GB2153126A - Self-monitoring flame monitor - Google Patents

Abstract

A flame monitor which can be connected to the mains voltage without using a transformer, with an ionization path (5, 46, 47) which is also supplied by the mains voltage, has an amplifier circuit (2), a flame relay circuit (1), an integrator (3) and a pulse generator (4). The ionization path causes charging of capacitor (6) which acts on an amplifier (8) of the amplifier circuit (2) by way of inputs (19, 20). The amplifier output simultaneously actuates two switching elements (9, 17) of which one is a short-circuit switch (9) which short-circuits a series circuit comprising diodes (13, 16), windings (11, 12) of a flame relay (10) and a charging capacitor (14). The other switching element (17) starts the integrator (3) which in turn triggers a switch (18) of the pulse generator (4). The pulse generator (4) short-circuits the inputs (19, 20) of the amplifier (8) in short intervals of time. When there is a flame (46), the flame relay (10) can only remain energized if the intervals occur in a given timing relationship. Any component failure interferes with that relationship and results in deenergization of the flame relay (10). <IMAGE>

Description

SPECIFICATION Self-monitoring flame monitor The invention relates to a self-monitoring flame monitor, particularly for monitoring the flame of an oil vaporization or gas burner.

Due to the increase in cost of the fossil fuels oil and gas, increasing attention is being paid to the level of efficiency of heating installations. For that purpose, efforts are being made to reduce the shutdown losses of boiler installations, intended to reduce the size of the boiler installations which previously were generally over-size, and by the use of burners whose output can be substantially matched to the instantaneous heat requirement by continuous regulation. Such burners operate in the on-off mode only under minimum load conditions, and are in continuous operation over the remaining load range. With the continuous mode of operation, it is no longer possible to use self-testing of the flame monitor for possible component defects, such testing operation hitherto taking place during the shutdown time or prior to the burner coming into operation on each occasion.Such an installation requires flame monitoring devices which are continuously self-monitoring for component defects, in the operational condition. Such flame monitors are known but still require an excessive level of expenditure for use in small-scale burners, or still suffer from technicai weak points.

Hitherto, there was also the disadvantage that, in conventional oil atomization burners, an inexpensive ionization electrode could not be used as a fail-safe flame sensor, because of carbonization thereof. However, that type of sensor will also gain in significance for oil burners, with increasing use of oil gasification or oil vaporization burners, insofar as in that case blue-burning flames which are suitable for ionization electrodes are produced, without sooting. There is then no carbonization of the sensor electrodes.

A known flame monitor, as disclosed German patent specification No. DE-C-3 026 787, is suitable both for ionization and ultraviolet (UV) modes of operation. In order that that monitor has a short signalling time when the flame goes out, from the operating condition of the installation, a relatively low-resistance operating voltage source with fail-safe voltage limitation is required. Together with the fact that the sensor voltage required for the UV-mode of operation is higher than the mains voltage, that arrangement requires a transformer, which is expensive and takes up a great deal of space.

If, in the case of the known flame monitor, the mode of operation involving an UV-sensor is omitted and a feed without transformer is selected, there is the danger that, in the event of a component defect, the feed voltage rises without that being detected in good time. In the flame relay circuit, that gives rise to an unacceptably long signalling time, in the event of accidental extinguishing of the flame.

In addition, a direct feed without transformer in the known circuit provides a lower ionization sensor voltage so that, with the same quality of flame, the sensor current is lower and is no longer adequate to ensure the necessary level of power at the threshold switch which is fed by the sensor current.

German laid-open application No. DE-A-2 430 328 discloses a flame monitor circuit which can be directly connected to an a.c.

system. However, it requires an extremely high-resistance sensor feed circuit, so that a good and therefore relatively low-resistance flame is not detected as being defective. Due to the high-resistance sensor feed, there is an extremely low level of load-carrying capacity with sensor line capacitances and just low insulation leakage currents have a serious adverse effect on the useful signal.

According to the invention, there is provided a self-monitoring flame monitor for monitoring the flame of an oil vaporization or gas burner, the monitor comprising an ionization flame sensor and a parallel circuit, connectable to an a.c. voltage. of the flame sensor with a first capacitor, the charging of which is dependent on the ionization current of the flame sensor, an amplifier which is influenced by a control voltage across the first capacitor and which has an output switching element forming part of a flame relay circuit and being connected in parallel with a series circuit of at least a diode. a component winding of a flame relay and a charging capacitor, while a further series circuit at least of a further diode with a further component winding of the flame relay and the charging capacitor is connected to a supply voltage which charges up the charging capacitor, a pulse generator for periodically opening and closing the output switching element when there is an ionization current, an integrator which is dependent of the condition of charging of the first capacitor but which is influenced by the output condition of the amplifier and which triggers a pulse generator with a pulse duration fixedly predetermined by a timing member, and a switching means which is actuated by the pulse generator during the pulse duration and which short-circuits the amplifier input and thus simulates for the amplifier a flame out" signal, wherein the flame monitor also includes a supply portion connectable to the a.c. voltage and which provides first and second voltages of two given magnitudes for correct operation of the apparatus.

Such a flame monitor can detect any component failure, can be operated without a transformer directly from the a.c. mains, and is substantially insensitive to insulation defects and line capacitances.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a circuit of a flame monitor according to a preferred embodiment of the invention, broken down into its basic operational groups; and Figure 2 shows a more detailed circuit diagram of one example of the Fig. 1 arrangement.

In the two figures of drawings which are described hereinafter, the same reference numerals are used to denote similar components of the flame monitor.

Referring to Fig. 1, a flame monitor comprises a flame relay circuit 1, an amplifier circuit 2, an integrator 3 and a pulse generator 4, as well as an ionization flame sensor, hereinafter referred to as the flame sensor 5.

With a first capacitor 6 in the amplifier circuit 2, the flame sensor 5 forms a parallel circuit which is connected by way of a further capacitor 7 to an a.c. voltage which is taken directly from the mains system. The amplifier circuit 2 also includes an amplifier 8 which can be influenced by the voltage across the capacitor 6 and whose output acts on a switching element 9 in the flame relay circuit 1 and on the integrator 3 which is described hereinafter.

The flame relay circuit 1 comprises a flame relay 10 having two windings 11 and 12, the first of which, being connected in series with a first diode 1 3 and a charging capacitor 14 as well as a load resistor 1 5, is connected to a d.c. voltage source (not shown in Fig. 1). The second winding 1 2 is also connected in series with the charging capacitor 1 4 and a second diode 1 6 which is opposite in polarity to the first diode 13, and the load resistor 15, and is thus also connected to the same d.c. voltage source.The switching element 9 is connected in parallel to the series circuit of the charging capacitor 1 4 with the two windings 11 and 1 2 and the diodes 1 3 and 1 6 respectively, and permits those circuits to be shunted, as will be described in greater detail hereinafter.

The output of the amplifier 8 also acts on a second switching element 1 7 which triggers the integrator 3. The integrator 3 serves as a timing member and actuates the pulse generator 4, when the supply portion which is connected to a mains a.c. voltage is operating correctly. after a period of time which expires independently of the condition of charge of the first capacitor 6. That procedure is also described in greater detail hereinafter with reference to Fig. 2. Triggering of the pulse generator 4 is indicated in Fig. 1 by a switch 1 8. Associated with the pulse generator 4 is a switching means 21 which short-circuits two inputs 1 9 and 20 of the amplifier 8 and which, in the closed condition, simulates a signal "flame out" for the amplifier 8.The pulse duration is fixed by a timing member (not shown in Fig. 1).

Before going into the details of the circuit shown in Fig. 2, the basic mode of operation will be described with reference to Fig. 1: In the rest condition, when there is no flame, the switching element 9 remains closed. The voltage at the flame relay circuit 1 appears at the load resistor 1 5 and the flame relay 10 is not energized. The voltage at the integrator 3 is zero and the switching means 21 is open. As long as there is no flame, the capacitor cannot be charged up by the applied a.c. voltage. There is no voltage applied to the inputs 1 9 and 20 of the amplifier 8. When the flame appears, the capacitor 6 is charged up and the amplifier 8 causes the switching element 9 to open and the switching element 1 7 to close.The charging capacitor 1 4 is charged up by way of the high-resistance winding 11 and the diode 13, the charging current being unable to cause the flame relay 10 to cut in. At the same time, the switching element 1 7 cut in the integrator 3 which, after reaching a predetermined threshold value, by way of the switch 18, actuates the pulse generator 3 which in turn actuates the switching means 21 and short-circuits the amplifier inputs 1 9 and 20.The capacitor 6 is discharged only to an insignificant degree during that short-circuiting procedure, by way of a limiting resistor 22, but the amplifier 8 actuates the switching elements 9 and 1 7. In the flame relay circuit 1, that causes the flame relay 10 to cut in, by the charging capacitor 14 being discharged, except for a residual voltage, by way of the low-resistance winding 1 2 and the diode 1 6. The duration of the discharging phase is set by the pulse duration of the pulse generator 4 and is so selected that the flame relay 10 is not deenergized.At the same time, the switching element 1 7 set the integrator 3 to zero and switches it on again at the end of the pulse duration of the pulse generator 4, whereupon the same cycle takes place afresh, with the flame relay 10 always remaining energized by virtue of the holding current through the winding 11.

When the flame goes out, then the capacitor 6 is no longer recharged, the amplifier 8 leaves the switching element 9 closed and the flame relay 10 switches off after the holding current falls below the holding value. When the flame is present, only the continuously occurring cycle can keep the flame relay 10 energized. However, so that a supply portion which is directly connected to the mains a.c.

voltage cannot cause the flame relay 10 to switch on when it should not, in the event of a component defect, the supply portion is so designed that it must supply two voltages of given magnitude, which are of opposite polarity, for correct operation of the flame monitor.

It is only when that is the case that the abovedescribed cycle takes place in a trouble-free fashion and, when a flame is present, permits the flame relay to be permanently switched on.

Further details will now be described with reference to Fig. 2.

Connected to a mains a.c. voltage UN, as a supply portion, between a phase conductor Ph and a neutral conductor N in a series circuit are a resistor 23, a capacitor 24, a diode 25 and a first storage capacitor 26. In addition, connected in parallel with the series circuit of the diode 25 and the storage capacitor 26 is a further series circuit of a first Zener diode 27 which is opposite in polarity to the diode 25, and a second storage capacitor 28, the second storage capacitor 28 additionally being shunted by a second Zener diode 29 which limits the charging voltage thereof. The first supply voltage U, is taken off across the first capacitor 26, that is to say, between the neutral conductor N and a conductor 30, while the second supply voltage U2 is taken off across the second storage capacitor 28, that is to say, between a connection 31 and the neutral conductor N.The two voltages U and U2 are of opposite polarity to each other, with respect to the neutral conductor N.

The flame relay circuit 1 with the elements 10 to 1 6 as shown in Fig. 1 is connected between the conductor 30 and the neutral conductor N, with the resistor 1 5 leading to the conductor 30. In that arrangement, the collector-emitter path of a transistor 32 replaces the switching element 9 in Fig. 1.

Connected to the first supply voltage U, is also the series circuit of a charging resistor 33 and an integration capacitor 34, the voltage across the integration capacitor 34 being passed to the base of the transistor 32f by way of a resistor 35 and the collector-emitter path of a further transistor 36. The base of the transistor 36 is also controlled, by way of a resistor 37. by the voltage U, at the conductor 30, and is also connected to the neutral conductor N by the drain-source path of an FET-transistor 38. Without a flame being present. the gate of the FET-transistor 38 is biased more negatively than the potential at the neutral conductor N, for which purpose there is a resistor 39 connected between the gate and the connection 31. In addition, the gate is connected to the neutral conductor N by way of the collector-emitter path of a bipolar transistor 40.

It will be seen from,the foregoing description that the first voltage U, serves both to supply the flame relay circuit 10 to 1 6 and also the amplifier circuit formed by the transistors 32, 36 and 38, and the integration capacitor 34, that is to say, the integrator 3.

The base of the transistor 40 forms the one input 1 9 of the amplifier 8 shown in Fig. 1, while the neutral conductor N forms the other input 20 of the amplifier 8. The voltage which is tapped off at the first capacitor 6 acts between those two inputs.

The capacitor 6 is connected to the mains a.c. voltage UN, in series with a resistor 42 and the capacitor 7, wherein the ionization path is taken off at the junction between the capacitor 7 and the resistor 42 by way of a resistor 43 and is thus connected in parallel with the first capacitor 6. The ionizatization path comprises the flame sensor 5 which extends into a flame 46, and a burner tip or nozzle 47 which is connected to ground or to the neutral conductor N.

The circuit also has a pulse generator capacitor 48 which is connected at one side to the junction between the load resistor 1 5 and the two diodes 1 3 and 1 6. Its second terminal is connected to a tapping 49 of a voltage divider comprising a resistor 50 and a diode 51 which is connected in the forward direction, and is connected between the conductor 30 and the neutral conductor N. The capacitor 48, together with the resistor 50, forms a timing member and the switching means which is controlled by that timing member comprises a transistor 53 whose emitter-collector path is connected between the base of the transistor 40 and the neutral conductor N.

The transistor 53 is controlled by the timing member 48, 50 and can be influenced from the second supply voltage U2. For that purpose, the tapping 49 is connected by way of a resistor 52 to the base of the transistor 53.

The transistor 53 corresponds to the switching means 21 in Fig. 1.

A further transistor 54 is connected with its base by way of a resistor 55 to the connection between the integration capacitor 34 and the two resistors 33 and 35. In addition, the emitter of the transistor 54 is connected by way of a Zener diode 56 to the conductor 30, while the collector thereof is connected by a resistor 57 to the connection 31 and thus to the second supply voltage U2. In addition, between the collector of the transistor 54 and the base of the transistor 53 is a connection by way of a diode 58 which, as viewed from the collector of the transistor 54, is connected in the non-conducting direction.

The above-described flame monitor as shown in Fig. 2 operates in the following manner: After application of the mains a.c. voltage UN. the two capacitors 26 and 28 are charged up by way of their associated diodes 25 and 27 respectively, and supply the two supply voltages U, and U2 which are of opposite polarity to each other with respect to the neutral conductor N, hereinafter referred to as voltage U, and voltage U2.

It is assumed that there is no flame for the time being.

The voltage U2, the magnitude of which is limited by the Zener diode 29, acts on the switching means which is controlled by the pulse generator described hereinafter, that is to say, on the base of the transistor 53 and thus also on the amplifier input 38a which is formed by the gate of the FET-transistor 38.

By virtue of using an FET-transistor, the negative voltage applied by way of the resistor 39 permits the transistor to be switched into a non-conducting condition. A positive voltage is now applied to the base of the transistor 36 which is connected to the output side of the FET-transistor, being applied thereto by way of the resistor 37, and the transistor 36 and consequently also the transistor 32 conduct.

That means that, without the presence of a flame signal, the series circuit in the flame relay circuit, comprising the diodes 1 3 and 16, the windings 11 and 1 2 and the charging capacitor 14, is short-circuited. That means that any flow of current through the flame relay 10 is prevented.

As a result of the transistor 32 being in a conducting condition, the voltage U1 is substantially across the resistor 1 5. and due to that load, the voltage U1 is for the time being at a relatively low value. Because the transistors 32 and 36 are conducting and the resistor 35 is of relatively low resistance, the base of the transistor 54 is almost at the potential of the neutral conductor N and the transistor 55 is conducting. The Zener voltage at the Zener diode 56 is exceeded and the voltage U, therefore produces a further current through 30, 56. 54, 57 and 29, the largest voltage drop being across the resistor 57. The potential at the cathode of the diode 58 thus becomes positive, and that prevents the transistor 53 from being influenced from that circuit.

In addition, the voltage U, across the resistor 50 causes charging of the pulse generator capacitor 48, the charging voltage of which is limited by the diode 51. The voltage drop across the diode 51. that is to say, the voltage at the tapping 49. by way of the resistor 52, holds the base of the transistor 53 so positive that the latter remains in a nonconducting condition.

The circuit remains in the condition as described up till now, until, after ignition of the flame 46, the capacitor 6 is charged up and the control voltage U3 thereof actuates the bipolar transistor 40, by way of the resistor 41. Accordingly, the transistor 40 transmits the flame signal to the amplifier input 38a. At the same time, the base-emitter diode of the transistor 40 limits a further rise in the control voltage U3. That is important because a higher control voltage U3, upon extinction of the flame, would result in an unacceptably long signalling time, while in this situation also there is a self-monitoring effect for any interruption in the base-emitter diode is detected. It would result in the flame relay 10 being deenergized. because the pulse cycle described hereinafter would then no longer occur.

Upon actuation of the transistor 40, the amplifier input 38a, that is to say, the gate of the FET-transistor 38, becomes more positive and the drain-source path thereof goes into the conducting condition, whereby the transistors 36 and 32 are switched off. The loading of the source of U1 by the resistor 35 disappears. The pulse generator capacitor 48 is charged up by way of the resistor 1 5 which is much smaller than the resistor 50, and by way of the diode 51. The integration capacitor 34 is also charged up, by way of the resistor 33. In addition, there is a charging current through the diode 1 3 and the winding 11 to the charging capacitor 14, that flow of current being inadequate to energize the flame relay 10.With increasing charge on the integration capacitor 34, the base of the transistor 54 assumes an increasingly more positive value, more specifically to such an extent that the voltage falls below the Zener voltage across the Zener diode 56, which causes the transistor 54 to switch off. The time for that to occur is set by the integration capacitor 34. The positive voltage at the resistor 57 disappears, due to the transistor 54 being switched off.

The transistor 53 is switched on by way of the diode 58. As it is now in a conducting condition, it forms a short-circuit switch for the base-emitter path of the transistor 40, apply- ing the base of the transistor 40 to the neutral conductor for a period of time which is determined by the pulse generator described hereinafter. Accordingly. the transistors 40 and 38 are non-conducting while the transistors 32 and 36 become conducting. The charging capacitor 14 discharges by way of 12, 1 6 and 32 and the current thereof causes energization of the flame relay 10 while the integration capacitor 34 discharges very rapidly by way of 35, 36 and 32.At the same time as the transistors 32 and 36 switch on, reversal of the charge of the capacitor 48 begins, by its electrode that is on the right in Fig. 2 being put to the potential of the neutral conductor N by way of the transistor 32. Due to the load which is produced as a result of the transistor 32 being switched on, the supply voltage U1 is reduced. Due to a positive feedback effect, that causes the transistor 54 to be switched off, and provides for the switching operation thereof, a desired tripping effect and a sufficiently long period for which it remains in the non-conducting condition.

The phase of charge reversal of the capacitor 48 lasts much longer than the very rapid discharge of the integration capacitor 34, the latter having the result that the positive voltage at the base of the transistor 54 falls quickly to such a degree that it becomes more negative than the voltage at the emitter. That causes the transistor 54 to be switched on again and the voltage U, again takes effect, by way of the resistor 57. The junction between the resistor 57 and the diode 58 is now at positive potential, which however can not influence the transistor 53, due to the blocking action of the diode 58. That is important because the period of time for which the transistor 54 is in a non-conducting condition is shorter than the desired pulse length of the pulse generator is to be.The charge reversal of the pulse generator capacitor 48, which takes place relatively slowly for that purpose, is effected by way of the resistor 50, with the voltage potential at the taping 49 first being negative and changing towards positive values, which is transmitted by way of the resistor 52 to the base of the transistor 53. Therefore, the transistor 53 does in fact become conducting when the transistor 54 becomes non-conducting, but it remains in a conducting condition for a longer period of time than the short blocking pulse of the transistor 54, more specifically until the new charge of the pulse generator capacitor 48 at the tapping 59 has risen again to a potential of about 0.5 volts. The transistor 53 is then switched off again.

During the period for which the transistor 53 was in a conducting condition, the capacitor 6 was discharged by way of the resistor 41 only to an immaterial degree. Therefore, after the transistor 53 is switched off, the magnitude of the control voltage U3 which can be taken off across the capacitor 6 is again the sole controlling factor at the base of the transistor 40, although, as described here inbefore, both the transistor 53 and the control voltage U3 act on the base of the transistor 40. In other words, assuming that the flame 46 is still present, the transistors 40 and 38 become conducting again, while the transistors 36 and 32 are again non-conducting.

The charging current for the charging capacitor 14, which flows through the winding 11, now holds the flame relay in the energized condition. The entire cycle is then repeated afresh.

The flame relay 10 can therefore only remain in the energized condition if the transistor 40 is periodically switched off for a short period of time. Any disturbance in the abovedescribed cycle which takes place in a given period of time permits deenergization of the flame relay 1 0. That therefore also provides for monitoring of the mains portion which supplies the two voltages U, and U2, for any failure of one of the elements thereof results in the above-indicated cycle being disturbed, and therefore results in the flame relay 10 being deenergized.

In contrast to the known art, in the abovedescribed flame monitor the transistor 32 which is connected in parallel with the flame relay circuit is so selected that it is conducting when there is no flame signal. By virtue of that arrangement, the signalling time is no longer determined by the high charging time constant of the charging capacitor 14, but only by the short discharging time constant thereof. Therefore, the source of the supply voltage U, can be selected to be a highresistance source and in consequence thereof can be operated with a relatively flat configuration in respect of the charging curve of the capacitor 14. That makes it possible to increase the cycle repetition time, that is to say, to make it more than one second.That in turn results in an improved relationship of the charging time to the discharging time of the capacitor 6 which is charged by the ionization circuit, that is to say, a smaller amount of energy is taken from that circuit, thus giving a lower loading in respect of the sensor signal and enhanced sensitivity. The signalling time is now primarily determined by the discharging time constant which is defined by the resistor 41 and the capacitor 6.

The above-described flame monitor has a continuous self-monitoring action, for any short-circuit or interruption of any semiconductor or capacitor and any interruption of any resistor breaks down the cycle required for the flame relay circuit, so that the flame relay is no longer energized.

In order to achieve that performance, it is important that triggering of the discharging operation at the charging capacitor 1 4 is derived not from the voltage at that capacitor but from the independent integration capacitor 34. It is also important that the duration of the discharging operation is dependent not on the voltage at the charging capacitor 14 but on the separate timing circuit of the pulse generator capacitor 48 and its resistor 50.

The above-described arrangement is inexpensive to produce and, in spite of the current supply without a transformer, ensures that, in the event of a component defect, there cannot be any unacceptable increase in the operating voltage and a consequential increase in the length of the signalling time.

Claims (9)

1. A self-monitoring flame monitor for monitoring the flame of an oil vaporization or gas burner, the monitor comprising an ionization flame sensor and a parallel circuit, connectable to an a.c. voltage. of the flame sensor with a first capacitor. the charging of which is dependent on the ionization current of the flame sensor, an amplifier which is influenced by a control voltage across the first capacitor and which has an output switching element forming part of a flame relay circuit and being connected in parallel with a series circuit of at least a diode, a component winding of a flame relay and a charging capacitor, while a further series circuit at least of a further diode with a further component winding of the flame relay and the charging capacitor is connected to a supply voltage which charges up the charging capacitor, a pulse generator for periodically opening and closing the output switching element when there is an ionization current, an integrator which is dependent of the condition of charging of the first capacitor but which is influenced by the output condition of the amplifier and which triggers a pulse generator with a pulse duration fixedly predetermined by a timing member, and a switching means which is actuated by the pulse generator during the pulse duration and which short-circuits the amplifier input and thus simulates for the amplifier a "flame out" signal, wherein the flame monitor also includes a supply portion connectable to the a.c. voltage and which provides first and second voltages of two given magnitudes for correct operation of the apparatus.

2. A flame monitor according to claim 1, wherein the first and second voltages of given magnitude are of opposite polarity to each other, the first voltage serves for feeding the flame relay circuit, the amplifier, the pulse generator and the integrator, and the second voltage acts on a switching means influenced by the pulse generator and thus on the amplifier input.

3. A flame monitor according to claim 1 or claim 2, wherein the supply portion comprises a series circuit, connected to the mains a.c. voltage, of a resistor, a second capacitor and a diode, and a first storage capacitor, and connected in parallel with the series circuit of the diode and the first storage capacitor is a further series circuit of a first Zener diode of opposite polarity to the series circuit diode, and a second storage capacitor. and wherein the second'storage capacitor is additionally shunted by a second Zener diode for limiting the changing voltage thereof, the first voltage is provided across the first storage capacitor, and the second storage capacitor.

4. A flame monitor according to claim 3, wherein the parallel circuit of the flame sensor and its ionization path with the first capacitor which provides the control voltage, is connectable to the mains a.c. voltage by way of at least one resistor in series with a further capacitor.

5. A flame monitor according to any one of the preceding claims, wherein the output switching element which is connected in parallel with the series circuit in the flame relay circuit that comprises the diodes, the windings and the charging capacitor, is a transistor which is in a conducting condition when there is no flame signal.

6. A flame monitor according to claim 5, wherein a bipolar transistor is provided for transmitting the flame signal to the amplifier input, the collector-emitter path of the bipolar transistor being connected between the amplifier input and a neutral conductor of the mains a.c. voltage, and the base of the bipolar transistor being acted upon both by the switching means which is actuable by the pulse generator, and also the control voltage which is provided across the first capacitor, the base-emitter path of the bipolar transistor serving at the same time to limit the control voltage.

7. A flame monitor according to claim 6, wherein the switching means is a further transistor whose emitter-collector path is connected between the base of the bipolar transistor and the neutral conductor and whose base, actuated by the timing member, can be influenced by the second voltage.

8. A flame monitor according to claim 7, wherein the input of the amplifier is the gate of an FET-transistor which can be controlled by the second voltage.

9. A self-monitoring flame monitor for monitoring the flame of an oil vaporization or gas burner, substantially as hereinbefore described, with reference to the accompanying drawings.