the other current path only supplies a holding current for the flame relay (34). 5. Flame monitor according to claim 4, characterized in that only the closed output switch (44) in the second Current path (35, 39, 40, 41) causes the flowing discharge current of the capacitor (35) to attract the flame relay (34), while the charging current of the capacitor (35) flowing in the first current path (36, 37, 38, 35) through the first winding (37) the flame relay (34) can only keep energized for a limited period of time. 130052/0369 3026797 6. Flame monitor according to claims 1 to 5, characterized in that the flame sensor (2) is an ionization sensor is. 7. Flame monitor according to claims 1 to 5, thereby characterized in that the flame sensor is a UV cell (47) that a shunt resistor (50) the first capacitor (7) and the threshold switch (74) connected in parallel to the capacitor (7) bridged and that also a flip-flop in the cycle d, er (42, 43, 44) of the flame relay circuit (4) effective circuit (5, 6) for the temporary suppression of the UV cell (47) controlled charging current to the first capacitor (7) is present. 8. Flame monitor according to claim 7, characterized in that that the circuit (5) contains a device (51) which interrupts the radiation to the UV cell (47) and which is operated by means of a The flip-flop (56, 55) is controlled depending on the voltage across a capacitor (60) that the capacitor (60) in Series with at least one charging resistor (61) and a diode (62) bridging the charging resistor (61) connected in parallel to the circuit (34 to 41) containing the flame relay (34) and thus from the output switch (44) in the flame relay circuit (4) can be short-circuited with the circuit (34 to 41) containing the flame relay (34). 9. Flame detector according to claim 7, characterized in that the circuit (6) consists of a transistor (64) connected in parallel with its em it ter-Ko I lektor path to the first capacitor (7) of the sensor circuit (1 ) and a capacitor (65 ) , which is connected in parallel to the base-emitter path of the transistor (64) in series with a Zener diode (66), that furthermore the capacitor (65) in series with at least one resistor (68, 70) and one the resistor (68) bridging diode (69) also containing the flame relay (34), from the output transistor (44) of the flame relay circuit (4) briefly 130052/0369 3026797 t closable circuit is connected in parallel, and that too a diode (73) with opposite polarity is connected to the base-emitter path of the transistor (64) and an alternating voltage acts in series with a further diode (72) at the base of the transistor (64) in such a way that its half-waves are in phase with the half-waves flowing through the UV cell (47). 10. Flame monitor according to claim 7, characterized in that in the series connection of the capacitor (7) with a Zener diode (13) is arranged next to the UV sensor (3). ^{PA 2108} 130052/0369