United States Patent [191 [11] Patent Number: 4,643,668 Geary [45] Date of Patent: Feb. 17, 1987

[54] HOT SURFACE DIRECI‘ IGNITION SYSTEM 4,459,097 7/1984 Riordan et al...... 431/69 X FOR GAS I URNACES FOREIGN PATENT DOCUMENTS [75] Inventor: Fredemk J- Gwy’ Hdlamh Mlch- 1957920 6/1970 Fed. Rep. of Germany ...... 431/70 [73] Asslgnee: g’mggv 52mm“ Company’ Primary Examiner-Margaret A. Focarino ’ ' Attorney, Agent, or Firm-candor, Candor & Tassone 21 A l. N .: 624,014 [ 1 pp ° [57] ABSTRACI‘ [22] Filed: Jun. 25, 1984 . . . . I A low cost hot surface ignition system particularly Int- (:1.4 ...... s adapted for use gas furnaces and the like’ the system UsS. Cl...... - low voltage control and circuitry cf. [58] F m f S h 431/27 66 1/6798 fective to control a gas valve and high voltage ignition ‘8 0 we """"""""""" " ’ ’ ’ ’ circuitry incorporating a hot surface silicon carbide 431/70, 71’ 78' 25’ 6’ 74’ 49; 361/253’ 2'54 igniter disposed in the path of incoming gas and in the [56] References Cited heat of the ?ame when ignition is achieved to effect Us. PATENT DOCUMENTS ?ame recti?cation resulting in ?ame sensing. The sys tem also provides fail-safe operation or safety shutdown 3,947,220 3/1976 Dietz ...... 431/27 X of the associated furnace if ignition is not achieved or in the event of a failure of a component of the system. 4,303,385 12/1981 Rudich et a]. 4,325,689 4/1982 Burke ...... 431/ 70 21 Claims, 4 Drawing Figures

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U.S. Patent Feb. 17, 1987 Sheet 2 of 2 4,643,668

6%! 4,643,668 1 2 FIG. 2 is a schematic circuit diagram illustrating the HOT SURFACE DIRECT IGNITION SYSTEM FOR circuitry for the system illustrated in FIG. 1; GAS FURNACES FIG. 3 is a schematic diagram illustrating the cir cuitry for the igniter circuit and auxiliary timer blocks BRIEF SUMMARY OF THE INVENTION 5 illustrated in FIG. 1; This invention relates to ignition systems and, more FIG. 4 is a schematic diagram illustrating the pro particularly, to an improved hot surface direct ignition grammable unijunction transistor timer circuitry por system particularly adapted for use in gas furnaces and tion of the system illustrated in FIG. 2. the like. DETAILED DESCRIPTION Heretofore, numerous ignition systems have been 10 utilized for the purpose of igniting either natural or Referring to the drawings, and more particularly to manufactured gas utilized in gas furnaces for heating FIG. 1 thereof, a schematic block diagram of a hot purposes, such ignition systems usually including a pilot surface direct ignition system, generally designated 10, ?ame and functioning under the control of a thermostat embodying the present invention is illustrated therein. situated in an area which is to be heated by the furnace. As shown in FIG. 1, the system 10 includes a conven An object of the present invention is to reduce the tional thermostat 12, a main timer circuit 14, an auxil cost of ignition systems of the indicated character, elim iary timer circuit 16, an igniter circuit 18, a hot surface inate the need for a pilot ?ame, and at the same time silicon carbide igniter 20 which is disposed in the path provide an improved hot surface direct ignition system of incoming gas and in the ?ame when ignition is ef incorporating improved means for providing fail safe fected, a ?ame sensing circuit 22, a main valve control operation of the furnace in which the system is incorpo circuit 24, and a conventional main gas valve 26. The rated. _ system 10 is adapted to provide control, supervision, Another object of the present invention is to provide ignition, automatic retry ignition in the event of com an improved ignition system incorporating hot surface bustion failure, and safety shutdown in the event reigni 25 tion is not established. ignition means effective to ignite directly natural or The electrical circuitry for the system 10, illustrated manufactured gas utilized in gas furnaces for heating in FIG. 1, is illustrated in FIGS. 2, 3 and 4. As shown in purposes. FIG. 2, in addition to the thermostat 12, the silicon Another object of the present invention is to provide carbide igniter 20 and the main gas valve 26, the system an improved hot surface direction ignition system incor 10 includes a relay K-l having normally open contacts porating improved low voltage control circuitry in K-lA and K-lB; a relay K-2 having normally open cluding unique, economical timing means and effective contacts K-2A and normally closed contacts K-2B; to control high voltage hot surface ignition circuitry resistors R-1 through R-20; diodes D-1 through D-10; incorporating hot surface direct ignition means effec transistors Q-l through Q-6; and capacitors C-1 through tive to ignite natural or manufactured gas. 35 C-11, the above described components all being electri Another object of the present invention is to provide cally connected by suitable conductors as illustrated in an improved hot surface direct ignition system for gas FIGS. 2, 3 and 4 and as will be described hereinafter in furnaces incorporating improved means which provides greater detail. As shown in FIG. 2, the system 10 is fail safe operation of safety shutdown of the furnace in adapted to be connected to a conventional source of line the event of a failure of a component of the ignition 40 voltage alternating current, such as 120 volts alternating system. current, by the leads L-1 and L-2, and the system 10 also Another object of the present invention is to provide includes low voltage control circuitry which includes an improved hot surface direct ignition system incorpo the thermostat 12 and which is adapted to be connected, rating improved means for retrying ignition in the event through the thermostat 12, to a source of low voltage of a power failure or gas interruption and for effecting 45 alternating current, such as 24 volts alternating current, safety shutdown of the furnace if reignition is not ef by the lead L-3 and to a low voltage return by the lead fected. L-4. Another object of the present invention is to provide In general, the system 10 operates in the following an improved hot surface direct ignition system for gas manner to perform three timing and two control func furnaces incorporating improved means for attempting tions. On closure of the contacts of the thermostat 12, a single retry for ignition in the event power is momen the relay K-l is energized thereby effecting closure of tarily removed or the ?ow of gas is interrupted during the contacts K-lA and K-lB and 120 volt AC current is a normal heating cycle and for effecting shutdown of supplied to the silicon carbide igniter 20 preferably for the ?ow of gas, if reignition is unsuccessful, until power a minimum of approximately 40 seconds. The system 10 is removed from the system for a predetermined period 55 then causes the main gas valve 26 to be opened for of time. approximately four seconds to initiate the ?ow of gas Another object of the present invention is to provide while simultaneously extending the igniter “on” time an improved hot surface direct ignition system for gas for an additional approximately two seconds into the furnaces that may be readily adapted to meet the igni valve trial for ignition time. The valve control is then tion requirements of a wide variety of gas furnaces. 60 relinquished to the ?ame sense circuit 22. If ?ame is The above as well as other objects and advantages of present, the main gas valve 26 will be allowed to remain the present invention will become apparent from the open. However, if ignition has not occurred, the main following description, the appended claims and accom gas valve 26 will close at the end of the trial for ignition panying drawings. period and stop the flow of gas. This lockout condition 65 BRIEF DESCRIPTION OF THE DRAWINGS can only be cancelled by power removal, i.e. opening of the contacts of the thermostat 12 for a minimum prede FIG. 1 is a schematic block diagram of a hot surface termined time. If in the normal heating cycle, power is direct ignition system embodying the present invention; momentarily removed, or gas ?ow is interrupted, the 4,643,668 3 4 system 10 will attempt reignition one time, and if not combination will reduce ripple voltage. Ripple voltage successful, will revert to lockout. will increase as input voltage increases. The load cur As shown in FIGS. 2 and 4, the main timing is pro rent demanded by the programmable unijunction tran vided by a unique programmable unijunction transistor sistor/Darlington circuit plus the bleeder R-15 varies. timing circuit that utilizes the main timing capacitor C4 Moreover, the ?lter capacitor 05 performs a dual func in both a charging and discharging mode to render the tion. When the programmable unijunction transistor circuitry fail safe from an open or short capacitor, and Q-1 times out and latches, it causes the main valve relay at the same time reduce the number and cost of compo K-2 to energize. When the relay K-2 energizes, it re nents required. The igniter circuit 18 and auxiliary timer moves power from the programmable unijunction tran circuit 16 are illustrated in FIGS. 2 and 3. The auxiliary sistor circuit by opening the contacts K-2B. The pro timer 16 is an R-C discharge circuit and is provided in grammable unijunction transistor Q-l must remain addition to the programmable unijunction transistor latched during the entire trial for ignition period so that bias circuit. The igniter circuit 18 includes the double in case ignition does not occur and the relay K-Z drops pole double throw relay K-l to switch in or out the out, reapplying power to the programmable unijunction igniter 18 and the ?ame sense circuit 22. The relay K-l transistor Q-l, it is still latched on and does not allow is sourced with a recti?ed, ?ltered voltage, and con retry. It only allows lockout. The only source of power trolled by the Darlington transistor Q-2 which is, in to the programmable unijunction transistor during this turn, controlled by the main timer circuit 14 and the cycle is the ?lter capacitor 05. As the power supply auxiliary timer 16. On timing up, the main timer circuit voltage decreases, the programmable unijunction tran 14 preferably times the hot surface igniter 20 for a mini sistor anode, valley and gate current all decrease in mum of approximately 40 seconds. On timing down, a proportion until the programmable unijunction transis signal is sent to the main valve circuit 24 for an approxi tor Q-1 reaches the limit of its holding current capabili mately four second ignition trial. If ignition is success ties, at which time it reverts to the blocking state. It will ful, the ?ame sense circuit 22 supplies a signal to the be understood that the latching time must be greater main valve circuit 24 to hold the main gas valve 26 open 25 than the trial for ignition time. In the embodiment of the and voltage is removed from the main timer circuit 14 invention illustrated and described, trial for ignition at to allow reset for a retry ignition, if such retry ignition room temperature will be approximately four seconds. becomes necessary. If ignition is not established within The capacitor C-3 between the gate and source voltage the trial period, even though voltage had been removed allows the ripple voltage to modulate the gate and make from the main timer circuit 14, lockout occurs. The 30 the threshhold voltage of the programmable unijunc reason that lockout occurs in that the discharge time tion transistor to appear higher. The capacitor C-3 aids constants of the programmable unijunction transistor in circuit reset and initial turn on, to drive the gate circuitry are longer than the trial for ignition period and higher in voltage than the anode. The capacitor C-3 also the programmable unijunction transistor circuitry is still supplies maximum surge current to the Darlington tran latched upon reapplication of voltage. The resistors R-4 sistor base to aid ignition operation. The time constant and R-14, the diode D-9, and the base emitter junction of the capacitor O3 in parallel with the gate to supply of the Darlington transistor Q-2 form the bias network resistor R-4 should be approximately the same as the for the programmable unijunction transistor main timer. ?lter capacitor load resistance time constant, as for The capacitor C6 and the resistor R-lit- form the auxil example 3. 5 seconds. On start up or application of iary timer. When the programmable unijunction transis 40 voltage to the timer, the gate voltage will be forced to ‘ > tor Q=1 (represented by the switch SW in FIG. 3) breaks the source value and will then bleed down to the volt over, the capacitor 06, blocked by the isolation diode age divider set value, as for example in approximately D-9, must discharge through the resistor R-14 into the 1.5 seconds. base of the Darlington transistor Q-2, delaying relay The initial function of the programmable unijunction dropout. In the embodiment of the invention illustrated, 45 transistor timer is to provide power source to the igniter the low voltage control circuitry is designed to operate 20. The main timing circuit provides the initial igniter over a voltage range of 18 volts AC minimum to 30 timing, and an additional approximately two seconds volts AC maximum and a temperature range of minus timing is also provided so that the igniter will still be 40 degrees C to plus 85 degrees C. powered up when the gas valve is initially opened. This The relay K-1 is supplied half wave AC voltage and 50 is accomplished by connecting the capacitor C-6 from therefore the capacitor C-7 is provided to sustain and ground to the junction of the isolation diode D-9 and maintain a DC voltage condition across the relay K-l. the resistor R-14 of the programmable unijunction tran The voltage supplied to the programmable unijunction sistor biasing network. When the programmable unin transistor timer and base circuit of the Darlington tran junction transistor Q-l is timing, the capacitor 06 sistor Q-2 is derived from a half wave recti?er D-5, charges to the bias voltage, and when the programma ?ltered by a single capacitor C-5, and regulated by the ble unijunction transistor times out, it discharges zener diode D=-10, and utilizes the bleeder resistor R-15 through the resistor R-14 into the base of the Darling to provide a discharge time to one volt preferably in ton transistor Q-2 causing the igniter relay K-l to re approximately 18 seconds. The ripple voltage associ— main energized for an additional time. ated with the zener diode D-10, capacitor ?lter O5, is 60 In the system 10 there is a diode D-11 in series with decreased with a given size capacitor, as the zener oper the timing capacitor G4 which allows the capacitor C4 ational current is decreased. This is the result of two to charge, but prevents it from discharging directly conditions, one, with a larger series resistor (lower through the programmable unijunction transistor Q-l, zener current), the capacitor cannot charge as close to but instead forces it through additional resistance R-10, or overshoot the peak (zener clamped) voltage as with 65 R-16, R-17 parallel to the diode D-11. The voltage a smaller resistor. Secondly, the zener diode D-10, oper developed across these resistors R-10, R-16, R-17 is ating closer to the knee, gives the capacitor less time to negative with respect to ground. This voltage is routed discharge through its relatively low impedance. This to the ?eld effect transistor Q-4 in the relay control 4,643,668 5 6 circuit and causes the relay K-2 to become energized, tive DC voltage. The DC voltage derived by the diode thus closing the contacts K-2A and opening the gas D-5 is ?ltered by the capacitor 05, regulated by the valve . When the capacitor C4 becomes discharged, zener diode D-10, and applied to the circuit of the pro the relay K-2 deenergizes. The effective ?eld effect grammable unijunction transistor Q-l for timing, and to transistor pinch off voltage may be in the range from the Darlington transistor Q-2 for biasing. The transistor 1.64 to 3.64 volts and so long as the main timing capaci Q-2 is allowed to conduct and causes the relay K-] to tor C-4 applies a negative voltage above this pinch off energize, the relay K-l being supplied by the DC volt value, the main valve will remain open. The system 10 age from the diode D-6 and the capacitor C-7. When the is capable of generating a wide range of trial for ignition relay K-l is energized, the contacts K-IA and K-lB timing. In the system 10, the discharge path for the main close and 120 volts AC current is applied to the igniter timing capacitor C4 is through two independent paths. element 20, which may be a conventional silicon car The dominant path is through the series resistor R-10, bide igniter element, thereby causing it to heat and the diode D-12 and the collector emitter junction of the glow, and additionally shorting out the ?ame sensing transistor Q-3, and since the semiconductor junctions capacitor C-l. Since direct current is applied to the develop voltage across them, they may be viewed as 15 circuit of the programmable unijunction transistor Q-l; equivalent resistances for purposes of timing. The sec the capacitor C4 charges to a preset voltage level and ondary discharge path exists through the series resistor causes the programmable unijunction transistor Q-l to R-10 and the redundant parallel resistors R-16 and R-17. change from an open circuit to a short circuit. When This secondary path is necessary because if at the in this condition occurs, two timing periods are initiated. stant of programmable unijunction transistor breakover 20 The ?rst timing period is caused by the removal of (highest voltage across the capacitor C4), the thermo voltage across the capacitor C-6 which then discharges stat contacts are opened, the primary discharge path into the Darlington transistor Q-2 through the resistor would be eliminated because the transistor Q-3 would R-14. This extends the igniter “on” period for approxi not be operating. As a consequence the capacitor C4 mately two seconds after the gas valve has been opened, would maintain its charge, and either turn on the main 25 and insures that gas will ?nd the igniter 20 at its hottest valve for a trial period, without previous ignition tim temperature optimum ignition condition. The second ing, when the thermostat was released, or give a short timing period is the trial for ignition period. When the ened i gniter timing period. Both could result in nuisance capacitor 04 changes the state of the programmable lockout. To avoid this situation and to allow a speci?c unijunction transistor Q-l, the capacitor C4 is left with thermostat reset time, the resistors from R-10 to ground a voltage remaining across it due to the blocking action are added to act as a bleeder for the capacitor C4. Since of the diode D-ll. Since the programmable unijunction this resistor now shares in valve timing, if it should fail transistor Q-l is essentially a short circuit, the capacitor in an open circuit mode, the valve trial for ignition C4 will discharge through the transistor Q-l, the resis period would increase. Placing a plurality of resistors in tor R-10 and the parallel combination of the resistors parallel reduces the percentage of time change versus R-16 and R-17 developing a negative voltage, with the number of potential failures. respect to ground, across the resistors R-16/R17. This _ Another failure that could affect valve trial for igni negative voltage is applied through the isolation diode tion time is the opening of the power supply diode D-12 to the gate of the ?eld effect transistor Q-4. When D-10. This would cause the capacitor C4 to charge to there is no voltage at the gate of the ?eld effect transis a higher voltage because the threshhold voltage of the 4-0 tor Q-4, the ?eld effect transistor Q-4 is essentially a programmable unijunction transistor would now shift short circuit. When the transistor Q-4 has a negative with input voltage. It would not cause much change in voltage applied to its gate, commonly known as pinch the igniter timing, but would increase both auxiliary off voltage, the ?eld effect transistor Q-4 is an open time and valve trial for ignition time without adversely circuit. The capacitor C4 is applying a negative voltage affecting the safety of the system. 4-5 to the gate of the transistor Q-4, making it an open The main valve circuit operates upon the principle of circuit. However, during every negative half cycle of energy transfer. On one-half cycle, the capacitor C-2 the AC voltage input, the transistor Q-3 conducts charges to a predetermined voltage. 0n the next half thereby removing this negative voltage and causing the cycle, this capacitor discharges into the relay K-2 and transistor Q-4 to act as a short circuit. The transistor the sustaining capacitors C-10 and C-11 across the relay 50 Q-4 goes from open circuit to short circuit as long as the K-2. The energy imparted from the discharge capacitor capacitor C4 supplies negative voltage to the gate of 02 into the relay K-2 and sustaining capacitors C-10 the transistor Q-4, or as long as any negative voltage is and C-11 is great enough to pull in and hold the relay applied to the gate, such as that supplied by ?ame recti K-2 until the next discharge cycle. To prevent relay ?cation. When the negative voltage isv removed from shatter from occurring, the two sustaining capacitors 55 the gate of the ?eld effect transistor Q4, the ?eld effect C-10 and C-11 are placed across the relay coil so that if transistor Q-4 reverts to a shorted state, whether or not one opens the other will sustain the relay K-Z. the transistor Q-3 is alternately opening and shorting The bleeder resistor R-2 is included across the dis the gate to ground. Negative voltage is removed when charge capacitor C-2 to eliminate a momentary pulse to the capacitor C-4 completes its discharge period (trial the main valve on thermostat reset. This pulse, although 60 for ignition period) or if ?ame recti?cation is not pres too short to release gas, could increase valve wear and ent. shorten its life. The ?eld effect transistor Q-4 alternately opening and In the operation of the system 10, when the thermo shorting in synchronism with the line voltage causes the stat 12 calls for heat, closure of the contacts of the ther transistor Q-5 and the transistor Q-6 to follow its action. mostat 12 applies low voltage alternating current to the 65 This changing state allows the capacitor C-2 to charge circuitry. There are four separate recti?er circuits that through the diode D-7, the resistor R-l, the diode D-1 convert the AC voltage to positive DC voltage and one and the diode D-2 on the positive half cycle and dis recti?er circuit that converts the AC voltage to a nega- ' charge into the relay K-2, the capacitor C-10 and the 4,643,668 7 8 capacitor C-ll through the transistor Q-6, the diode state, the capacitor C-4 also caused the gas valve 26 to D-8 and the resistor R-13 on the negative half cycle. open through its action on the ?eld effect transistor Q4. This causes the relay K-2 to be energized as long as The igniter 20 was caused to remain energized by the negative voltage is applied to the gate of the ?eld effect auxiliary timer for an additional approximately two transistor Q-4. Energization of the relay K-2 effects 5 seconds after main gas was released by opening of the closure of the normally open contacts K-ZA thereby gas valve to insure ignition. During this two second causing the main gas valve 26 to open. Energization of igniter extension, the flame sense capacitor 01 was the relay K-2 also opens the normally closed contacts shorted, preventing any type of flame recti?cation from K-2B and removes the source of voltage to the pro occurring. grammable unijunction transistor Q-l and the bias of the 0 When the relay K-l is deenergized, the contacts Darlington transistor Q-2. K-lA and K-lB open thereby disconnecting the igniter Even though the voltage is removed from the pro 20 from the line voltage, and the capacitor C-l becomes grammable unijunction transistor Q-l, it is important to unshorted. The igniter itself, being exposed to the ?ame, note that the programmable unijunction transistor Q-l becomes the ?ame sense probe. This, through the diode was in conduction (shorted) when this voltage was is action caused by the ?ame impinging on the igniter 20 removed and the programmable unijunction transistor (?ame recti?cation) causes the capacitor C4 to transfer Q-l will remain in this condition (latched) until the a negative voltage charge to the capacitor 08. A nega capacitors C-3, C-4 and C-5 have discharged. When the tive voltage develops across the capacitor C-8 and is capacitors C-3, C-4 and C-5 have discharged, the pro supplied to the gate of the ?eld effect transistor Q-4 grammable unijunction transistor Q-1 will again revert 20 through the resistor R-12, causing the relay K-Z to ' to an open condition as it was when the thermostat remain energized and the contacts K-lA and K-lB to contacts were ?rst closed. If the relay K-2 deenergizes remain open. After the relay K-2 has been energized before the capacitors C-3, C-4 and C-5 discharge, the long enough for the capacitor C-5 to discharge, the relay K-2 would reapply voltage to the circuit of the programmable unijunction transistor Q-l timing circuit transistor Q-l while it was still shorted, causing the 25 unlatches, or rearms, and if a power or gas interruption transistor Q-1 to remain shorted and not allowing it to occurs, the system will attempt a retry for ignition. If recycle. This constitutes a lockout condition and can the retry for ignition is not successful, lockout will only be changed by removing voltage from the circuit occur. by opening the contacts of the thermostat 12 until all of Typical values for the components of the system 10 the capacitors have discharged. The time required to 30 described hereinabove are as follows, although it will be effect such discharge may be approximately 30 seconds understood that these values may be varied depending under some conditions. upon the particular application of the principles of the When the capacitor C4 ?rst caused the programma present invention: ble unijunction transistor Q-l to revert to a shorted

SYMBOL DESCRIPTION R-l 79 ohm 7 W 5% vWire Wound R-2 5.6K l W 5% Carbon ?lm R-3 430K l W 2% Carbon ?lm R-4 2.7 m l W 2% Carbon ?lm R-S 2.2 M l W 5% Carbon ?lm R-6 2.2K l W 5% Carbon ?lm R-7 39K l W 5% Carbon ?lm R-B 4.3K ; W 5% Carbon ?lm R-9 1K 5 W 5% Carbon ?lm R-lO 12K } W 5% Carbon ?lm R-ll 2.2 M k W 5% Carbon ?lm R-l2 4.3 M l W 2% Carbon ?lm R-l3 33 ohm l W 5% Carbon ?lm R-l4 4.3 M l W 2% Carbon ?lm R-l5 270K ' l W 5% Carbon ?lm R-l6 240K } W 5% Carbon ?lm R-l7 240K l W 5% Carbon ?lm R-l8 470 hm l W 5% Carbon ?lm R-l9 22 M i W 5% Carbon composition R-20 22 M 5 W 5% Carbon composition D-l lN4004 Recti?er D-2 IN4004 Recti?er D-4 lN4004 Recti?er D-6 lN4004 Recti?er D-3 IN4148 Diode D-5 IN4148 Diode D-7 lN4148 Diode D-8 IN4l48 Diode D-9 lN4148 Diode D-ll IN4148 Diode D-l2 IN4148 Diode D-lO IN6008B Zener Diode Q-l 2N6028 PUT Q-2 2N5308 Darlington Transistor Q-3 2N2907 PNP Transistor Q-4 2N5639 Field Effect Transistor Q¢5 2N2222A NPN Transistor Q.6 MPS-W-OlA NPN PWR Transistor K-l Relay, 24 VDC, DPDT 4,643,668 9 10 -continued SYMBOL DESCRIPTION K-2 Relay, 12 VDC, SPDT C-l .0068 MFD @ 400 VDC 10% C-2 47 MFD @ 35 VDC 10% G3 1.5 MFD @ 50 VDC, Low Leakage, Nichicon # 50~U-KB-l.5-M C-4 100 MFD @ 35 VDC 10% Low Leakage C-5 47 MFD @ 50 VDC, Nichicon # U-LB-lI-I-470~M C-6 .15 MFD @ 63 VDC 10%, EVOX (Secor) Type SMMKO C-7 33 MFD @ 50 VDC, Nichicon # U-LB-lH-330-M C-8 .068 MFD @ 250 VDC 10% C-9 10 MFD @ 50 VDC 20% C10 6.8 MFD @ 50 VDC 20% G1] 6.8 MFD @ 50 VDC 20%

The ?ame sense circuitry operates on voltage and 15 tern will continue to operate within the timings speci does not have to draw current, and is much more sensi ?ed and that in this event, failure of any one additional tive than prior known controls, and the system 10 pro functioning part will cause the system or component to vides fail safe operation or safety shutdown of the asso continue to operate within the timings speci?ed or ciated furnace in the event of a failure of a component cause the system to interrupt ?ow of gas under its con of the system. The following tables depict possible fail- 20 trol, or the system will complete the cycle of operation ure modes of the various components of the system 10, and will fail to start or will lockout on the subsequent the effects and consequences of such possible failures, cycle. The designation column “b” indicates that the and include remarks respecting particular components. system will act to interrupt ?ow of gas under its control. In the columns designated “a” and “b”, under the desig nation “Consequences”, column “a” indicates the sys

FAILURE MODES AND EFFECTS ANALYSIS FAILURE W COMPONENT MODE EFFECT a b REMARKS R-l Open Lockout X R-2 Open None X Bleeder Resistor Short Lockout X R-3 Open Lockout X Short Lockout X R-4 Open Lockout X Short Lockout X R-S Open Lockout X Short None X R~6 Open Lockout X Short Lockout X R-7 Open Lockout X Short Lockout X R-8 Open Lockout X Short Lockout X R-9 Open Lockout X Short Lockout X Destroys D-5 and D-lO R-lO Open Lockout X Short Lockout X R-l 1 Open Lockout X Short None X R-S & R-ll Short Lockout X R-l2 Open " Lockout X Short Lockout X R-13 Open Lockout X Short None X Protects D-8 R-l4 Open Lockout X Short Lockout X R-15 Open None X Bleeder Resistor Short Lockout X R-l6 Open Inc Lkout time X 9% inc in time Short Lockout X R-l7 Open Inc Lkout time X 9% inc in time Short Lockout X R-16 & R-l7 Open Inc Lkout time X 21% inc in time D-l0 & R-l6 Open Inc Lkout time X 30.5% inc in time D-l0 & R-17 Open Inc Lkout time X 30.5% inc in time R-18 Open Lockout X Short None X Protective Resis. R-l9 Open X Short Lockout X R-2O Open X Short Lockout X D-l Open Lockout X Short None X D-2 Open Lockout X Short None X 4,643,668 11 12 -continued FAILURE MODES AND EFFECTS ANALYSIS FAILURE w COMPONENT MODE EFFECT a b REMARKS D-l & D-2 Short None X D-3 Open Lockout X Short None X Protects Q-3 D-4 Open Lockout X Short Lockout X D-5 Open Lockout X Short Lockout X D-6 Open Lockout X Short Lockout X Melds K-l, no ?ame sense D-7 Open Lockout X Short Lockout X D-8 Open Lockout X Short None X D-9 Open Lockout X Short None X No Aux Time D-lO Open Inc Lkout time X 18% inc in time Short Lockout X D-ll Open None X Dec lkout time Short Lockout X ' D-12 Open Lockout X No Flame Sense Short Lockout X No Flame Sense C-l Open Lockout X No Flame Sense Short Lockout X No Flame Sense C-2 Open Lockout X Short Lockout X G?» Open None X Therm Reset 4 Min. Short Lockout X Permanent ignition 04 Open Lockout X Short Lockout X 05 Open Lockout X Short Lockout X C-6 Open None X No Aux Time Ign. Short Lockout X 07 Open Lockout X Buzz/No Ign Short Lockout X Destroys D-6 C-8 Open Lockout X No Flame Sense Short Lockout X No Flame Sense C-9 Open Lockout X Short Lockout X Destroys D-4 C-lO Open None X Short - Lockout X C—1l Open None X Short Lockout X C-l0 & C-ll Open Lockout X K-Z may Buzz K-l Open Lockout X No Ign Short Lockout X Destroys Q-2 K-2 Open Lockout X Short Lockout X 9i G-K Open Lockout X G-K Short Lockout X G-A Open Lockout X G-A Short Lockout X A-K Open Lockout X A-K Short Lockout X 91 B-E Open Lockout X B-E Short Lockout X B-C Open Lockout X B-C Short Lockout X C-E Open Lockout X C-E Short Lockout X 91 BE Open Lockout X B-E Short Lockout X B-C Open Lockout X B-C Short Lockout X C-E Open Lockout X C-E Short Lockout X 9i G-S Open Lockout X G-S Short Lockout X G-D Open Lockout X G-D Short Lockout X D-S Open Lockout X D-S Short Lockout X 4,643,668 13 14 -continued FAILURE MODES AND EFFECTS ANALYSIS FAILURE CONSE UENCES COMPONENT MODE EFFECT a b REMARKS 9i B-E Open Lockout X B-E Short Lockout X B-C Open Lockout X B-C Short Lockout X C-E Open Lockout X C-E Short Lockout X 9i B-E Open Lockout X B-E Short Lockout X B-C Open Lockout X B-C Short Lockout X C-E Open Lockout X C-E Short Lockout X

5. The combination as set forth in claim 4 including 20 recti?er means converting the voltage derived from While a preferred embodiment of the invention has said low voltage source to DC voltage. been illustrated and described, it will be understood that 6. The combination as set forth in claim 5 including various changes and modi?cations may be made with zener diode means regulating and additional capacitor out departing from the spirit of the invention. means ?ltering the recti?ed DC voltage derived from What is claimed is: 25 said recti?er means. 1. A hot surface direct ignition system for gas fur 7. A hot surface direct ignition system for gas fur naces and the like, said system comprising, in combina naces and the like, said system comprising, in combina tion, a high voltage circuit adapted to be connected to a tion, a high voltage circuit adapted to be connected to a source of high voltage AC current, said high voltage source of high voltage AC current, said high voltage circuit including hot surface igniter means disposed in circuit including hot surface igniter means disposed in the path of incoming gas and being subjected to the heat the path of incoming gas and being subjected to the heat of the gas ?ame when gas ignition is obtained whereby of the gas ?ame when gas ignition is obtained whereby ?ame recti?cation is effected by said igniter means, a ?ame recti?cation is effected by said igniter means, a low voltage circuit adapted to be connected to a source ?ame sensing circult including a ?rst capacitor always of low voltage AC current, said low voltage circuit 35 connected in series with said igniter means, a low volt including thermostatic switch means controlling the age control and timing circuit adapted to be connected energization of said low voltage circuit, an electrically to a source of low voltage AC current, said low voltage operable gas valve, ?rst relay means having contacts in circuit including thermostatic switch means, an electri said high voltage circuit controlling the energization of cally operable gas valve and ?rst relay means having said igniter means, second relay means controlling the 40 contacts in said high voltage circuit always connected energization of said gas valve, said low voltage circuit in parallel with said ?rst capacitor and in series with also including ?rst timing means controlling the energi said igniter means and controlling the energization of zation of said ?rst relay means, second timing means said igniter means, said low voltage circuit also includ controlling the energization of said second relay means, ing second relay means having normally open contacts and third timing means operable upon the energization 45 controlling the energization of said gas valve and nor of said second relay means for maintaining the energiza mally closed contacts controlling the application of low tion of said ?rst relay means and said igniter means for voltage AC current from said source to said low volt a predetermined period of time, ?ame sensing means age control circuit, said low voltage circuit including including a ?rst capacitor connected always in parallel sequentially operable ?rst and second timing means across said ?rst relay contacts and always in series with controlling the energization of said ?rst relay means, said igniter means and effective to maintain energization said second timing means being effective to extend the of said second relay means when gas ignition is ob energization of said ?rst relay means and said igniter tained, said second relay means also having contacts in means for a predetermined period of time subsequent to said low voltage circuit controlling the energization of energization of said gas valve. said ?rst, second and third timing means. 55 8. The combination as set forth in claim 7 including a 2. The combination as set forth in claim 1 including Darlington transistor controlling the energization of Darlington transistor means controlling the energiza said ?rst relay means. tion of said first relay means and ?eld effect transistor 9. The combination as set forth in claim 8 including a means controlling the energization of said second relay programmable unijunction transistor controlling the means. energization of said Darlington transistor. 3. The combination as set forth in claim 2v including a 10. The combination as set forth in claim 7 including programmable unijunction transistor controlling the a ?eld effect transistor controlling the energization of energization of said Darlington transistor means and said second relay means. said ?eld effect transistor means. 11. The combination as set forth in claim 7 including 4. The combination as set forth in claim 3 including a 65 recti?er means converting the voltage derived from second capacitor, said programmable unijunction tran said low voltage source to DC voltage. sistor and said ?eld effect transistor being connected to 12. The combination as set forth in claim 11 including opposite sides of said second capacitor. zener diode means regulating the voltage derived from 4,643,668 15 16 said recti?er means, and capacitor means ?ltering the time to initiate the flow of gas, means for extending the voltage derived from said recti?er means. energization of said igniter after the opening of said gas 13. The combination as set forth in claim 7, each of valve, and means for energizing said igniter means upon said timing means including capacitive-resistance momentary interruption of power to said system. 15. The combination as set forth in claim 14 including means. 14. A hot surface direct ignition system for gas fur means for energizing said igniter means in the event gas naces and the like, said system comprising, in combina ?ow is momentarily interrupted. tion, a high voltage circuit adapted to be connected to a 16. The combination as set forth in claim 14 including source of high voltage AC current, said high voltage a Darlington transistor controlling the energization of circuit including a silicon carbide hot surface igniter said ?rst relay means and a ?eld effect transistor con disposed in the path of incoming gas and being sub trolling the energization of said second relay means. jected to the heat of the gas ?ame when gas ignition is 17. The combination as set forth in claim 16 including obtained whereby ?ame recti?cation is effected by said a programmable unijunction transistor controlling the igniter, a'?rst capacitor always connected in series with energization of said Darlington transistor and said ?eld said igniter, a low voltage circuit adapted to be con 5 effect transistor. nected to a source of low voltage AC current, said low 18. The combination as set forth in claim 17 including voltage circuit including thermostatic switch means a second capacitor controlling the conductivity of said controlling the energization of said low voltage circuit, programmable unijunction transistor and said ?eld ef electrically operable gas valve means and ?rst and sec fect transistor. ond relay means, said ?rst relay means having contacts 19. The combination as set forth in claim 18 including in said high voltage circuit always connected in parallel recti?er means converting the voltage derived from with said capacitor and in series with said igniter and said low voltage source to DC voltage. controlling the energization of said igniter, said second 20. The combination as set forth in claim 19 including relay means having normally open contacts controlling zener diode means regulating the voltage derived from the energization of said gas valve and normally closed 25 said recti?er means, and a third capacitor ?ltering the contacts controlling the application of low voltage cur voltage derived from said recti?er means. rent to said low voltage circuit, ?rst timing means con 21. The combination as set forth in claim 14, said ?rst trolling the energization of said igniter for a predeter and second timing means including capacitive-resist mined period of time, second timing means effective to ance means. open said gas valve after said predetermined period of * 1i 1 * *

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