GB2089975A - Flame Monitors - Google Patents

Abstract

A fail-safe flame monitor includes a flame relay circuit (15) having an amplifier (17) that holds a flame relay (18) in a position thereof indicating a flame only when an alternating signal in a given frequency range, which signal is influenced by a UV-cell flame sensor (1), is applied to an input (14). A shutter means (2, 3), which oscillates synchronously with the mains frequency, interrupts the radiation to the UV-cell (1) always during the same half-cycle of the a.c. supply (11). The amplifier (17) actuates a pole changing switch (13) by way of a control output (8) at a rate in the given frequency range, the pole changing switch changing the synchronisation between the shutter position and the pulses of a half-wave rectified d.c. voltage (UF) applied to the UV-cell circuit. In a light phase, the pulses (UF) appear if the shutter (3) is open, and in a dark phase, when it is closed. During the light phase, current pulses occur at the input (14) if a flame is present, while in the dark phase no such current pulses will occur at the input (14) unless the UV- cell is defective. The alternating signal is produced by the successive light and dark phases. When pulses are produced in the dark period as well as in the light period, the signal frequency changes to a value outside the given frequency range and the monitor will not indicate the presence of a flame. The pole changing switch (13) may comprise thyristors (Figures 4 and 5, not shown). <IMAGE>

Description

SPECIFICATION Flame Monitors This invention relates to flame monitors.

Flame monitors are required for setting oil or gas burners in operation in a fully automatic manner, and for contiuously monitoring such burners, the purpose of the flame monitor being to indicate the presence of a flame at any time.

The flame monitors, together with their flame sensors, must be fail-safe, that is to say any possible defect in a component must be indicated by the signai 'flame extinguished'. This is required in particular when using flame sensors which are responsive to uitra-violet radiation (referred to hereinafter as UV-cells), the advantage of which, over other flame sensors, is that they respond only to UV-light emitted by burning flames and not to radiation from glowing pieces of fire clay or visible light. In this connection, it is of course necessary to accept the disadvantage that, due to ageing phenomena, UV-cells tend in time also to respond without a flame being present, and therefore to simulate such a flame.Therefore, UVcells must be periodically checked for satisfactory functionability thereof, and this has to be carried out during normal operation when using burners which are constantly in operation.

For this purpose, Austrian Patent Specification No. 307 605 discloses a flame monitoring device in which, in a beam path a a UV-cell, there is installed a movable shutter member which, when an a.c. supply voltage is always of the same polarity, interrupts the beam path synchronously therewith. A control device connected to this device then only needs to distinguish between direct current on the one hand and no voltage or alternating current on the other hand for the purposes of flame detection and testing of the UV-cell. In this apparatus, opposite directions of current flow are used for operation and for testing of the UV-celI.

It has been found in practice, however, that the performance of a UV-cell is not the same in both directions of current flow so that defective sparking through the device can occur in the direction of current flow in the operating condition, long before that also occurs in the opposite direction of current flow which is used for testing purposes. Reliable defect detection is therefore not possible.

German Patent Application No. DE 2 809 993 A discloses the series connection of a flame sensor with a capacitor, wherein a trigger circuit which detects the voltage across the capacitor forms an input of an amplifier. However, the amplifier does not have the required fail-safe character.

According to the present invention there is provided a flame monitor comprising a flame relay circuit which includes a flame relay and which responds only to an alternating signal, a UV-cell flame sensor which acts on an amplifier, a shutter arrangement operative to interrupt radiation to the flame sensor at the frequency of an a.c.

voltage and in half-cycles of the a.c. voltage which are always of the same polarity, at least one switch arranged to be actuated by a control output of the flame relay circuit and operative to alter the synchronisation between a voltage feeding the shutter arrangement and a pulsating d.c. voltage feeding a UV-cell circuit in such a way that, in a first switching condition, voltage is applied to the UV-ceil when the shutter arrangement is closed (dark phase) and, in a second switching condition, voltage is applied to the UV-cell when the shutter arrangement is open (light phase), the arrangement being such that signals which are applied to an input of the flame relay circuit during the light and dark phases produce in the amplifier the alternating signal which can hold the flame relay in an energized state.

In a fail-safe flame monitor embodying the invention and described below a periodically checked UV-cell is tested under the same electrical conditions as in an operative condition thereof, in which it must be capable of detecting a flame.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a flame monitor embodying the invention; Figures 2 and 3 are two operating waveform diagrams for the circuit shown in Figure 1; Figure 4 is a circuit diagram of a second embodiment; and Figure 5 is a circuit diagram of a further embodiment.

Figure 1 shows a UV-cell 1 which is exposed to ultra-violet radiation UVfrom a burner flame (not shown). Disposed between the flame and the UVcell 1 is a fixed shutter member 2, which is in this case in the form of an apertured shutter member, and a movable shutter member 3 which can cover an opening 4 in the xixed shutter member 2. The movable shutter member 3 comprises a springly sheet metal member 5 which is fixedly clamped at one end 6 and which carries a pole plate 7. The pole plate 7 is disposed opposite a fixed yoke 9 of a solenoid 10. A winding 11 of the solenoid is connected to an a.c. voltage U by way of a diode 12.The same a.c. voltage U is supplied, by way of a pole changing switch 13, to a series circuit comprising an input 14 of a flame relay circuit 15, the UV-cell 1 and a diode 1 6. The pole changing switch 3 is actuated by a control output 8 of the flame relay circuit 15 and, in the embodiment shown in Figure 1, comprises two individual change-over switches. In its two possible positions, the switch changes the synchronisation between a pulsating half-wave rectified d.c.

voltage U5 fed to the shutter arrangement 2, 3 and a halfwave rectified d.c. voltage UF fed to the UV-cell circuit. Both the d.c. voltages U5 and UF pulsate at the same frequency because they are produced by rectification of the same a.c. voltage U by the diodes 12 and 16, respectively. In a first position of the pole changing switch 13, namely the position shown in Figure 1, the pulse maxima or peaks of the voltages U6 and UF occur at the same time because both the diodes 12 and 16 are of the same polarity with respect to the a.c.

voltage U. In this switching condition, voltage is applied t the UV-cell 1 whenever the shutter arrangement 2, 3 is covered. No UV-radiation reaches the UV-cell 1 and the applied voltage thus cannot produce a sensor current. This condition is referred to hereinafter as 'the dark phase'.

In its second position, the pole changing switch 1 3 has the effect that the peaks or pulse maxima of the two d.c. voltage U6 and UF are uniformly shifted in time relative to each other, since in that condition the diodes 12 and 16 are of opposite polarities with respect to the a.c. voltage U. In this case, voltage is applied to the UV-cell 1 whenever the shutter arrangement 2, 3 is open and, when UV-radiation is received, the UV-cell 1 permits a flow of current through the input 14. This condition is referred to hereinafter as 'the light phase'. The light phase and the dark phase can each extend over a number of cycles or periods of the a.c. voltage U. The signals which are applied alternately to the input 14 during the light and the dark phases form an alternating signal at a given frequency in an amplifier 17.The signal can hold a flame relay 1 8 in an energized condition when the UV-cell 1 is functioning correctly and when UV-radiation is present. In addition, the amplifier 1 7 actuates the pole changing switch 1 3 at the cycle rate or frequency of the alternating signal, by way of the control output 8, while the flame relay 1 8 indicates the presence of a flame by means of its contacts (not shown). For that purpose, the flame relay 18 is so arranged in the circuit that it can be held in its condition of indicating a flame only by an alternating signal in a given frequency range. Any other condition prevents a flame from being indicated.The frequency of the alternating signal is determined by the performance of the UV-cell 1 and the amplifier 1 7 during the light and dark phases. In the present example, that frequency is so selected that the light and dark phases each always extend over a plurality of periods of the a.c. voltage U.

The frequency of the alternating signal is limited in an upward direction only by the frequency of the a.c. voltage U.

the co-operation between the shutter arrangement 2, 3 and the pole changing switch 1 3 will now be described in greater detail with reference to Figures 2 and 3 of which Figure 2 shows waveforms present in the light phase and Figure 3 shows waveforms present in the dark phase. Both figures show four curves 19, 20, 21, 22 and 23, 24 respectively, the common absicissa of which is time and in regard to which the curves 19 and 20 are the same in both Figures 2 and 3. The curve 1 9 indicates the nature of the pulsating half-wave rectified d.c.

voltage U9 at the solenoid 10 while the curve 20 denotes the condition of the shutter member 2 which is open at 0 and closed at C by the movable shutter member 3. The curve 21 or 23 shows the nature of the d.c. voltage UF through the flame sensor circuit while the curve 22 or 24 shows the voltage across the input 14. It will be seen from the curve 21 that, in the light phase (Figure 2), the voltage UF reaches a maximum whenever the shutter member is open, as shown by the curve 20. When UV-radiation from a flame is present, through-striking phenomenon which are produced by the UV-cell 1 therefore occur, which appear at the input 14 as shown by the curve 22, and control the amplifier 17. In comparison, in the dark phase (see Figure 3), the voltage maxima across the UV-cell 1 always coincide with the closed position of the shutter member 2 (see curve 23).Therefore, no throughstriking phenomena occur and no voltage is applied to the input 14 (see curve 24).

In trouble-free operation, d.c. voltage pulses occur at the input 14 during the light phase, as shown by the curve 22, and no voltage may occur during the dark phase. The continuous changing between these two conditions produces the alternating signal of the required frequency at the amplifier 17. If now, as a result of ageing phenomena, through-striking phenomena also occur at the UV-cell in the dark phase, even though there is no radaition, then the frequency of tlie alternting signal is interfered with and the flame relay 18 is released. The same also applied in regard to any other fault, such as for example the failure of the diode 12 or the diode 16.

The pole changing switch 1 3 is advantageously formed from semiconductor components, as shown in the examples of Figure 4 and 5. Reference numerals used in Figures 4 and 5 that are the same as reference numerals used in Figure 1 are used to denote the same components as in Figure 1.

In both the embodiments of Figures 4 and 5, a first winding 25 of a transformer 26 applies the required a.c. voltage U for operating the UV-cell 1 while the voltage across a second winding 27 supplies a shutter drive means 28 (shown only in diagrammatic form) and electronic circuits described hereinafter.

Connected to the first winding 25, to provide protection from over-voltage, is the series circuit of a resistor 29 and a voltage-dependent resistor (VDR) 30, the voltage which is tapped off across the VDR 30 corresponding to the voltage U at the pole changing switch 13 shown in Figure 1. In the embodiments shown in Figures 4 and 5 the switch 1 3 comprises a Graetz rectifier circuit (32, 33, 37, 38) which is supplied by the a.c. volage U and which, between a d.c. voltage output 31 and a d.c. voltage input, supplies the d.c. voltage UF, wherein two output diodes of the rectifier circuit, on the d.c. voltage side, each comprise a respective thyristor 32 and 33, the control or gate terminals 34 and 35 of which are influenced by a control output 36 of the flame relay circuit 1 5 (Figure 4) or 57 (Figure 5). In addition, the rectifier circuit comprises two series-connected diodes 37 and 38 which are connected in parallel with the series-connected thyristors 32 and 33. A connection 39 between the diodes 37 and 38 forms the d.c. voltage input of the rectifier circuit.

This is emphasised in Figures 4 and 5 by thicker lines.

A further d.c. voltage source comprises a capacitor 40 which is connected to the winding 27 by way of a diode 41. This source serves to supply the flame relay circuit 15 or 57, although this is not shown in Figure 4, and also, in the embodiment shown in Figure 4, to influence the control or gate terminal 34 which is connected to the d.c. voltage source by way of two resistors 42 and 43 and which switches the thyristor 32 into a conducting condition.

The circuit shown in Figure 4 will now be described in greater detail. The connection 39 is connected to one terminal of the UV-ceil 1, the second terminal of which is taken to one terminal of the input 14 of the flame relay circuit 15, while the second terminal of the input 14 is connected to the d.c. voltage output 31 and to a collector line 44. The control or gate terminal 35 is connected to the collector line 44 by way of a resistor 45 and is connected to the control output 36 by way of a resistor 46. This coupling arrangement is so designed that the thyristor 33 is constantly non-conducting without voltage at the control output 36, which corresponds to the light phase. During the light phase, the voltage UF must again be applied to the UV-cell circuit when the shutter member 3 is open.This is the case when a positive potential of the a.c. voltage U is applied to one terminal 47 of the winding 25, which is referred to hereinafter as the positive half-cycle or haif-period. Because the thyristor 32 is initially continuously in a conducting condition, a circuit is formed from the terminal 47 to the conducting thyristor 32 by way of the collector line 44 and the input 14 to the UV-cell 1 and from there through the diode 38 and the resistor 29 back to the winding 25. This flow of current, the magnitude of which is influenced by the radiation impinging on the UV-cell 1, occurs in any of the following positive half-cycles, as long as there is no voltage applied at the control output 36. In the negative half-cycles, the UV-cell 1 has no voltage applied thereto, this being ensured by the thyristor 33 and the diode 38.

Being influenced by the performance of the UV-celi 1, the flame relay circuit 1 5 switches over into the dark phase after a given period of time.

When this happens, similarly to actuation of the pole changing switch 13 in the embodiment shown in Figure 1, the control output 36 changes its electrical condition at the frequency or cycle rate of the alternating signal produced by the amplifier 17. A voltage which is positive with respect to the collector line 44 is produced at the control output 36 and switches the thyristor 33 into a conducting condition by way of the resistor 46. Because of this, the voltage UF appears across the UV-cell circuit during the negative half-cycle during which the shutter member 3 is closed, more particularly, on the one hand from the winding 25 by way of the resistor 29, the thyristor 33 and the collector line 44, and on the other hand from the connection 39 by way of the diode 37 to the terminal 47. The voltage UF cannot trigger any UV-cell current in the described condition.However, even during the positive halfcycles when the shutter member 3 is open, no UV-cell current may flow during the dark phase, and for that reason the thyristor 32 must then be in a non-conducting condition. This is achieved by a blocking circuit which nullifies the effect of the d.c. voltage at the control terminal 34 of the thyristor 32, as will now be described.

A voltage divider comprising two resistor 48 and 49 is provided between the connection 39 and the collector line 44. The voltage across the resistor 48 serves to charge a capacitor 50. For that purpose, starting from the collector line 44 and parallel to the resistor 48, there is a series circuit comprising a diode 51, the capacitor 50, a Zener diode 52 and a further resistor 53. In this arrangement, the connection between the Zener diode 52 and the resistor 53 is connected to the base of a first transistor 54, the collector of which is connected to the collector line 44 and the emitter of which is connected by way of a resistor 55 to the junction between the capacitor 50 and the Zener diode 52.The base-emitter path of a further transistor 56 is connected in parallel with and of opposite polarity with respect to the diode 51, while the collector of the transistor 56 is connected to the junction between the resistors 42 and 43.

The blocking circuit operates in the following manner: At the same time as the voltage UF appears across the UV-cell circuit, a current flows to the connection 39 from the collector line 44 by way of the resistors 48 and 49. This occurs during the light phase in each positive half-cycle and during the dark phase in each negative half-cycle. The voltage drop across the resistor 48 causes a flow of current through the diode 51, the capacitor 50, the Zener diode 52 and the resistor 53, which charges up the capacitor 50. The two transistors 54 and 56 are non-conducting during the charging process. During each subsequent halfcycle, the voltage disappears across the resistor 48, the transistor 54 goes into a conducting condition and the capacitor 50 discharges by way of the base-emitter path of the transistor 56, the collector-emitter path of the transistor 54 and the resistor 55.This causes the transistor 56 to conduct and it connects the junction of the two transistors 42 and 43 to the potential of the line 44. It thereby reduces the voltage at the control or gate terminal 34 of the thyristor 32 to a value which renders the thyristor 32 non-conducting during the light phase in each negative half-cycle, although this is irrelevant, and during the dark phase, in each positive half-cycle. As a result of this, the UV-cell circuit receives no voltage within the dark phase during the positive half-cycles, and no current flows at the input 14. When the flame relay circuit 1 5 again becomes without voltage at its control output 36, the light phase occurs again and the cycle begins from the beginning.

In Figure 5, reference numerals which are the same as those used in Figures 1 and 4 are used to denote the same components as in Figures 1 and 4. The embodiment of Figure 5 includes the flame relay circuit 57, for the operation of which the UVcell 1 is connected in series with a capacitor 58, and in which a trigger circuit 59 for detecting the voltage across the capacitor 58 serves as an amplifier. The signal from the trigger circuit 59, which controls the flame relay 18, is applied at the same time to the control output 36.

As in the embodiment shown in Figure 4, the flame relay circuit 57 is supplied by the voltage across the capacitor 40, for which purpose there are two conductors 60 and 61 which are shown in Figure 5, but are omitted from Figure 4. The conductor 61 is connected to the d.c. voltage output 31, that is to say, to the line 44, and to the capacitor 58 which is connected in series with the UV-cell 1. A further input 62 to the flame relay circuit 57 is connected to the junction between the capacitor 58 and he UV-cell 1. Together with the line 61, the input 62 of the trigger circuit 59 serves to detect the voltage across the resistor 58.

The two control or gate terminals 34 and 35 of the two thyristors 32 and 33 are each connected by a respective resistor 63 and 64 to the d.c.

voltage output 31. The winding 27, the diode 41 and the capacitor 40 serve as a d.c. voltage source, the voltage of which biases the two control terminals 34 and 35 with respect to the d.c. voltage output 31. For that purpose, the control terminal 34 is connected to the line 60 by way of two resistors 65 and 66 while the control terminal 35 is connected by way of the emittercollector path of a transistor 67 to a tapping between the two resistors 65 and 66. The base of the transistor 67 is connected by a diode 68 to the line 44, in such a way that it is non conducting in respect of the d.c. voltage.A further resistor 69 connects the base of the transistor 67 to the line 60 and permits the passage of a basecurrent which initially renders the transistor 67 conducting, which has the effect that the thyristor 33 is also conducting, while under those conditions the thyristor 32 is in a non-conducting condition. That condition can be altered by a timing element or arrangement which is controlled by the control output 36 and which will now be described.

The aforesaid timing element or arrangement includes the transistor 67, the base of which is additionally connected by way of a Zener diode 70 and a resistor 71 to the voltage across a capacitor 72. Together with a further resistor 73, the capacitor 72 forms an RC-arrangement which is supplied by the pulsating d.c. voltage UF across the UV-cell circuit 1, 58. For that purpose, the resistor 73 is connected to the connector 39 and the capacitor 72 is connected to the line 44. The tapping of the resistor 71 on the RC-arrangement occurs at the junction between the capacitor 72 and the resistor 73. The capacitor 72 can also be short-circuited by way of the current path of a further transistor 74, by the series circuit of the emitter-collector path of the transistor 74 with a resistor 75 being connected in parallel with the resistor 72.The control output 39 acts on the transitor 74 and is connected to the base thereof by way of a resistor 76, while a diode 77 is connected in parallel with and in opposition to the emitter-base path.

The circuit shown in Figure 5 operates in the following manner: As soon as a mains a.c. voltage UN is present on the transformer 26, a d.c. voltage occurs on the lines 60 and 61. By way of the resistor 69, this has the effect that the transistor 67 and thus also the thyristor 33 conduct, while the control signal at the thyristor 32 remains so small that the thyristor is in a non-conducting condition during the negative half-cycles of the a.c. voltage U. There is no voltage at the control output 36, with respect to the line 44, and the transistor 74 is a non-conducting condition. The arrangement is in the dark phase. In this condition, a current flows from the winding 25 through the components 29, 33, 31,44, 72, 73, 39, 37 and 47 in each negative half-cycle.That current charges the capacitor 72 negatively, until the Zener diode 70 switches into a conducting condition. When this occurs, the transistor 67 goes into a non-conducting condition as its base becomes negative. The transistor 77 controls the magnitude of the d.c. voltage at the control terminals 34 and 35 of the thyristors 32 and 33 in such a way that the thyristor 32 conducts during the positive half-cycles, and the thyristor 33 is then non-conducting. This produces the light phase.

In contrast to the example shown in Figures 1 and 4, the constant duration of the dark phase is determined by the timing element or arrangement constituted by the components 72, 73, 71, 70 and 67 and not simply by the performance of the flame relay circuit 57.

During the light phase, the current flowing through the UV-cell 1 in the positive half-cycles charges the capacitor 58 until the trigger circuit 59 responds. If this is not the case due to the absence of UV-radiation or because of a defect, then the trigger circuit 59 remains in its previous condition and the flame relay 18 cannot be actuated or is released. A negative voltage pulse occurs at the control output 36, in the correct mode of operation, and terminate the light phase, as described hereinafter. The duration of the light phase is thus determined by the UV-cell current.

The negative voltage at'the control output 36 renders the transistor 74 conducting until the capacitor 72 can be entirely discharged. The Zener diode 70 is in a non-conducting condition and the transistor 67 is in a conducting condition again. The result of this is that the thyristor 33 conducts again and the thyristor 32 is nonconducting again during the negative half-cycles, so that the dark phase is initiated again. If, due to some defect, the dark phase is not terminated again by the timing element or arrangement constituted by the components 72, 73, 71, 70 and 67, then the trigger circuit 59 retains its condition and the flame relay 1 8 is released.

The timing element or arrangement, which can be produced at low cost and which has an additional effect in comparison with the embodiment shown in Figure 4, provides an additional improvement in the fail-safe character of the arrangement, and that can be achieved virtually without an increase in cost.

The circuits shown in Figures 4 and 5 permit inertia-free electrical change-over switching between the light and dark phases, that is to say, between a measuring phase in respect of UVradiation and at test phase in respect of the UVcell, and the time succession in respect thereof can be increased to very close to the mains frequency. This therefore provides very short response times both when a flame goes out and also when an apparatus defect occurs, during operation of the arrangement. The latter is possible because each component of the circuit is involved in properly maintaining the light/dark cycle and any trouble in that cycle causes the flame relay to be released. The circuits are therefore also always tested under permanent burner operating conditions, and are therefore fail-safe.

Claims (9)

Claims

1. A flame monitor comprising a flame relay circuit which includes a flame relay and which responds only to an alternating signal, a UV-cell flame sensor which acts on an amplifier, a shutter arrangement operative to interrupt radiation to the flame sensor at the frequency of an a.c. voltage and in half-cycles of the a.c. voltage which are always of the same polarity, at least one switch arranged to be actuated by a control output of the flame relay circuit and operative to alter the synchronisation between a voltage feeding the shutter arrangement and a pulsating d.c, voltage feeding a UV-cell circuit in such a way that, in a first switching condition, voltage is applied to the UV-cell when the shutter arrangement is closed (dark phase) and, in a second switching condition, voltage is applied to the UV-cell when the shutter arrangement is open (light phase), the arrangement being such that signals which are applied to an input of the flame relaty circuit during the light and dark phases produce in the amplifier the alternating signal which can hold the flame relay in an energized state.

2. A flame monitor according to claim 1, wherein said switch is formed from semiconductor components.

3. A flame monitor according to claim 2, wherein said switch is formed by a Graetz rectifier circuit which is connected to be fed by said a.c.

voltage and which, between a d.c. voltage output and a d.c. voltage input, supplies said d.c. voltage for the UV-cell circuit, and wherein two output diodes of the rectifier circuit, on the d.c. voltage side, each comprise a thyristor, control terminals of which are influenced by the control output of the flame relay circuit, the flame monitor further comprising a d.c. voltage source for feeding the flame relay circuit and for influencing the control terminal at least of one said thyristor.

4. A flame monitor according to claim 3, wherein a control terminal of one of the thyristors is connected to the control output of the flame relay circuit, wherein the flame relay circuit is operative to alter the electrical condition of its control output the frequency of the alternating signal produced by the amplifier while said d.c.

voltage source acts by way of at least one resistor at the control terminal of the other of the thyristors and renders such other thyristor conducting, the flame monitor comprising a blocking circuit operative to nullify the effect of the d.c. voltage at the other thyristor and to render the other thyristor non-conducting during the dark phase at least in the positive half-cycle of the a.c. voltage in which positive potential is applied to one terminal of the a.c. voltage.

5. A flame monitor according to claim 4, wherein the blocking circuit in respect of the thyristor comprises a capacitor and two transistors, the arrangement being such that the capacitor is charged up in each case by the voltage across the UV-cell circuit while in the subsequent half-cycle when there is no voltage one of the transistors closes a discharging circuit of the capacitor and the other of the transistors renders the thyristor non-conducting.

6. A flame monitor according to claim 3, wherein the UV-cell flame sensor is connected in series with a capacitor and a trigger circuit operative to detect the voltage across the capacitor serves as the amplifier, the two control terminals of the thyristors are each connected by respective resistors to the d.c. voltage output of the Graetz rectifier circuit, the voltage of said cc.c.

voltage source is operative to bias the two control terminals with respect to the d,c. voltage output, and said bias is variable by a timing element which is influenced by the control output of the flame relay circuit.

7. A flame monitor according to claim 6, wherein the timing element includes a transistor operative to influence the magnitude of the d.c.

voltage acting at the control terminals of the thyristors, the base of such transistor is connected by way of a Zener diode to the voltage across the capacitor of an RC-arrangement, and such capacitor can be short-circuited by way of the current path of a further transistor, the base of such further transistor being connected to the control output of the flame relay circuit.

9. A flame monitor substantially as herein described with reference to Figures 1 to 3, Figure 4 or Figure 5 of the accompanying drawings.