US3393966A - Burner control - Google Patents

Description

y 23, 1963 D. CLARK BURNER CONTROL Filed Nov. 23, 1966 INVENTOR LORIS D. CLARK A TT'ORNEYS United States Patent Office 3,393,966 Patented July 23, 1968 3,393,966 BURNER CONTROL Loris D. Clark, Dayton, Ohio, assignor, by mesne assignments, to Koehring Company, Milwaukee, Wis., a corporation of Wisconsin Filed Nov. 23, 1966, Ser. No. 596,656 4 Claims. (Cl. 431-44) ABSTRACT OF THE DISCLOSURE A failsafe solid state burner safety control uses a bidirectional semiconductor to control the application of electric current to a burner. The semiconductor is controlled by a photocell and a silicon controlled rectifier which apply a start signal to the semiconductor and which also apply an overload current to a thermal cutout switch to trip the switch and disable the circuit if no flame is sensed by the photocell within a predetermined short time interval.

Brie summary of the invention This invention relates to burner safety controls and more particularly to a control circuit embodying solid state control elements.

An important object of this invention is the provision of a solid state burner safety control for controlling the application of an AC source of power to a burner actuator, such as an electric valve, electric motor, ignition transformer or the like.

A further object of this invention is the provision of a burner safety control in which a solid state symmetrical or bi-directional conducting device controls the application of electric power to the burner actuator, and a cadmium sulfide cell in combination with a silicon controlled rectifier senses the presence of a flame and disables the operation of the burner if a flame does not occur within a predetermined short time interval following initiation of burner or motor operation.

A still further object of this invention is the provision of a burner safety control in which the usual sensitive relay parts are eliminated, and which is quiet and fail-safe in operation, and is not subject to erratic performance from vibration, air flow or position.

Another object of this invention is the provision of a solid state burner control which either shuts off, or prevents the initiation of, burner operation in the event of component shorting or failure.

These and other objects and advantages of the present invention will be apparent from the following description, the accompanying drawing and the appended claims.

The single figure of the drawing is a Wiring diagram of a solid state burner safety control constructed according to this invention.

Referring to the figure of the drawing which represents a preferred embodiment of the invention, a fuel burner is indicated generally at in FIG. 1 as including a motor 12 and an ignition transformer 13. While the burner 10 may be of any type, such as oil, gas or coal stoker, which is electrically controlled, the invention has particular applicability to the control of the operation of compact portable space heaters such as shown and claimed in the patent of Varvel No. 3,101,193, assigned to the same assignee as this invention. In this heater, an electric motor 12 provides the air for combustion and furtherprovides for delivery of fuel oil to a nozzle, while a continuously operating transformer 13 provides ignition current to starting electrodes.

The power for the burner 10 is provided by an alternating current supply 15 including supply leads 16 and 17 as controlled by a line switch or thermostat 18. The flow of electricity from the power supply 15 to the burner 10 is controlled by a bi-directional triode semiconducting device 20 which may be gate triggered from a blocked state to a conducting state for either polarity of the applied voltage. The device 20 is positioned effectively in the lead 16 in current controlling relation to the motor 12 and the transformer 13. Any suitable symmetrical or bi-directional semiconductor may be used for the device 20 which, when gated on, permits the AC current to flow through the motor and transformer, but when gated off provides a high impedance so that no current flows. Suitable devices are silicon gate-controlled AC switches or thyristers commonly called by a generic name triac and sold by General Electric Company, Semi-Conductor Products Department, Syracuse, N.Y., under the designations of types SC40D and $0468. Alternately, two silicon controlled rectifiers connected in reverse parallel may be used in place of the single device 20.

Means for disabling the control circuit and stopping the operation of the burner 10 includes a thermally actuated cutout switch 22 including a heater element 23 and normally closed switch contacts 24. The switch 22 is preferably of the manual-reset circuit breaker type in which the contacts 24 are opened by reason of excessive current through the heater 23 and which remain open until manually reset. The switch contacts 24 are preferably placed in one of the lines 16 or 17 leading to the burner 10 and the current control device 20, and are shown in the drawing as being placed in the line 17. Alternately, the contacts 24 may be connected to ground the gate 21 of the device 20 to its cathode so as to bias the device into a non-conducting state, but the arrangement shown in the drawing is preferred by reason of its greater reliability in the event of malfunction of the device 20, since they remove power to the full control circuit.

The circuit further includes means for providing a start signal to the gate 21 of the device 20, and, at the same time, to provide a predetermined overload current to the heater 23 of the switch 22 until a flame has been detected, indicating that the burner is operating normally. For this purpose, a photoconductor fiame detector 30, preferably a cadmium sulfide cell, is physically positioned to respond to the presence of the flame at the burner 10. The cell 30 has a high resistance in the order of megohms in the absence of a flame and has a substantially de creased resistance in the order of approximately 1,000 ohms in the presence of light from the flame of the burner 10.

The cell 30 is connected to control the operation of a further or second AC control switch, preferably a silicon controlled rectifier (SCR) 32. However, a bidirectional device, such as RCA type T2893, can be used in place of the SCR32. The cell 30 has one lead connected to the lead 16 and another lead connected tothe gate 33 of the SCR32. It will further be seen that the gate 33 is connected in a bridge circuit including a resistor 34 to the line 16 and a further resistor 35 to a common junction point 36 leading to the heater 23. The heater resistance, which may be in the order of approximately 250 ohms, when combined with that of the resistor 35, form a total resistance sufliciently high compared to the resistor 34, that the SCR32 is biased into a conductive state as soon as power is applied to the line 16 and 17. A decoupling capacitor 37 shunts the gate of SCR32, the resistor 34 and the cell 30 to line 16 and acts as a transient filter across lines 15 and 17 to protect the SCR32 and keep spurious line switching transients use of the control circuit.

The anode and cathode circuit of the SCR32 is, in turn, connected to the common junction 36 through a resistor 38, and although the SCR32 conducts only during half.

cycles of the alternating current input, the values of the resistance of the heater 23 and the resistor 38 are such that a substantial overload current is immediately applied to the heater 23 of the switch 22, which may be in the order of approximately 150% to 250% of the rated current of the switch 22. If no flame is sensed by the cell 30 Within a predetermined short time period of approximately to seconds, this overload current will cause the switch 22 to open its contacts 24, thereby shutting off power to the burner 10. Since the contacts 24 are positioned in the line 17 leading from the heater of the switch 22 to the power source 15, the power is similarly interrupted to the cell 30, the SCR32. and the heater 23.

If the burner 10 functions normally, a flame will be detected by the cell 30, and its resistance will be substantially reduced. Since the cell is effectively in parallel with the resistor 34, the gate 33 is shorted to the cathode and the SCR32 is biased into a non-conductive state.

The initial signal through SCR32 is also applied to the gate 21 by conduction through a resistor 40 and a coupling capacitor 42. Since the signal at the junction 36 is effectively an alternating current (actually, it is a halfwave rectified current with a smaller AC signal superimposed by reason of the bridge elements 34, and 23) a gating signal is applied through the capacitor 42 to the gate 21 causing the device 20 to conduct symmetrically, passing AC power from the source 15 to the burner 10. As soon as a flame is detected, the half-wave rectified signal is removed.

Once the device 20 is triggered, the voltage developed on the capacitor 42 by conduction through the heater 23 and resistor triggers the device on succeeding cycles, and assures continued conduction of the device and continued operation of the burner control of the burner 10 until such time as the power is again interrupted.

If no flame is sensed within a 15 to 20 second period by the cell 30, the thermal cut-out switch 22 will open due to the excessive or overload current applied through the SCR32, and the opening of the contacts 24 will thus disable the entire circuit until manually reset.

The circuit of this invention is fail-safe from the standpoint of component failure. If the cadmium sulfide cell 30 fails, or the leads to the cell 30 become disconnected or open, the cutout switch 22 will be opened within a specified period due to the overload current, in the manner described above as if no flame had occurred. If the cadmium sulfide cell 30 sees light before the heater unit is started, or if the leads to the cell 30 are shorted, then the SCR32 will be biased off and cannot fire. Under these circumstances, there is no starting signal voltage applied to the junction 36 to fire the device 20, and the heater cannot begin to function, since the current is blocked by the device 20. Similarly, if the SCR32 should fail, no start signal would be applied to the device 20.

The apparatus of this invention is particularly useful where reliability and quiet operation are desired. It is not sensitive to vibration, in the manner of the usual cut-out relay devices, nor is it sensitive to air flow or position of mounting on the heater. It is also fail-safe, providing full safety protection for the burner.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. A solid state burner safety control for controlling the application of an AC source of power to a burner actuator in response to the presence or absence of a flame,

comprising means defining a bi-directional semiconductor device connected in current controlling relation from said source to said burner actuator and having a control gate, a thermally operated cutout switch having a heater and contacts, means connecting said contacts to disable the operation of said bi-directional device upon the occurrence of excessive current through said heater, a biasing circuit for said bi-directional device upon the occurrence of excessive current through said heater, a biasing circuit for said bi-directional device and for controlling the operation of said cut-out switch including a photoconductor responsive to the presence of a flame in said burner, a further semiconductor device controlled by said photoconductor and connected to apply a substantial overload current to said heater to cause said contacts to open if the burner does not ignite within a predetermined short time period, said photoconductor being connected to the control gate of said further device to bias said further device off in the presence of a flame, means including said further device providing a start signal to said gate of said bi-directional device providing for the initiation of conduction of said bi-directional device concurrent with the conduction of said further device, a coupling capacitor, and means connecting said heater and said capacitor to said bi-directional device gate forming a keep alive circuit for said bi-directional device.

2. The safety control of claim 1 wherein said silicon controlled rectifier is connected to said heater substantially at the junction of said heater and capacitor for applying an overload current to said heater and said start signal to said bi-directional device.

3. The safety control of claim 1 wherein said further device is a silicon controlled rectifier, and said photoconductor is a cadmium sulfide cell.

4. A solid state burner safety control for controlling the application of an AC source of power to a burner actuator in response to the presence or absence of a flame, comprising a controllable symmetrical conducting device connected in current controlling relation to said burner actuator and having a control gate, a thermally operated cutout switch having a heater and normally closed contacts, circuit means connecting said heater to said gate and to one side of said source, means connecting said contacts to disable the operation of said device by interrupting the power thereto, a biasing circuit for said device and for controlling the operation of said cut-out switch including a cadmium sulfide cell responsive to the presence of a flame in said burner, a silicon controlled rectifier controlled by said cadmium sulfide cell and connected to apply a substantial overload current to said heater at said junction of said heater and said device control gate during conduction of said silicon controlled rectifier to cause said contacts to open if the burner does not ignite within a predetermined short period, said cadmium sulfide cell being connected to the control gate of said silicon controlled rectifier to bias said silicon controlled rectifier ofl? in the presence of a flame, and circuit means including said heater providing a signal to said gate of said device providing for the continued operation of said device subsequent to the sensing of a flame by said cadmium sulfide cell.

References Cited UNITED STATES PATENTS 3,149,224 9/1964 Horne et al. 236 3,204,113 8/1965 Snygg 236 3,306,339 2/1967 Barton et al. l5828 3,335,781 8/1967 Krump ct al.

JAMES W. WESTHAVER, Primary Examiner.

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