Combustion Control Circuit For A Fuel Burner

Abstract

A combustion control circuit for an oil burner installation includes a triac for controlling the energization of both a burner motor and an ignition circuit. A control circuit carries out a safe start component check and then operates the triac when a thermostat closes and when no flame is detected at the burner. If ignition fails to take place in a trial-for-ignition period of predetermined time, a thermal switch is heated by a heater in the control circuit and opens to shut off the burner motor. However, if ignition takes place, flame is detected within the predetermined period by an improved, positively acting, reliable resistance bridge flame detector circuit. The flame detector circuit serves to render nonconductive a silicon controlled rectifier in the control circuit in order to disconnect the thermal switch heater while maintaining operation of the burner motor. Simultaneously, the ignition circuit is deenergized by a novel ignition disabling circuit connected between the silicon controlled rectifier and the ignition circuit. Failure of the triac in a short circuit condition prevents an unsafe condition due to connection of the thermal switch to the motor and due to the use of a second heater in circuit with the ignition circuit.

Description

The present invention relates to an improved combustion control circuit for reliably and safely controlling the operation of a fuel burner.

A conventional fuel oil burner heating installation of a type widely used in the past includes an ignition device such as a transformer or other ignitor for igniting fuel at the burner, together with a primary control using two or more relays for operating the burner in response to a thermostat. Upon closing of the thermostat in normal operation, the burner motor and the ignition device are energized during a trial-for-ignition period. If burner flame is detected, the burner motor is continued in operation and, if the installation is of the intermittent ignition type, the ignition device is deenergized. On the other hand, if flame is not detected during the initial period, a thermal delay safety switch shuts down the installation.

The complexity and expense of conventional control systems has made it desirable to provide a safe, economical and reliable combustion control circuit making use of inexpensive solid state components in place of expensive relays, large transformer windings, and the like. Devices of this general class have been proposed, and one example may be found in U. S. Pat. No. 3,624,407 -- Bauer. However, solid state devices developed to date have suffered from several disadvantages such as lack of complete safety in operation, unreliability under some circumstances, the need for expensive components, and excessive cost and/or complexity.

Objects of the present invention are to provide an improved combustion control circuit for controlling the operation of a fuel burner installation including a burner motor and an ignition circuit; to provide an improved combustion control circuit having desirable safety features including a novel arrangement for protecting against component failure; to provide an improved combustion control circuit in which the necessity for expensive components is reduced; to provide an improved, highly reliable flame detection circuit for use in a combustion control circuit; and to provide a combustion control circuit having an improved arrangement for discontinuing the operation of the ignition circuit when burner ignition is accomplished.

In brief, the above and other objects and advantages of the invention are realized by the provision of an improved combustion control circuit for an oil burner installation of the type including a burner motor and an ignition circuit. A triac switching device is connected for simultaneously energizing both the burner motor and the ignition circuit. A thermostat is located in the area to be heated by the oil burner installation, and a light sensitive variable resistance flame detector is disposed adjacent the fuel burner. A control circuit operates to place the triac in a conductive condition to energize both the burner motor and the ignition device when the thermostat closes upon demand for heat. When ignition is accomplished, the control circuit operates in response to the flame detector for discontinuing operation of the ignition circuit. On the other hand, if ignition is not accomplished during a predetermined period of time, a thermal delay switch connected to the burner motor prevents further operation of the burner motor upon being heated to a predetermined extent. A first heater in the control circuit operates from the time that the thermostat closes until such time as a flame is detected. A second heater for the thermal switch protects against failure of the triac and heats the thermal switch at all times that the ignition circuit is in operation.

In accordance with another feature of the invention, there is provided a novel flame detection circuit comprising a resistance bridge provided with a regulated operating potential and including a light sensitive variable resistance flame detector in one leg thereof. In the absence of flame the resistance bridge is maintained in an unbalanced condition. Upon the detection of flame, the resistance bridge is moved through the null point to an oppositely unbalanced condition, and the control circuit is operated positively and effectively.

An aspect of the invention resides in a novel circuit for disabling the ignition circuit in response to the detection of flame. In one embodiment, a transformer coupled between a switching device in the control circuit and a controlled conduction device in the ignition circuit enables or prevents operation of the ignition circuit in response to presence or absence of flame. In other embodiments, a relay and a light and light sensitive device are used.

The invention and its objects and advantages may be best understood from consideration of the following detailed description of embodiments of the invention shown in the accompanying drawing, wherein:

FIG. 1 is a schematic and diagrammatic illustration of a combustion control circuit embodying the principles of the present invention; and

FIGS. 2 and 3 are fragmentary schematic illustrations of alternative embodiments of the invention.

With reference now to FIG. 1 of the drawing, there is illustrated a combustion control circuit designated generally by the reference numeral 10 and constructed in accordance with the principles of the present invention. The circuit 10 controls the operation of a typical oil burner installation including a burner 12, a burner motor 14 and an ignition circuit generally designated by the reference numeral 16. In normal operation, the burner motor 14 and the ignition circuit 16 are operated upon a demand for heat by a burner circuit generally designated as 18 under the control of a control circuit generally designated as 20. Demand for heat is signalled in conventional manner by a line voltage thermostat switch 22 or by a low voltage thermostat switch 24 located to sense temperature in a region heated by operation of the burner 12. In any given installation only one of the switches 22 or 24 is used, the other being replaced by a permanent closed circuit.

Upon closing of the controlling thermostat switch 22 or 24, fuel at the burner 12 is ignited by the ignition circuit 16 and flame is detected by a flame detection circuit generally designated as 26. Upon detection of flame, the control circuit 20 serves to continue operation of the burner motor 14 while an ignition disabling circuit generally designated as 28 discontinues further operation of the ignition circuit 16. Should ignition fail to take place within a predetermined time, a thermal delay device designated as a whole by the reference numeral 30 discontinues operation of the fuel burner installation. In accordance with a feature of the invention, the device 30 also discontinues operation of the installation in the event of certain undesirable malfunctions of components in the circuit 10.

Referring now in more detail to the construction and operation of the combustion control circuit 10, the circuit includes a pair of power supply terminals 32 and 34 adapted to be connected to a standard nominal 115 volt, 60 hertz power supply. The control circuit 20 and the flame detection circuit 26 are supplied with operating potential by means of a pair of secondary windings 36 and 38 of a transformer 40 having a primary winding 42 adapted to be coupled to the power supply terminals 32 and 34. In addition to the line voltage thermostat switch 22 (when used), energization of the burner circuit 18 and the control circuit 20 can be interrupted by a conventional over-temperature limit switch and fan switch 44 and by operation of the thermal switch structure 30.

In operation of the circuit 10, control of the energization of the burner circuit 18 is provided by a gated, bidirectional solid state controlled conduction device 46 in the form of a triac. The output electrodes of triac 46 are in current supplying relation with both the burner motor 14 and with the ignition circuit 16. Thus triac 46 must be rendered conductive by coupling of its gate electrode to one of its output electrodes in order to bring about operation of the burner circuit 18.

In FIG. 1 the combustion control circuit 10 is illustrated in its normal, temperature satisfied condition wherein the thermostat switch 22 or 24 is open and wherein the triac 46 is in its high resistance or open circuit condition so that neither the burner motor 14 nor the ignition circuit 16 is energized. When a demand for heat is sensed by closing of the controlling thermostat switch 22 or 24, a heating cycle is initiated under the control of the control circuit 20. Upon initiation of this heating cycle, a safe start component check takes place, and assuming the circuit components to be operating properly, the control circuit 20 serves to render the triac 46 conductive thereby simultaneously energizing the burner motor 14 and the ignition circuit 16 throughout a trial-for-ignition period having a duration determined by the operation of the thermal device 30.

In order to operate the triac 46, control circuit 20 includes a pair of relay windings 48 and 50, which windings control a normally open set of relay contacts 52 connected in series with a resistor 54 and with the gate electrode of the triac 46. Windings 48 and 50 are arranged in known manner relative to contacts 52 through adjustment of variable resistances 49 and 51 respectively so that energization of both windings is necessary to move or pull in the set of contacts from the open to the closed position, while energization of only the winding 50 suffices to hold the contacts in the closed position. Energization of the windings 48 and 50 is controlled by operation of switch 22 or 24, while energization of winding 48 is further controlled by a gated solid state controlled conduction device 56 in the form of an SCR having its gate electrode controlled by the flame detection circuit 26.

With reference now to the flame detection circuit 26, the function of this circuit is to control the condition of the SCR 56 in accordance with the presence or absence of flame at the burner 12. In accordance with important features of the invention, the circuit 26 carries out a stable and reliable control operation despite such unfavorable factors as supply voltage fluctuations, intermittent or unstable ignition, intermittent extraneous light, or the like.

More specifically, circuit 26 includes a zener diode 58 serving together with a resistor 60 to supply a voltage regulated driving signal to a resistance bridge 62. Due to the inclusion of diode 58, the bridge 62 operates substantially independently of supply voltage variations. Bridge 62 includes a pair of input terminals 64 and 66 connected across the zener diode 58 and a pair of output terminals 68 and 70 connected to the gate and cathode electrodes of the SCR 56 and shunted by a capacitor 72.

One leg of bridge 62 includes a variable resistance photoelectric device 74 disposed adjacent the burner 12. In the absence of flame the device 74 is in a high resistance condition, and when illuminated by the presence of flame the device 74 assumes a low resistance condition. The remaining three legs of bridge 62 include resistances 76, 78 and 80 having values such that the bridge 62 is unbalanced in opposite directions when the device 74 is alternatively in its high and low resistance conditions.

The novel flame detection circuit 26 has important advantages in operation because it reliably and positively controls the condition of the SCR 56. When no flame is present at burner 12, device 74 is in its high resistance condition and as a result the bridge 62 is unbalanced and bridge terminal 70 is maintained at a positive potential relative to bridge terminal 68. This potential appears across capacitor 72 in such a way as to maintain the gate electrode of the SCR 56 positive relative to its cathode. Accordingly, the SCR is maintained in condition for conduction of current through its output circuit. When flame is detected the resistance of device 74 drops to a low level and the bridge 62 moves through its null point or balance condition to an oppositely unbalanced condition wherein terminal 70 is negative relative to terminal 68, thus biasing the gate electrode of the SCR 56 negative relative to its cathode to assure that SCR 56 ceases conduction and remains nonconducting.

Capacitor 72 provides a smoothing or filtering effect which prevents unwanted response of the flame detection circuit to various possible transient effects. Moreover, since the incident light level at device 74 must change substantially enough to change the condition of the bridge from one unbalanced condition to the oppositely unbalanced condition, positive and reliable operation is achieved. Furthermore, stability of operation is aided by the use of the zener diode 58 which provides a regulated driving voltage for the bridge 62 substantially independent of variations in the supply voltage.

As indicated above, upon a demand for heat the operation of the combustion control circuit 10 commences with a safe start component check. When switch 22 or 24 closes, a circuit is completed for energization of the control circuit 20 from the transformer 40. At this time the triac 46 is rendered conductive only if the components of the flame detection circuit 26 and control circuit 20 are operational. If the photoelectric device 74 provides a false indication of flame, the SCR 56 is nonconductive and winding 48 cannot be energized to close normally open relay contacts 52. Similarly if SCR 56 is inoperative and cannot conduct current, winding 48 cannot be energized. The same result follows if because of any other fault the control circuit 20 or flame detector circuit 26 fails to operate.

If all components are operational, closing of the thermostat switch causes both windings 48 and 50 to be energized. More specifically, winding 50 is energized at a continuous DC level by a diode 82 and capacitor 84 connected across secondary winding 38 in parallel with a resistor 86. Winding 48 is energized simultaneously by current flowing through the conductive SCR 56 and through a resistor 87 associated with the thermal delay circuit 30.

Simultaneous energization of windings 48 and 50 causes contacts 52 to move to their closed condition thereby gating the triac 46 to its conductive condition. Voltage fluctuations due to the inductive nature of the load and switching transients and the like are shunted by a circuit including a capacitor 88 and resistor 90 providing reliable turn-off of the triac 46. A further aspect of the safe start component check is that the triac 46 cannot be rendered conductive following a failure of the relay which prevents contacts 52 from closing, or if the triac itself fails in the open circuit condition.

Operation of both the burner motor 14 and the ignition circuit 16 takes place when the triac 46 is rendered conductive. When the burner motor operates, a combustible mixture of fuel and air is emitted at the burner, and is ignited by the ignition circuit 16 to produce a stable burner flame.

Referring more specifically to the structure of the ignition circuit 16, this circuit may take any desired form, and as illustrated includes a pair of spark electrodes 92 and 94 located in ignition relation to the combustible air - fuel mixture at the burner 12. Although other types could be used, the ignition circuit 16 is similar to the ignition circuit illustrated and described in detail in U. S. Pat. No. 3,556,706 -- Campbell, to which reference may be had for a more complete disclosure. Briefly, the circuit 16 includes a capacitor 96 which charges repetitively during alternate half-cycles of the power supply waveform through a diode rectifier 98, a resistance 100 associated with the thermal delay circuit 30, and an inductance 102. Repeatedly during the half-cycle, as the voltage across the capacitor 96 reaches a predetermined threshold level, a trigger circuit including a pair of voltage divider resistors 104 and 106 and a phase shift capacitor 108 applies an operating voltage to the gate electrode of an ignition circuit SCR 110. At this point the SCR 110 is placed in a conductive condition and rapidly discharges the capacitor 96 in a pulse or surge through the primary winding 112 of a transformer 114 having a secondary winding 116 connected to the spark gap electrodes 92 and 94. As a result, during alternate half-cycles of the power supply waveform there are produced adjacent the burner 12 a high frequency series of discrete, high energy ignition sparks.

Following initial energization of the burner motor 14 and ignition circuit 16, the combustion control circuit carries out a trial-for-ignition operation during a predetermined time period the duration of which is established by operation of the thermal delay device 30. In normal operation, ignition takes place during this period. If ignition fails to occur, then continued emission of fuel from the burner 12 could create an undesirable and potentially unsafe condition. For this reason, the thermal delay device functions to shut down the burner motor 14 and the combustion control circuit 10 at the end of the trail-for-ignition period if ignition is not detected by the flame detecting circuit 26.

Proceeding now to a description of the thermal delay device 30, this device includes a normally closed set of switch contacts 118 controlling current flowing via the triac 46 to the burner motor 14 and ignition circuit 16. Contacts 118 are controlled by a bimetallic switch actuator 120 which opens the contacts 118 when the actuator is heated to a predetermined extent. Preferably, the contacts are arranged so that they are latched open and cannot reclose until released by a service man or operator. Heating of the actuator 120 is carried out at a relatively high rate of thermal transfer by the resistor 87 in series with the relay winding 48 and the control circuit SCR 56. Heating at a slower rate is carried out by the resistor 100 in series with the triac 46. The heat transferred to the actuator 120 by the resistor 87 is such that contacts 118 open in about 30 seconds.

Although the thermal delay device 30 may take various forms, one device highly suitable for the purpose is disclosed in U. S. Pat. application Ser. No. 204,491, filed Dec. 3, 1971, of Donald F. Dalziel and Charles H. Heide. Reference may be had to that application for a further description of the structure and operation of the device 30.

Throughout the trial-for-ignition period, the heater resistor 87 is energized because it is in series with the relay winding 48. If ignition fails to occur within the predetermined time, contacts 118 open and deenergize the burner motor 14 and the ignition circuit 16. The burner installation remains shut down until such time as the cause for ignition failure is corrected and the contacts 118 are reclosed.

In normal operation, ignition occurs before opening of the contacts 118 and the control circuit 20 places the combustion control circuit 10 in an operating condition wherein the burner motor 14 is continuously energized and the ignition circuit 16 is disabled. The latter function is carried out by the novel ignition disabling circuit 28.

More specifically, upon ignition a flame at the burner 12 is detected by the photoelectric flame detecting device 74. In the manner described above, this results in the SCR 56 being placed in a nonconductive condition. Accordingly, the heater resistor 87 of the thermal delay device is deenergized and actuator 120 is no longer heated by this resistor. Although relay winding 48 is deenergized at this time, contacts 52 remain closed due to the continuing energization of the relay winding 50. The burner motor 14 continues to operate because the triac 46 is maintained in its conductive condition.

When the control circuit SCR 56 becomes nonconductive, the ignition circuit SCR is disabled by the circuit 28. Ignition disabling circuit 28 includes a coupling transformer 122 having a primary winding 124 connected to the anode and cathode of the SCR 56 and a secondary winding 126 connected to the gate electrode and cathode of the SCR 110. When SCR 56 is nonconductive, current flows through the primary winding 124 in half cycles when the ignition circuit 116 would otherwise operate, and the gate electrode and cathode of the SCR 110 are reverse biased. Due to this reverse bias the SCR 110 cannot be rendered conductive and no capacitor discharge takes place to produce ignition sparks. Conversely, when SCR 56 is conductive, it acts as a low resistance shunt of primary 124 to prevent reverse biasing of the ignition circuit SCR 110.

FIG. 2 illustrates an alternative ignition disabling circuit generally designated as 28A. In this arrangement a relay winding 128 is energized by a DC potential produced by a diode rectifier 130 and capacitor 132 to close a normally open set of relay contacts 134. Closing of contacts 134 effectively connects the gate electrode of SCR 110 to its cathode and prevents firing of the SCR 110. When SCR 56 conducts, it provides a low resistance bypass of the winding 128 and contacts 134 remain open, permitting SCR 110 to operate in its normal fashion in ignition circuit 16.

FIG. 3 illustrates yet another alternative ignition disabling circuit generally designated as 28B. In this arrangement a light sensitive resistance 140 is coupled between the gate electrode and cathode of the ignition circuit SCR 110. A lamp 142 is disposed to illuminate the resistance 140 and is connected across the output electrodes of the SCR 56. When SCR 56 conducts, it shunts the lamp 142 so that resistance 140 is in its high resistance condition and the ignition circuit operates. When SCR 56 is biased to its nonconductive condition, the lamp 142 is energized to reduce the resistance of the device 142 and disable the ignition circuit.

If at any time during normal operation the burner flame should be extinguished, another trial-for-ignition period is commenced. If flame detecting device 74 does not detect flame, the SCR 56 is operated once again. The ignition circuit 16 operates and the heater resistor 87 is energized until such time as ignition is reestablished, or, alternatively, until the contacts 118 open.

Returning to a description of a heating cycle carried out by the combustion control circuit 10, after ignition the burner motor 14 continues to operate and the flame at the burner 12 heats the region including the thermostat 22 or 24. When the demand for heat is satisfied, the triac 46 is rendered nonconductive and the installation returns to its initial temperature satisfied condition.

One important feature of the present invention is that the circuit 10 includes novel structure to prevent against unsafe conditions arising from failure of the triac 46 in a short circuit condition wherein it remains conductive regardless of the control signal applied to its gate electrode. This is a common failure condition of such devices. In prior art units if a controlling triac shorts out, the burner motor is energized without any monitoring by the flame detector, and without control of the thermostat in some cases, giving rise to a highly undesirable and potentially unsafe condition. Thus, in prior circuits it has been necessary to purchase expensive triac devices of a high quality in order to reduce the probability of failure as much as possible.

In the circuit 10, failure of the triac 46 does not lead to an unsafe condition. Should such a failure occur with a control circuit thermostat 24 in the open position, the burner motor 14 and ignition circuit 16 are energized, and the control circuit 20 exercises no monitoring function. Moreover, the ignition circuit 16 is not disabled by the SCR 56 and the disabling circuit 28 or 28A. However, the continuously operating ignition circuit 16 draws current through the heater resistor 100 of the thermal delay device 30. After a period of several minutes or so, the resistor 100 heats the actuator 120 and the contacts 118 open to shut down the installation in a safe condition.

Should the triac 46 fail with a low voltage thermostat 24 in the closed circuit condition, the circuit 10 operates normally until the thermostat opens. At this time the burner motor 14 continues to operate and the ignition circuit 16 begins to operate. After a delay period, heater resistor 100 heats the actuator 120 and the circuit 10 goes out on safety.

When a line voltage thermostat is used, a potentially unsafe condition exists in prior circuits wherein the thermal switch contacts are located in a low voltage circuit and not in series with the triac. In such an arrangement, should ignition fail to occur or should the flame be extinguished, the burner motor is maintained energized even if no flame is present because the thermal switch contacts are not in circuit with the burner motor. The present invention avoids this unsafe condition because if flame is not present and the thermal switch contacts open, current to the burner motor is positively discontinued without reliance upon the operability of the triac 46.

Although the present invention has been described with reference to the details of the illustrated embodiments, it should be understood that other modifications and embodiments will be apparent to those skilled in the art. The details of the illustrated embodiments are not intended to limit the scope of the present invention as set forth in the following claims.

Claims

What is claimed and desired to be protected by Letters Patent of the United States is:

1. A combustion control circuit for controlling the operation of a fuel burner having a burner motor and an ignition device and comprising:

a triac in circuit with both the burner motor and the ignition device;

a thermostat;

a control circuit for placing the triac in a conductive condition to energize both the burner motor and the ignition device in response to operation of the thermostat;

a flame detector coupled to the control circuit for providing a flame indication in response to operation of the burner;

means coupled to said control circuit for disabling the ignition device in response to a flame indication;

a thermal delay switch for disabling the burner motor in response to being heated to a given extent;

a first heater connected in said control circuit for heating said thermal delay switch during the time period beginning with operation of the thermostat and ending with production of a flame indication;

and a second heater connected in circuit with the ignition device for heating said thermal delay switch during operation of the ignition device.

2. The combustion control circuit of claim 1, said thermal delay switch being connected in series with said triac.

3. A combustion control circuit for a fuel burner used to heat a space and including a burner motor and an ignition device, said combustion control circuit comprising:

a switching means coupled to both the burner motor and the ignition device and operable to energize the burner motor and ignition device simultaneously;

a flame detector disposed adjacent the fuel burner to provide a flame indication;

a thermostatically controlled switch in the heated space movable to an operating position in response to a demand for heat;

a first control means coupled to said switching means and to said thermostatically controlled switch for operating said switching means in response to movement of said thermostatically controlled switch to the operating position;

a second control means coupled to said ignition device and to said flame detector for discontinuing operation of said ignition device in response to a flame indication;

a thermal delay switch connected to the burner motor for preventing operation of the burner motor in response to being heated to a predetermined extent;

a first heater adjacent said thermal delay switch and connected to said first and second control means for energization during operation of said first control means prior to operation of said second control means; and

a second heater adjacent said thermal delay switch and connected to said ignition device for energization during operation of said ignition device.

4. The combustion control circuit of claim 3, said switching means comprising a triac.

5. The combustion control circuit of claim 4, said thermal delay switch being connected in series with said triac.

6. The combustion control circuit of claim 5, said first control means including relay means having a winding means and having a contact means connected in controlling relation to said triac, and an SCR having output electrodes and a gate electrode, said thermostatically controlled switch being coupled to said output electrodes, and said flame detector being connected to said gate electrode.

7. The combustion control circuit of claim 6, said flame detector comprising a light sensitive variable resistor, a bridge circuit including said detector in one leg, means for driving said bridge with a regulated driving signal, and a capacitor coupled to the bridge output and to the gate electrode of the SCR.

8. The combustion control circuit of claim 5, said second control means comprising a coupling transformer connected between said flame detector and said ignition device.

9. The combustion control circuit of claim 5, said second control means comprising a relay connected between said flame detector and said ignition device.

10. The combustion control circuit of claim 5, said second control means comprising a lamp and light sensitive resistor connected between said flame detector and said ignition device.

11. A combustion control circuit for a fuel burner comprising:

control means for operating the burner in response to energization of the control means;

a controlled conduction device including electrodes forming an output electrode pair and a control electrode pair;

said output electrode pair being coupled to said control means for controlling the energization thereof;

a variable resistance flame detection device disposed in flame sensing relation to the burner;

a resistance bridge having opposed input and output terminals and having one leg including said flame detection device and having three legs including only fixed resistance; and

driving signal means coupled between a source of operating potential and said bridge input terminals;

said bridge output terminals being coupled to said control electrode pair of said controlled conduction device.

12. The combustion control circuit of claim 11, said controlled conductive device comprising an SCR.

13. The combustion control circuit of claim 11, said detection device comprising a photoelectric cell.

14. The combustion control circuit of claim 11, said driving signal means including a zener diode.

15. The combustion control circuit of claim 11, said flame detection device having dark and light resistance conditions, said fixed resistances being chosen so that the bridge is unbalanced in response to the absence of flame and oppositely unbalanced in response to the presence of flame.

16. A combustion control circuit for controlling a fuel burner installation including a burner motor and an ignition circuit of the type including an ignition circuit gated device periodically rendered conductive by a gating signal to create a burner ignition condition, said combustion control circuit including:

motor start means and motor hold means coupled to the burner motor for sequentially initiating and then maintaining operation of the burner motor;

a normally conductive controlled conduction device including electrodes forming an output electrode pair and a control electrode pair;

said output electrode pair being coupled to said motor start means for controlling the energization thereof;

a variable resistance flame detection device disposed in flame sensing relation to the burner;

a resistance bridge having opposed input and output terminals and having one leg including said flame detection device; and

driving signal means coupled between a source of operating potential and said bridge input terminals;

said bridge output terminals being coupled to said control electrode pair of said controlled conduction device for rendering said controlled conduction device nonconductive in response to detection of flame to disable said motor start means;

and an ignition disabling circuit coupled between said output electrode pair and the ignition circuit gated device for disabling the gated device in response to detection of flame.

17. A combustion control circuit for a fuel burner installation of the type including a burner motor and an ignition circuit having an active element, said combustion control circuit comprising in combination:

a triac having a gate electrode and having output electrodes connected in current supplying relation between a power source and both said motor and said ignition circuit;

a thermal switch connected in current supplying relation with the motor;

a flame sensitive variable resistance device adjacent the burner;

a bridge circuit including said device in one leg;

means for driving said bridge with a regulated voltage;

a capacitor connected to said bridge for storing a potential corresponding to the balance condition of the bridge;

a controlled conduction device having a control electrode coupled to said capacitor for conduction in response to the absence of flame and for nonconduction in response to the absence of flame;

a relay having pull-in winding means and hold winding means and having a normally open set of contacts coupled to the gate electrode of said triac;

means including a thermostat for energizing said hold winding means and said controlled conduction device in response to a demand for heat;

said pull-in winding means being connected for energization by said controlling conduction device;

an ignition disabling means coupled between said controlled conduction device and the ignition circuit active element for disabling the active element in response to nonconduction of said controlled conduction device;

a first heater for said thermal switch connected in circuit with said pull-in winding means; and

a second heater for said thermal switch connected in circuit with said ignition circuit.

18. The circuit of claim 17, said thermal switch being in series with said triac.

19. The circuit of claim 17, said controlled conduction device comprising an SCR.

20. The circuit of claim 17, said thermostat being in series with said triac.

21. The circuit of claim 17, said thermostat being connected in current supplying relation to said pull-in winding means and said hold winding means.

22. The circuit of claim 17, said disabling means comprising a transformer.

23. The circuit of claim 17, said disabling means comprising a relay.

24. The circuit of claim 17, said disabling means comprising a lamp and a light sensitive resistance.

25. A combustion control circuit for a fuel burner and for an ignition circuit including an ignition circuit SCR and including a gating circuit for periodically applying a gating signal to the cathode and gate electrodes of the ignition circuit SCR for producing an ignition condition, said combustion control circuit comprising:

control means for operating the burner in response to energization of the control means;

a solid state controlled conduction device having a control electrode and having output electrodes coupled to said control means for controlling the energization thereof;

a flame detection circuit coupled to the control electrode of said solid state controlled conductive device for rendering said device nonconductive in response to detection of burner flame; and

an ignition disabling circuit coupled between said solid state controlled conduction device and said ignition circuit;

said disabling circuit including a transformer having one winding connected across the output electrodes of the solid state controlled conduction device and having another winding connected across the cathode and gate electrodes of the ignition circuit SCR to reverse bias the ignition circuit SCR in response to nonconduction of said solid state controlled conductive device. A combustion control circuit for a fuel burner and for an ignition circuit including an ignition circuit SCR and including a gating circuit for periodically applying a gating signal to the cathode and gate electrodes of the ignition circuit SCR for producing an ignition condition, said combustion control circuit comprising;

control means for operating the burner in response to energization of the control means;

a solid state controlled conduction device having output electrodes coupled to said control means for controlling the energization thereof;

a flame detection circuit coupled to the control electrode of said solid state controlled conduction device for rendering said device nonconductive in response to detection of burner flame; and

an ignition disabling circuit between said solid state controlled conduction device and said ignition circuit;

said disabling circuit including a lamp connected across the output electrodes of the solid state controlled conduction device and a light sensitive resistance connected across the cathode and gate electrodes of the ignition circuit SCR to prevent gating of the ignition circuit SCR in response to nonconduction of said solid state controlled conduction device.