US3666392A - Safety shutoff - Google Patents

Abstract

CIRCUITRY INITIATES TWO SHORTER AND LONG TIMED BUT VARIABLE INTERVALS WHEN A FURNACE THERMOSTATE SWITCHES TO CALL FOR HEAT. THE CIRCUITRY EFFECTS THE SWITCHING TO INITIATE COMBUSTION AT THE END OF THE SHORTER TIMED INTERVAL; BUT, IN THE EVENT THAT NO COMBUSTION HAS COMMENCED BY THE END OF THE LONGER TIMED INTERVAL, THE CIRCUITRY PREVENTS THE COMBUSTION THEREAFTER UNTIL A RESET OPERATION HAS BEEN PREFORMED. PREFERABLY THE RESET IS MECHANICAL. IF COMBUSTION START BEFORE THE ENT OF THE LONGER TIMED INTERVAL, THE TIMIING OF THE LONGER TIMED INTERVAL CEASES.

Description

H. D. JULINOT SAFETY SHUTOFF May 30, 1972 2 Sheets-Sheet 1 Filed July 16, 1970 Inventor HEL MU] 0. JUL //V07 Attorney May 30, 1972 H. D. JULINOT SAFETY SHUTOFF Filed July 16, 1970 2 Sheets-Sheet 2 7b Re/qy P E Therm/3m" 5% Inventor HEL/VUTD JUL/N07- A Home y United States Patent 3,666,392 SAFETY SHUTOFF Helmut D. Jnlinot, Toronto, Ontario, Canada, assignor to International Telephone and Telegraph Corporation,

New York, N.Y.

Filed July 16, 1970, Ser. No. 55,393 Claims priority, application Canada, Mar. 2, 1970, 076,241 Int. Cl. F23n 5/14 U.S. Cl. 431-78 4 Claims ABSTRACT OF THE DISCLOSURE Circuitry initiates two shorter and longer timed but variable intervals when a furnace thermostat switches to call for heat. The circuitry effects the switching to initiate combustion at the end of the shorter timed interval; but, in the event that no combustion has commenced by the end of the longer timed interval, the circuitry prevents the combustion thereafter until a reset operation has been performed. Preferably the reset is mechanical. If combustion starts before the end of the longer timed interval, the timing of the longer timed interval ceases.

BACKGROUND OF THE INVENTION This invention relates to controls for equipment and, more particularly to an automatic control for shutting down equipment such as an oil or gas furnace when the burner fails to ignite.

It will be appreciated that when a thermostat calls for ignition and combustion and if such ignition does not take place at the time called for or within a reasonable interval thereafter, many disadvantageous situations can rise, including the danger of oil spreading and the consequent danger of fire and/or explosion.

SUMMARY OF THE INVENTION Therefore, it is an object of the invention to provide a furnace control circuit, where if ignition and combustion are called for by the thermostat but do not occur within a predetermined interval, the necessary switching will be performed to prevent ignition thereafter. Means are preferably provided to ensure as far as possible that the prevention of combustion continues until the causes of the original combustion delay have been corrected.

It is also an object of the invention to provide furnace control circuitry as described in the previous paragraph where, when ignition is prevented electrically and due to a detected delay in such ignition, a mechanical reset is required before the control circuitry may again cause ignition and combustion. Thus, electronic release of the protective circuit is prevented and manipulations of the circuitry for electric resetting of the control circuitry before the trouble is fixed are inhibited.

When the thermostat calls for heat, it is desirable to know that all elements in the control system are functioning properly. It is an object of this invention to provide a circuit wherein the more important of such elements must function properly between the period the thermostat calls for heat and ignition; otherwise, ignition will not take place.

It should be emphasized that the circuit has other failsafe features so that other elements not affected as described in the preceding paragraph are connected so that their failure will be evident or will also prevent operation of the circuit.

The above-described and other advantages of the invention will be better understood from the following description when considered in connection with the accompanying drawings.

3,666,392 Patented May 30, 1972 BRIEF DESCRIPTION OF run DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings in FIG. 1, lines 10 and 12 are shown carrying standard AC power (usually volts). Lines 10 and 12 are shown as connected across ignition device 11 and oil supply motor 13 in series through normally open contact P1. The indication of ignition device 11 and motor lines for switching on and off by contact P1 are exemplary only. In accordance with conventional arrangements for oil furnace operation, when the circuit is closed, the ignition will intermittently operate until combustion has begun; and the motor will supply oil from the time of ignition onward.

Lines 10 and 12 are also connected across the primary of the stepdown transformer T1 whose two ends are connected to a positive line :14 through rectifiers 16 and 18 respectively and to a negative line 20 through rectifiers 22 and 24 respectively. The center tap of the secondary of transformer T1 is connected to line 26. Line 14 is connected through resistor R1 to a line 30. Line 30 is con nected to line 20 through the following elements in series: a thermostat 32, a normally closed contact L1 of a relay L, a relay coil P, a relay coil L, a thermistor 34 to an anode of a silicon controlled rectifier (SCR) 36, a cathode of SCR 35 being connected to line 20.

It will be noted that the relay L and the contact L1 are unconventional and of a type whereupon if there is sutficient current flow through relay L, contact L1 will open in the conventional manner for a relay. But on opening, contact L1 mechanically latches so that it can only be closed by a mechanical reset. Although the above is shown in FIG. 1, it is shown in detail in FIGS. 2, 3 and 4 and is later described.

It will be noted that the thermistor is a device having a negative temperature coefi'icient. Thus, on initiation of flow through .SCR 36, primarily self-heating thermistor 34 will gradually heat up; and its resistance will consequently lower in an amount varying with the time the intermitttent flow continues with the current in the circuit R1, 32, L1, P, L, 34, and 36 and correspondingly increasing. Resistor 38 is provided optionally in parallel with thermistor 34 and provides an adjustment of the current through thermistor 34.

If required for desired heating rates, a resistor R2 is provided which is connected between line 30 and line 26 but is physically juxtaposed to thermostat 32 to supply heating thereto. As stated, thermistor 34' is primarily self-heating, and the resistor R2 is only used and connected if auxiliary heating is required due to cold, ambient temperature conditions or other causes. The levels of operation of relays P and L are selected .so that they will not operate at the level of initial flow through thermistor 34 at environmental temperatures therein. However, relay P is selected and designed to operate at a lower current level (implying a smaller degree and shorter interval of heating of thermistor 34) than relay L. The connection between relays P and L is connected to line 26 through the normally open contact P2 of relay P which acts in circumstances to be described, as latching contacts for the relay P. A Zener diode 40 and a condenser 42 are connected in parallel with each other and in parallel with relays P and L and thermistor 34 in series. Zener diode 40 is oriented to provide its design voltage drop for potentials higher at contact L1 than at the anode of SOR 36. Line 30 is connected through resistor 44 to the base of a transistor 48, and the base of transistor 48 is connected to line 20 through a resistor 46. Line 14 is connected through a variable resistor 50 to the emitter of transistor 48. The emitter of transistor 48 is connected to line 20 through a flame detector 52 which would be located in the furnace in the vicinity of a burner flame. The flame detector 52 is a conventional type which has a high resistance when no flame exists at the burner and has a low resistance when a flame does exist in the furnace. Although exact values will vary with other design parameters, the type of flame detector which has been used (Type CT 97 manufactured by General Motors) has greater than one megohm resistance when the furnace is off and less than one kilohm when the furnace is on. Resistor 50 is chosen along with the other circuit parameters affecting transistor 48 so that if line 14 is at a voltage reflecting an unduly low voltage across lines '10 and 12, the transistor 48 will not turn on. This prevents dangers inherent in ignition of the furnace under unduly low voltages across lines and 12. The flame detector may, of course, be replaced within the scope of the invention by a different type of combustion detector as long as equivalent resistive effects are available for circuit biasing.

In operation then, it will be assumed that the furnace is off with contact P1 open. Also, it is assumed that the temperature is at the desired temperature so that thermostat 32 provides an open circuit through relays P and L and thermistor 34. Because the furnace is off, the resistance of flame detector 5-2 is high. It will be noted that the power supply provides full wave supply to lines 14 and 20 and, hence, to the circuitry of transistor 48. Resistor R1 is selected so that the base of transistor 48, when turned on, will be low enough to conduct for all values of current flow through the circuits, including thermostat 32. Thus, with the furnace ofi, transistor 48 will conduct on each half-cycle. Also, a pulse from the collector of transistor 48 which is developed across resistor 54 will be applied to the gate of SCR 36. However, SCR 36 will not conduct as thermostat 32 provides an open circuit to the anode thereof.

This situation will continue until the temperature measured by thermostat 32 is below the predetermined setting; at which time, the thermostat will close the circuit between resistor R2 and closed contact L1- hence, through to the anode of SCR 36. Then, SCR 36 is triggered on each half-cycle by the pulse from transistor 48. Thus, intermittent conduction through a portion of each half-cycle takes place in SCR 36 and causes thermistor 34 to gain in temperature. It will be noted that to the extent that the thermistors own initial temperature is not sufficient in view of ambient conditions at the thermistor 34 and/or to the extent that the rate of heating is not sufficiently fast, resistor R2 may be provided between line 30 and line 26 to provide auxiliary heating. In any event, the operating levels of the relays P and L are selected so that initial conduction through the unheated thermistor will not energize either relay. At a later time determined by the heating rate of thermistor 34 and its initial and continuing ambient temperature, the thermistor resistance falls to a sufficiently low value that the current level in the series circuit between line 30 and to and including SCR 36 is suflicient to operate relay P, which closes normally open contacts P1 and P2. Coutact P2 acts to latch in relay P to maintain it energized independently of relay L, thermistor 34, and SCR 36 for as long as contact L1 and thermostat 32 remain closed. Contact P1 closes to initiate the operation of the ignition and the motor circuits. Like the motor and ignition circuits shown, the contact P1 may be considered as exemplary.

At the time that the current through relay P is suificient 4 to operate its contacts, it will be noted that relay L has not yet operated.

A timed interval is the time from the initiation of conduction of thermistor 34 until the current therethrough reaches a level to operate relay P. The exact length of such interval will vary with the initial temperature of the thermistor 34.

In accordance with the present invention, the closure of contact P1 is a sequential step in the successful operation of the furnace control. It will be noted that the device so far is substantially fail-safe in that in the event of low voltage, transistor 48 would not conduct so that no pulse would be provided to ignite SCR 36. Also, energizing current for relay P would not be initiated.

Further, it will be noted that if any of relays P or L or thermistor 34 have failed open, the circuit will not operate; and the furnace cannot be ignited. Moreover, should condenser 42 or Zener diode 40 fail, the relays will not operate; and the furnace similarly will not be lighted. If condenser 42 fails open, the ripple through the relays P and L and thermistor 34 will cause noise and will tend to shorten the time at which relay L is operated (as hereinafter described) to shut off the ignition and motor circuits. Zener diode 40 tends to prevent high voltage fluctuations through the relays and thermistor. Should it fail on short circuit, relay P, inter alia, will not operate, and the furnace will not light. Should Zener diode 40 fail at open circuit, then the presence of high current fluctuations will shorten the time of operation of relay L which, as hereinafter described, will operate to prevent or to turn off the ignition and motor circuits as hereinbefore described. Thus, the circuitry is fail-safe.

Where the closure of contact P1 has caused ignition of the furnace, heat will continue until the resistance of detector 52 falls below the value at which conduction can take place in transistor 48. This will open or stop conduction in SCR 36, and the heating of thermistor 34 will cease. Thus, relay L cannot operate. However, the operation of the furnace is not impaired since relay P is latched closed through contact P2. When the heat in the ambient area reaches the preset temperature level, thermostat 32 opens which then opens the latching circuit for relay P, allowing it to open opening contact P2 and disconnecting the contact P1 which will turn off the furnace. This constitutes the completion of a normal cycle of operation of the furnace and control.

Going back to the state of the operation where contact P1 has just closed, let it be assumed that the ignition or the motor or both do not operate so that with no combustion taking place, a dangerous condition may arise. One of these dangerous situations may occur where oil is supplied to the furnace, but no ignition takes place which causes a spread of oil. This is a serious situation if ignition takes place at that time. In any event, with contact P1 closed, the thermostat 32 previously closed, and no ignition, current flow continues through thermistor 34 on intermittently triggered half-cycles from the transistor 48. With the heating of thermistor 34, the resistance thereof decreases, and the current through the thermistor and relay L increases. When the resistance of thermistor 34 decreases to allow sufficient current flow, relay L is operated which opens contact L1thus, disconnecting the latching circuit of relay P and preventing later ignition or combustion in the furnace by opening the contact P1.

When contact L1 has opened, as previously described because of a fault condition, it is desirable that contacts L1 not be easily reclosed. Thus, contact L1 is provided preferably with means to be mechanically looked open. Once opened, they can only be reset mechanically which prevents the chance or electric reclosing of contact P1.

It will be noted that the circuit is also designed to disconnect the motor and/or ignition circuits in the event that the flame goes out after proper ignition. In this event with contacts P1, P2, and L1 closed, the lack of flame will cause the resistance of detector 52 to rise until transistor 48 turns on, supplying pulses to SCR 36. Therefore, with such pulses, conduction resumes through thermistor 34. When thermistor 34 heats to a suflicient degree, relay L operatesopening contact L1 (which locks itself open as hereinafter described), de-energizing relay LP, and opening contacts P1 and P2.

It should be noted that resistor 38 will be selected or set at values too high to bring in relays L or P if the thermistor 34 should fail as an open circuit.

In FIGS. 2, 3, and 4, the preferred design for relay L to achieve the desired feature of manual reset is shown. In these figures, relay L and its connections are conventional. The frame 100 which holds the relay L also provides a fulcrum on which armature 104 may pivot. Armature 104 is biased away from relay L by tension spring 106 attached at one end to arm 108 which projects from armature 104 and at the other end to a projection from frame 100.

Incorporating arm 108 extends upwardly on each side of fulcrum 102 to provide base plate 110 located over the relay. Mounting spacers 112 and top plate 114 are attached to base plate 110 in any desired manner, such as by rivets 116. Spacers 112 maintain the desired spacial relationship between contact arms 118 and 122 which are conventional. Arms 118 and 122 have contact points 124 and 126 respectively.

Contact arm 122 is mechanically linked to armature 104, as shown, for movement therewith. Thus, in the absence of current in relay L, contact 126 is held touching contact 124 under the control of armature 104 as biased by spring 106. Plate 114, approximately over contacts 124 and 126 is provided with a downwardly projecting prong 128. Armature 104 is provided with a spring metal extension 130 having an upwardly turned free end 131. End 131 is designed to rest inwardly of prong 128 when contacts 124 and 126 are closed and is shaped and resiliently constructed to move outwardly of prong 128 when extension 130 and armature 104 move downwardly together on the operation of relay L.

End 131 is further designed in combination with prong 128 that on any attempt by armature 104 and arm 122 to move upwardly on de-energization of relay L, it catches on the outside of prong 128 and interlocks therewith to prevent upward movement of contact 126 and movement of arm 122 toward closure of contact 126 with contact 124. Thus, contacts 124 and 126 cannot be closed electrically once opened. Mechanical means for resetting the relay to the normally closed position of the contacts will now be described.

A push button 136 is spring-mounted on plate 114 to overlay extension 130 in the vicinity of prong 128. A projection 140 from push button 136 extends toward extension 130 but is biased (in the absence of pressure on the push button) just out of contact with extension 130. However, with extension 130 bearing against prong 128 as shown in FIG. 3, contacts 124 and 126 may be caused to close by depression of button 136, whereupon extension 130 near prong 128 is depressed. On release of button 136, end 131 moves inwardly of prong 128 whereupon contacts 124 and 126 may close under the impulsion of spring 106.

Button 126 may be located in the immediate vicinity of the furnace, requiring attendence thereat before resetting can be performed.

Instead of the thermistor, any device may be used which has self-heating means, a negative temperature coefficient, and operable within the current and time limits required for the two intervals. Where a thermistor is used, it is preferred that Model B8-320-01A, 500 ohm NTC manufactured by Philips Electron Devices be used.

What is claimed is:

1. In a combustible fuel ignition system, the combination comprising: first and second power input leads; a thermostat switch actuable in response to the temperature of a space to be heated; a first relay having a winding and a pair of mutually engagable normally open contacts; a second relay having a winding and a pair of mutually engagable normally closed contacts; a negative temperature coefficient thermistor; a silicon controlled rectifier having an anode-cathode circuit including an anode and a cathode, said rectifier also having a gate; means responsive to the closure of said first relay contacts to supply fuel to a combustion space and to ignite the fuel; said thermostat switch, said second relay contacts, said first and second relay windings, said thermistor, and the anode-cathode circuit of said rectifier being connected in series between said power input leads; a flame detector; a safety control circuit connected from said power input leads and from said detector to said rectifier gate to control the current flowing in the anode-cathode circuit of said rectifier in accordance with the output of said detector, said second relay having a construction requiring manual closing of said contacts thereof after they have been opened, closure of said thermostat switch causing immediate energization of said first relay and conduction in the anode-cathode circuit of said rectifier, said thermistor being constructed so as to heat during said rectifier conduction, said second relay and said thermistor having a construction so as not to energize said second relay when said first relay is first energized, said control circuit limiting the current through said rectifier and preventing energization of said second relay if fuel ignition is detected by said detector within a predetermined time interval, said control circuit allowing sufiicient current to flow through said rectifier and said thermistor to energize said second relay and open the said contacts thereof when fuel ignition is not detected by said detector within said predetermined time interval, energization of said second relay causing deenergization 'of said first relay, opening of said first relay contacts and the shutoff of said fuel supply to prevent a hazardous buildup of fuel in the absence of the ignition thereof.

2. The invention as defined in claim 1, wherein a capacitor is connected in parallel with at least said second relay.

3. The invention as defined in claim 1, wherein a Zener diode is connected in parallel with the series connection of said first and second relay windings and said thermistor, said diode being poled in a direction opposite that from the cathode to the anode of said rectifier.

4. The invention as defined in claim 3, wherein a capacitor is connected in parallel with said Zener diode.

References Cited UNITED STATES PATENTS 3,380,796 4/ 1968 Kompelien 43169 X 3,364,972 1/ 1968 Deubel et a1. 43169 FOREIGN PATENTS 940,836 11/ 1963 Great Britain 431-69 CHARLES SUKALO, Primary Examiner W. C. ANDERSON, Assistant Examiner

Images