Patent Publication Number: US-6222719-B1

Title: Ignition boost and rectification flame detection circuit

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to gas burners such as the type found in gas furnaces, and is more particularly concerned with means for electronically igniting the burner and for detecting or proving the existence of flame after ignition. 
     A number of electric igniter systems have been proposed for use with gas burners, including igniters that employ a high voltage spark, and igniters that involve a hot surface. In a mobile environment, in which the power for the furnace or heater is derived from a 12 volt DC or a 24 volt DC source, it has been common to employ a spark igniter, as heated surface type igniters have a high failure rate. The spark igniter requires some source of AC or pulsating voltage, and an inverter can be used to generate a wave which is then fed to an ignition transformer. Because of the relatively low voltage available in the mobile environment (i.e., 12 or 24 VDC), the turns ratio of the ignition transformer needs to be quite high. This means that the cost of the transformer is quite high, and also that the transformer can experience inter-turn arcing if fine wire is used in the secondary winding. 
     In any gas furnace it is mandatory to detect a successful ignition as a safety measure. If gas is permitted to flow to an unlit burner, explosive vapors can fill the dwelling and create a hazardous situation. Accordingly, a flame detection or flame proving means needs to be employed at the gas burner. One simple means for doing this is with a flame rectification probe. This technique is based on the fact that an active flame acts as a plasma diode. A unidirectional current can flow from a probe within the flame to the metal casing of the burner, i.e., the firebox. The flame itself thus acts like a resistance and diode connected in series. By applying an alternating current to the rectification probe, it is possible to detect the presence of flame. Rectification flame proving requires a source of alternating current, but in a mobile environment, where the power comes from 12 or 24 VDC, an inverter or other AC source has to be included in the burner control circuitry. This increases the cost of the circuitry. Moreover, the additional circuit elements increase the risk of failure. 
     Accordingly, a low cost ignition circuit and a flame detection circuit that would be suitable in a DC control system have been sought without success. A DC furnace control circuit that combines a burner igniter and a flame rectification probe has also been unavailable, without use of an on-board transformer. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an igniter and rectification flame detection circuit which avoids the drawbacks of the prior art. 
     It is another object to provide a ignition circuit that employs flyback current from a furnace relay coil to develop a primary ignition current, and which permits the turns ratio of the ignition transformer to be kept relatively low. 
     It is a further object of the invention to provide a rectification flame detection circuit that derives an alternating current for flame detection from a furnace relay actuator coil. 
     It is a still further object of this invention to provide a combination burner ignition and flame proving circuit. 
     According to one aspect of this invention, an igniter circuit for a furnace gas burner employs a pulsating current applied to a relay coil (such as the relay actuator coil for the inducer motor) to generate high flyback voltage. A flyback rectifier has its anode connected to the relay coil and its cathode feeds flyback pulses to a charge storage capacitor arrangement, where the flyback voltage accumulates. A step-up transformer has a primary winding and a secondary winding, with the secondary winding being connected to the igniter. High voltage at the igniter causes arcing to ignite the flame in the gas burner. A hysteresis switch is coupled between the charge storage capacitor and the primary winding of the step-up transformer. When the voltage on the storage capacitor arrangement exceeds some predetermined voltage threshold, e.g., 300 volts, the stored voltage is discharged through the primary winding, and this generates the high voltage arc on the igniter probe. With this arrangement, an intermediate or booster transformer is not needed. Also, this arrangement makes it possible to use an ignition transformer with a relatively low turns ratio, which increases the reliability and reduces the cost. 
     The charge storage capacitor arrangement can employ only a single capacitor coupled between the diode and a point of DC reference voltage, such as ground. In a preferred embodiment, the capacitor arrangement can be configured as a voltage doubler, with a pair of capacitors and a diode connected in series between points of positive and negative DC voltage 
     The hysteresis switch can include a controlled switching device, such as an SCR, having main electrodes, e.g., anode and cathode, connected respectively to the diode and to the primary winding of said step-up transformer. A zener device can be positioned between the gate or control electrode and one of the main electrodes of the SCR. A filter capacitor can be connected between the cathode and gate. 
     According to another embodiment of this invention, a rectification flame detection circuit is constructed for detecting the presence of flame in the burner of the gas furnace. Again, a pulsating current is employed, which is applied to a relay coil (e.g., the gas valve relay) in order to actuate the furnace. A capacitor has one electrode connected to the relay coil, and derives an AC voltage that is used for rectification flame detection. A detection transistor has its gate or control electrode connected through a resistive network to the flame detection conductor, a common or source electrode tied to ground, and a power or drain electrode connected via a signal impedance to a DC source. The drain and signal impedance define an output terminal therebetween. In the resistor network a first resistor has one end connected to the capacitor, its other end being connected to the control or gate electrode of said transistor. A second resistor is connected between the control electrode and common electrode, i.e., ground, of the transistor. The flame detection probe, which is located within the gas burner, is electrically connected to the capacitor and first resistor. In this arrangement, the output of the transistor oscillates between a high state and a low state, e.g., if flame is present, but remains locked in one state, i.e., the low state, if flame is not present in the burner. In a preferred embodiment, the transistor can be a depletion mode FET. 
     According to a further aspect of the invention, a control circuit combines a gas burner igniter circuit and a rectification flame detection circuit. There are pulsating current signals applied respectively to first and second relay coils in order to actuate the furnace. The combination igniter and flame detection circuit employs a flyback rectifier and charge storage means coupled to the flyback rectifier to accumulate flyback voltage. A step-up transformer has a primary winding and a secondary winding, with the secondary winding being connected to the igniter and flame detection probe to provide a high voltage for generating an arc for ignition. A hysteresis switch is coupled between the charge storage means and the primary winding of the step-up transformer and acts to discharge the current from the charge storage means through the primary winding whenever the stored flyback voltage reaches a predetermined threshold. There is also a capacitor connected to one end of the second relay coil. A flame detection transistor has a signal impedance connected with its drain or power electrode to define an output terminal. A resistor network has a first resistor with one end connected to the capacitor and a its other end connected to the gate or control electrode of the transistor. A second resistor is connected between the gate (control) and source (common) electrodes of the transistor. In this embodiment, one end of the ignition transformer secondary is connected to the one end of the first resistor, so that the igniter and flame detection conductor is connected through said transformer secondary and through the first resistor to the transistor. In this case, the output of the transistor terminal is oscillating if flame is present, and in a low state if flame is not present in the burner. Where the inducer relay coil is used to for generating the ignition voltage, and a microprocessor generates actuation pulses to energize the coil, the duty cycle of these pulses can be changed after ignition so as not to interfere with flame detection. 
    
    
     The above and many other objects, features, and advantages of this invention will present themselves to persons skilled in the art from the ensuing detailed description of a preferred embodiment of the invention, when read in conjunction with the accompanying Drawing. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of an ignition circuit according to an embodiment of this invention. 
     FIG. 2 is a schematic diagram of a rectification flame proving circuit according to an embodiment of this invention. 
     FIG. 3 is a circuit diagram of a combination ignition and flame proving circuit according to an embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the Drawing, FIG. 1 schematically illustrates an ignition circuit  10  according to one possible embodiment of this invention. Here an inducer relay actuator coil  12  is employed for switching on an inducer motor (not shown). This coil is in series with a switching transistor  14 , and a microprocessor  16  supplies square-wave gating pulses to the base of the transistor  14 . A flyback diode  18  has its anode connected with the collector of the transistor  14  and the lower end of the coil  12 . Flyback pulses, of relatively high voltage, e.g., +180 VDC, pass through the diode  18  to a storage capacitor  20 . Another diode  19  between coil  12  and ground charges another capacitor  22 . A network formed of capacitors  20  and  22  and a diode  24 . The capacitors  20  and  22  are connected in series with the diodes  18  and  24  between the positive and negative rails (+12 and ground) and serve as a voltage doubler. The diode  18  connects between the capacitors  20  and  22 , so that flyback voltage across the capacitor  22  builds up towards +360 VDC. 
     A hysteresis switch arrangement is formed of a gated switching device, e.g., an SCR  26 , whose anode is connected to the high end of the capacitors  20 ,  22 , and a zener  28  that is connected between the gate and the anode of the SCR  26 . A filter capacitor  30  spans between the cathode and gate of the SCR In this embodiment, the zener has a threshold value of +300 volts, so that the SCR turns on when the flyback voltage reaches that level, and then turns off at some lower voltage when the capacitors  20  and  22  are discharged. In an alternative arrangement, the SCR could be controlled from another output (not shown) from the microprocessor  16 . A neon bulb or other negative resistance device could replace the SCR. 
     An ignition transformer  32  is shown here with its primary winding  34  coupled between the cathode of the SCR  26  and the junction of the capacitor  22  and the diode  24 . When the SCR is switched on, the accumulated charge on the capacitive network  20 ,  22  is dumped through the primary winding at about 300 volts. This produces a high voltage, e.g., 20,000 volts, from the transformer secondary winding  36 , which feeds an igniter probe  38  within the gas burner. The high voltage generates an arc that causes the flame to light in the burner. After flame is detected, the microprocessor  16  can change the waveform of the gating pulses to the coil  12 , i.e., change the duty cycle, so that the circuit ceases producing a high ignition voltage. 
     Because the flyback voltage is considerably higher than the 12 volt working DC supply voltage, the stored flyback voltage can be discharged directly into the primary  34  of the ignition transformer  32 , and there is no need for an intervening or booster transformer. Also, with the relatively high voltage (300 volts) supplied from the capacitors  20 ,  22 , the turns ratio of the transformer  32  can be kept small. This permits the transformer  32  to be provided at low cost, and yet can be provided with high reliability insulation in the secondary winding  36  so that the risk of inter-turn arcing is minimized. 
     FIG. 2 schematically illustrates a flame detection circuit or flame proving circuit  40  according to a possible embodiment of this invention. Here a gas valve relay actuator coil  42  is employed, which is also used to actuate the gas valve that supplies a combustible gas to the gas burner (not shown). A switching transistor  44 , which receives a square-wave gating signal from the microprocessor  16 , interrupts the current flow through the actuator coil  42 . A capacitor is connected to the collector electrode of the transistor  44 , and derives an AC signal that is fed to a resistive network. This network is formed of a resistor  48  (here with a value of 10 megohms) and a resistor  50  (with a value of 2 megohms). A third resistor  52  has one end connected to the junction of the resistor  48  and capacitor  46  and its other end connected to a flame detection conductor within the burner or firebox  54 . In the Drawing, the schematic representation of a diode and resistor in series within the firebox  54  represents the fact that the flame behaves like a diode and resistor, and produce a weak rectified current. A depletion mode MOSFET transistor  56  detects the presence of flame. Here the MOSFET  56  has its source or common terminal connected to ground, and its gate connected to one end of the resistor  48 . The other resistor  50  is connected between the gate and source terminals of the MOSFET  56 . A load or signal resistor  58  is connected between the drain of the MOSFET  56  and a supply of signal voltage (+5 VDC), with an output terminal  60  being defined by the junction of the load resistor  58  and the MOSFET drain. 
     The AC signal from the coil  42  is supplied through the resistor  48  to the gate of the transistor  56 . However, if flame is present, the capacitor will charge through the rectification conductor in the firebox  54 , and this drives the voltage down at the gate of the transistor This means if flame is present, then the depletion mode transistor  56  will change states, and this will oscillate at the frequency of the forcing function at the base of the transistor  44 , producing an oscillating change of level at the output electrode  60 . 
     FIG. 3 illustrates an embodiment of a combined ignition and flame detection  100  circuit of this invention. Here, elements that correspond to elements in the FIG.  1  and FIG. 2 embodiments are identified with the same reference characters, but raised by  100 . A detailed description of each of these elements should not be necessary. 
     The flame ignition portion  110  of the circuit is tied here to the inducer relay coil  112  and the switch transistor  114 , with flyback diodes  118  and  119  connected to the transistor end of the coil  112 . As in the FIG. 1 embodiment, capacitors  120  and  122  are connected with a diode  124  to form a voltage doubler, and an SCR  126  and zener diode  128  are coupled to form a hysteresis switch. When the flyback voltage stored on the capacitors  120 ,  122  reaches the voltage defined by the zener  128 , the SCR conducts and discharges through the primary winding  134  of the ignition transformer  132 . This creates a high ignition voltage on the secondary winding  126  that in turn forms a spark on the ignition probe  138  in the firebox  154 . 
     The rectification flame proving section  140  is tied to the gas valve relay  142  and the associated switching transistor  144 . A capacitor  146  is tied to the transistor end of the coil  142 , and passes flyback pulses to resistor network formed of resistors  148  and  150 . The capacitor  146  also supplies the flyback pulses through a resistor  152  and through the secondary winding  136  of the ignition transformer  132  to the probe  138  within the firebox  154 . As is well known, when flame is present in the gas burner, the flame itself acts as a weak rectifier, here represented within the firebox  154  by a diode in series with a resistor to ground. The junction of the resistors  148 ,  150  is tied to the gate terminal of a depletion mode MOSFET  156 . A drain resistor  158  is tied to a source DC voltage (+5 V), and the drain electrode of the MOSFET  156  defines an output electrode  160 . 
     When flame is not present, the flyback pulses do not pass through the flame diode, and so the gate of the depletion mode MOSFET remains high. This produces a steady low at the output terminal  160 . On the other hand, when flame is present, there is flame rectification of the flyback pulses, and each occurrence of the flyback pulse will produce a low at the gate of MOSFET  156 , resulting in a pulsating signal, as illustrated. This pulsating signal can be easily detected in the microprocessor. 
     Here, the circuit is implemented with various transistors, resistors, capacitors, and other discrete elements. However, the circuit as shown here could be implemented using a microprocessor to carry out many of the same functions. Also, while the invention has been described for use in connection with low voltage DC environments (i.e., 12 or 24 volts) the invention can be applied in other environments as well. 
     While the invention has been described here with reference to several preferred embodiments, it should be recognized that the invention is not limited to those precise embodiments. Rather, many modifications and variations will present themselves to persons skilled in the art without departing from the scope and spirit of this invention, as defined in the appended claims.