1. In a spark ignition circuit, a low voltage direct current power source, a storage capacitor, a pulsing oscillator circuit operating on said power source and operative to incrementally charge said storage capacitor to a considerably higher voltage than said power source, said oscillator circuit comprising a transistor having its collector-emitter circuit connected across said power source, starting means, a transformer having a primary winding in the output of said transistor and a secondary winding of considerably greater inductance than said primary winding, parallel resistance-capacitance means connected between one end of said secondary winding and the base of said transistor and operative to drive said transistor to saturation and to abruptly cut off said transistor at saturation, thereby to effect an induced high voltage pulse in said secondary winding, circuit means including impedance means connecting the other end of said secondary winding to a point of fixed potential, and circuit means including a diode connecting said storage capacitor in parallel with said impedance means, an igniter transformer comprising a primary winding, a secondary winding, and a secondary circuit including spaced spark electrodes, circuit means including voltage responsive switching means connecting said igniter transformer primary winding across said storage capacitor, and said switching means being responsive to a predetermined charge on said storage capacitor to effect its discharge through said igniter transformer primary winding.
2. The spark ignition circuit claimed in claim 1 in which said impedance means connected between said other end of said secondary winding and said point of fixed potential is a small capacitor.
3. The spark ignition circuit claimed in claim 1 including a diode connected between the output side of said transistor and its base.
4. The spark ignition circuit claimed in claim 1 in which the time constants of the oscillator circuit are such that its oscillates at a frequency of between 100 and 400 kilocycles per second.
5. The spark ignition circuit claimed in claim 4 in which said storage capacitor is discharged one to three times per second.
6. The spark ignition circuit claimed in claim 1 in which said voltage responsive switching means is a controlled solid-state switch having gating circuit means therefor including resistance-capacitance means series connected across said storage capacitor and operative to permit the application of a firing signal to said switch through said resistance means only when said storage capacitor attains a predetermined charge.
7. The spark ignition circuit claimed in claim 6 in which said gating circuit means further includes a neon bulb between said resistance capacitance means and the control electrode of said controlled solid-state switch.
8. The spark ignition circuit claimed in claim 6 which further includes shunting circuit means connected between said gating circuit means and said point of fixed potential and including said spaced spark electrodes operative to shunt the application of a firing signal to said controlled solid-state switch when the impedance across said spaced spark electrodes is diminished by a bridging burner flame.
This invention relates to electronic spark ignition circuits which operate on direct current supply and employ oscillating circuit means to producce energy pulses to effect sparking across spaced electrodes.
There is a need for a simple and inexpensive spark ignition circuit which will operate on a direct current supply as low as 12 volts to produce adequate sparking which will reliably ignite a gas burner. An igniter circuit of this kind, when provided with suitable rectifying means, may be directly incorporated in the room thermostat circuit of a space heating system operating on a commercial a.c. power supply. Conventionally, the thermostat circuits of space heating systems are, for reasons of safety, operated on a stepped down supply of 24 volts. Also, a spark ignition circuit of this kind could be operated on a 12-volt wet or dry cell storage battery when the commercial alternating current supply fails or is not available, as may occur in space heaters for recreational vehicles.
Accordingly, it is an object of this invention to provide a generally new and improved spark igniter circuit, operable on a low voltage direct current or rectified low voltage alternating current supply, having oscillator means operative to produce high voltage pulses at relatively high frequency, a storage capacitor which is incrementally charged by these pulses, and means to discharge the storage capacitor through an igniter transformer primary winding at a relatively low frequency to effect strong sparking across the spaced electrodes in the igniter transformer secondary circuit.
A further object is to provide a spark igniter, as characterized in the preceding paragraph, in which the high frequency, high voltage pulses are generated by alternately driving a transistor to saturation through the primary winding of a voltage step-up transformer and abruptly cutting off current flow through the transistor and primary winding at saturation to induce high voltage pulses in a second winding.
A further object is to provide a spark igniter circuit, as characterized in the penultimate paragraph, which further includes means to automatically cut off sparking when a burner is ignited and to re-institute sparking in event the burner flame fails while the burner control system is in heat demand condition.
Further objects and advantages will appear from the following description when read in connection with the accompanying drawing.
The single FIGURE of the drawing is a diagrammatic illustration of a spark igniter circuit constructed in accordance with the invention. a gaseous fuel burner and fuel flow control means therefor is also illustrated in operative association with the spark igniter circuit.
Referring to the drawing, a voltage step-down transformer indicated at 10 has a primary winding 12 connected across a commercial alternating current power supply 14 through a manual switch 16. When manual switch 16 is closed, a rectified and filtered low voltage supply is provided across terminals 18 and 20 by a secondary winding 22 and the provision of a diode 24, a resistor 26, and a capacitor 28.
A NPN transistor 30 has its collector 32 connected to terminal 18 through a room thermostat 34 and its emitter 36 connected to terminal 20 through the primary winding 38 of a coupling transformer 40. A resistor 42 connected between terminal 18 and the base 44 of transistor 30 applies a limited forward bias which is just sufficient to initiate conduction through transistor 30 when thermostat 34 is closed.
A secondary winding 46 of transformer 40 is connected at its lower end at a point 60 to the base 44 of transistor 30 by a lead 62 and through a capacitor 56 and parallel connected resistor 58. The lower end of secondary winding 46 is also connected to terminal 20 through a voltage dividing resistor 48 and a lead 61. The upper end of secondary winding 46 is connected to terminal 20 through a small capacitor 50 and the lead 61. The upper end of secondary winding 46 is also connected to terminal 20 through a diode 62, a storage capacitor 54, and the lead 61. The diode 62 and storage capacitor 54 are connected in parallel with capacitor 50.
An ignition transformer 64 has a primary winding 66 connected across the storage capacitor 54 through a SCR 68. Gating means for SCR 68 comprises a resistor 70 and a capacitor 72 series connected across the storage capacitor 54, and a triggering neon bulb 74 connected between the SCR control electrode and a point 76 between reistor 70 and capacitor 72. The resistor 72 is connected between the SCR control electrode and the anode side thereof.
An ignition transformer secondary winding 78 is connected at one end to a point 80 in the SCR gate lead between neon bulb 74 and point 76. The other end of secondary winding 78 is connected to a spark electrode 82 positioned adjacent a conductive metal gas burner indicated at 84. The spacing of electrode 82 with respect to the metal burner 84 is such that a suitable spark gap is provided across which sparking will occur. Also, the electrode 82 is positioned so that the spark gap between it and the burner will be bridged by flame when the burner is ignited. The gas burner 84 and the igniter circuit are grounded at 86 and 88. It is to be understood that the burner 84 may be either a main burner or a pilot burner arranged to ignite a main burner.
An alternate, direct current power source for operating the igniter circuit is provided by a storage battery indicated at 90. Battery 90 is connected across terminals 18 and 20 through a manual switch 92. Selection of a power source may be made by closing one or the other of switches 16 or 92. Switches 16 and 92 may be interconnected so that only one may be closed at a time.
If the igniter circuit is to be operated on a commercial alternating current power supply, the switch 16 is closed and a low voltage rectified and filtered supply is available across terminals 18 and 20. If the igniter is to be operated on battery-supplied direct current, the switch 16 is opened and switch 92 is closed.
When one or the other of switches 16 or 92 is closed and thermostat 34 closes in response to a temperature drop, a normally closed, solenoid operated valve 94 is energized through leads 96 and 98 and opens to admit gas to flow through a gas supply conduit 100 to the burner 84. At the same time, a limited forward starting bias is supplied to transistor base 44 through resistor 42 to initiate conduction through transistor 30 and primary winding 38 of transformer 40. This initial current flow through winding 38 induces a voltage in secondary winding 46. This induced voltage is of such polarity that the lower end of secondary winding 46 is positive and causes current to flow through capacitor 56, causing it to charge, and through the parallel resistor 58 and the base-emitter circuit 44-36, thereby to increase current flow through the collector-emitter circuit of transistor 30 and through primary winding 38. Regenerative feedback therefore occurs, and the transistor 30 is rapidly driven to saturation.
During this period of increasing current flow through the transistor and winding 38, capacitor 56 is charged by the induced voltage in winding 46. Secondary winding 46 has a considerably greater number of turns than primary winding 38, so that the voltage induced therein when transistor 30 approaches saturation is considerably greater than the supply voltage across terminals 18 and 20. When saturation of transistor 30 occurs and current flow through primary winding 38 ceases to increase, the induced voltage in winding 46 drops to zero and its field collapses. As a result, capacitor 56 now discharges and a voltage of opposite polarity appears across winding 46.
The collapse of the field around winding 46 and discharge of capacitor 56 reverse biases transistor 30 and abruptly cuts it off at maximum output. The cutoff transistor 30 at saturation causes the field around winding 38 to collapse and by mutual induction causes a high voltage pulse to appear across winding 46 of the same polarity as the pulse induced therein upon collapse of its own field.
The mutually induced high voltage pulse in winding 46 now charges capacitor 50 and charges capacitor 56 in an opposite direction through a diode 101. This high voltage winding 46 is also simultaneously discharging through diode 52 to apaply an increment of charge on the storage capacitor 54. As this high voltage pulse discharges into capacitor 54, small capacitor 50 and capacitor 56 now discharge in a direction to again forward bias the base-emitter circuit to start another cycle. The diode 101 also functions to protect the transistor against reverse high voltage at the time the field around windidng 38 collapses.
The frequency at which the circuit oscillates is not critical except to the extent that it is desirable to stabilize the frequency at some point well below the frequencies assigned to commercial broadcasting channels. This is accomplished by predetermination of the R-C time constant of coupling capacitor 56 and resistor 58 and by small capacitor 50.
When the storage capacitor 54 attains a predetermined charge, the capacitor 72 will become sufficiently charged through the relatively high resistor 70 to permit the application of a break-down voltage across neon bulb 74 through resistor 70. When neon bulb 74 fires and conducts, the SCR 68 is fired and the storage capacitor discharges through primary winding 66 of igniter transformer 64. This induces a high voltage pulse in secondary winding 78, causing a spark to occur across the spark gap. The igniter transformer 64 is a voltage step-up transformer, the secondary winding having many more turns than the primary.
Upon discharge of storage capacitor 54, neon bulb 74 again becomes non-conductive, and the swing of the storage capacitor 54 following its discharge cuts off conduction through the SCR 68. The time constants of capacitor 72 and resistor 70, as related to the charge developed on storage capacitor 54, are such that the frequency of the storage capacitor discharge and, therefore, the occurrence of spark is in the order of two cycles per second.
It has been found that a strong spark occurring at relatively low frequency functions more reliably to ignite gas than a high frequency or continuous sparking. When gas is ignited at the burner 84, flame bridges the spark gap between electrode 82 and burner 84, thereby considerably reducing the gap impedance to the extent that lead-off from point 80 in the SCR gating circuit through secondary winding 78 and across the spark gap to ground is sufficient to preclude the charging of capacitor 72 through resistor 70 to the break-down voltage of neon bulb 74. Sparking across the gap will therefore cease when flame is present. If, however, the flame is extinguished while thermostat 34 is closed, sparking will be immediately resumed.
When gating of the SCR 68 is shunted by conduction through burner flame, the circuit will continue to oscillate at a somewhat higher amplitude, but with less power consumption. Under these conditions, as the accumulated charge on the storage capacitors 54 approaches the voltage of the charging pulses, the inductive and capacitive reactance will increase. Some of the charge applied to capacitor 54 will, however, under these conditions, leak off through resistor 70, igniter transformer secondary 78, and across the spark gap through the flame to ground.
An igniter circuit constructed in accordance with the foregoing description, and comprising the following component values, oscillated at approximately 250 kilocycles when operated on a 12-volt storage battery and provided adequate sparking at approximately two cycles per second to reliably ignite a gas burner:
Transistor 30 Type 2N3642 Resistor 42 56 kilohms Capacitor 56 .0047 microfarad Resistor 58 1.8 kilohms Resistor 48 220 kilohms Capacitor 50 220 picofarad Capacitor 54 1 microfarad Resistor 70 22 megohms Capacitor 72 .022 microfarad Diode 101 Type IN 4004 Diode 62 Type IN 4004 Neon Bulb 74 Signalite A-322 SCR 68 G. E. Type C106B2 Winding 38 .132 millihenrys Winding 46 2.93 millihenrys
It is to be understood that the circuit described will operate on direct current or rectified alternating current supply of considerably higher voltage than 24 volts with suitable changes in the values of circuit components. Also, the circuit described, having the above-listed component, values, has been successfully operated on both 12-volt direct current and 24-volt rectified and filtered alternating current supply.
Other modifications within the spirit of the invention will occur to those skilled in the art, such as substitution of a PNP transistor for the NPN transistor described.
The foregoing description and drawing is intended to be illustrative, and not limiting, the scope of the invention being defined in the appended claims.