GB2120807A - Burner control - Google Patents

Abstract

A control system for an oil burner comprises TRIACs 51, 57, 62 for switching on an air blower, an igniter and a solenoid valve for oil feed; solid- state drive circuitry for the TRIACs; circuitry for inhibiting the drive circuitry in response to a lock-out signal; and logic circuitry for providing control signals to the drive circuitry and a lock-out signal to the inhibiting circuitry. It determines, (a) the period for which the blower and igniter operate together to purge the nozzle, (b) the period for which the igniter remains operating with the solenoid valve also open (when a flame is normally established), and (c) the period for which the system is locked-out if a flame is not established when the igniter is turned off (or if a flame, once established, becomes extinguished). An indicator is energised when the drive circuitry is inhibited and a manual reset enables the ignition sequence to be restarted. <IMAGE>

Description

SPECIFICATION Burner control The invention relates to systemsforcontrolling oil burners.

According to the invention an oil burner control system comprises a thermostat which energises the system upon a call for heat; first, second and third solid-state switching means for coupling an air blower motor, an ignition device and a solenoid valve, respectively, to a power source; first solid-state drive means arranged to actuate the first switching means upon energising ofthe system; second and third solid-state drive meansfor actuating the second and third switching means, respectively, in response to respectivedrivesignals; afirsttiming meansresponsiveto energising ofthesystem and arranged to provide the drive signal forthethird drive means after a first periodandto trigger a second timing means arrangedto provide the drive signal forthe second drive means from energising of the system until the end of a second period; means responsive to the drive signal provided bythe second timing means and to the signal from a flame sensor and arranged to provide a first inhibiting signalforinhibiting all three drive means when the ignition sequence hasfailed to establish a flame or when an establishedflame goes out; third timing means responsive to the first inhibiting signal for providing a second inhibiting signal for inhibiting all three drive means for a third period (known as a lock-out period); and a manuallyoperable reset means arranged to resetthefirsttiming meanstostartafresh ignition sequence.

There may be an interlock logic circuit to prevent the first timing means from being reset until after the end ofthe lock-out period.

The means arranged to providethefirst inhibiting signal may be in the form of anothertiming means which provides the first inhibiting signal at the end of another period unless disabled by a "flame absent" signal from the flame sensor.

There may be provided an indicatorarrangedto be energised whilstthe three drive means are inhibited.

Each ofthe drive means may comprise a resistor arrangedtosupplyto a control electrode of its associated switching means from a voltagesupply line, the resistor being coupled by a respective clamping diode to the output of a transistor switch responsive to an inhibiting signal. There may be first and second such transistor switches responsive to the first and second inhibiting signals respectively. Alternatively, there may be a singletransistorswitch with logic gating circuitryfor receiving the first and second inhibiting signals.

Each of the switching means may comprise a TRIAC.

One oil burner control system embodying the invention will now be described with reference to the drawings, in which: Figure 1 shows the logic circuitry of the system; Figure 2 shows interface circuitrywhich controls the burner ignition, motor and valve in response to signals from the logic circuitry; and Figure 3 is a circuit diagram of the power supply and flame sensor.

The control system illustrated is suitable for use in a central heating system. It is only one example of forms which the invention mighttake.

A brief description of the operation of the control system will be given first, followed by a more detailed description of the circuitry.

The system is energised by means of a thermostat which can be positioned in a room to sense air temperature or on a water cylinder ortankto sense the temperature of the water. Initially the motor of the air blower and the ignition are energised for a period of about 15 secondswhilstthe solenoid valve remains closed, in orderto purge the nozzle region. The solenoid valve is then opened and a steadyflame is established atthe burner nozzle, its presence being indicated by the signal from an optical sensor. About 12 seconds after the solenoid valve was opened the ignition is switched off, provided that the flame is established, andthe burner continues until the desired temperature is reached and the thermostat switches the system off.

If the flame is not present atthe end of the 12-second period, then the ignition, the motor and the solenoid valve are switched off and an indicator is brought on to showthat no flame is present. Afeedback signal keeps these elements switched off for a period of about 75 seconds, after which the control sequence can be started by means of a reset button. The system is also switched off and disabled in the eventthatthe flame, once established, is extinguished, for example by excessive down draught.

Considering first Figure 3, in normal operation the system is initially energised bythe closing ofthermostat contacts 10 which are connected to one end of the primary winding 11 of a mains transformer 12 and to a neutral mains terminal 13 via a resistor 14. The other end of winding 11 is connected to a live mains terminal 15. Also connected to terminal 15 are output terminals 16, 17 and 18 which are connected to the ignition device (not shown), the motor (not shown) and the solenoid valve (not shown), respectively, which are connected to respectiveTRIACs referred to later in the interface circuitry of Figure 2.

The transformer 12 has a centre-tapped secondary winding 20 which feeds a pair of diodes 21 arranged forfull-wave rectification with their anodes connected to an outputterminal 22. A capacitor 23 is connected between terminal 22 and an output terminal 24 which is connected to the centre-tap of the secondary winding 20. Afurther secondarywinding 25 is also provided: this feeds a bridge rectifier 26 whose output is connected to an output terminal 27 and the output terminal 22. A smoothing capacitor 28 is connected between output terminals 22 and 27.

The transformer is selected so that, relative to terminal 24,terminal 22 provides a 3-volts negative power supply line and terminal 27 provides a 12-volts negative power supply line.

A bridge rectifier 30 has its input terminals con nected across the resistor 14 and its output terminals connectedtoasmoothing capacitor31. Connected in a series circuit across the capacitor31 are a lightresponsive resistor 32, a LED transmitter 33 of an optical coupler, and a resistor34, comprising a flame sensor circuit.

If desired, resistor 32 can be responsive to infra-red radiation.

Thus, upon closing of the thermostat contacts 10 the powersupply lines are energised, and so is the flame sensor circuit.

Considering now Figures 1 and 2, line 35 is connected to terminal 24, line 36 is connected to terminal 27 (-12 volts) and line 37 is connected to terminal 22 (-3 volts).

Capacitor 38 is directly connected to line 36 and is connected to line 35 via series-connected resistor 40 and variable resistor 41. The junction of capacitor 38 and resistor 40 is connected via resistor 42 to the inputs of NAND gate 43 whose output is connected via resistor 44to the inputs of NAND gate 45 and via resistor46to the inputs of NAND gate 47. The output of gate 47 is connected to a line Vwhich is connected via resistor 48 to the base of a control transistor 49 whose emitter is connected to line 35 and whose collector is connected via a load resistor 50to line 37.

ATRIAC 51 is connected between line 35 and a terminal 52 to which the solenoid valve (not shown) is connected. The gate of TRIAC 51 is connected via a pair of series-connected forward-biased diodes 53 to the collector transistor 49 via a resistor 54to line 35.

Avoltage-dependent resistor 55 is connected between terminal 52 and line 35.

Initially, the capacitor 38 is in a discharged state having discharged through the power supply via a diode 56 connected across resistors 40 and 41. This renders the inputs of gate 43 high (i.e. towards line36 relative to line 35) fora time delay determined by capacitor38 and resistors 40 and 41. This delay is set at approximately 15 seconds in this particular embodiment.

Thus,the output of gate 47 is high during this delay, and transistor 49 is conducting, so that there is no currentflowthrough diodes 53to switch on TRIAC 51 and the solenoid valve remains unenergised.

AsimilarTRIAC control forthe blower motor (not shown) comprises TRIAC 57, resistor 58, voltagedependent resistor 59, a pair of diodes 60 and terminal 61; and a further similarTRIAC control forthe ignition device (not shown) comprises TRIAC 62, resistor 63, voltage-dependent resistor 64, a pair of diodes 65 and terminal 66.

Diodes 65 are connected to the collector of a transistor 67 having a load resistor 68 and a base resistor 69 in the same manner astransistor49, but diodes 60 are coupled to line 37 via a load resistor70 with no corresponding control transistor so the blower motor becomes energised as soon asthe power lines become established upon closing of switch 10.

The output of gate 45 is connected via a resistor 71 anda clamping diode 72to another RC delay circuit comprising capacitor73, resistor74, and variable resistor 75, which is set to provide a delay of approximately l2seconds. The junction of capacitor 73 and resistor 74 is connected via resistor 76 to the inputs of a NAND gate 77 whose output is connected to a lineTwhich is connected to base resistor 69 of control transistor 67.

During thefirst delay period of 15 seconds the output of gate 45 is high and clamps the input of gate 77 high so that its output on line Tis low and control transistor 67 is non-conducting. TRIAC 62 is thus on and the ignition device is energised.

Gate 77 is also connected via a resistor 78 to the inputs of a NAND gate 79 whose output is connected via a resistor 80 and a series-connected clamping diode 81 to a capacitor 82 connected to line 36. The junction of diode 81 and capacitor 82 is connected via resistor 83 to the inputs of a NAND gate 84 whose output is connected via resistor 85 to the inputs of a NAND gate 86 whose output is connected to a line B; and this junction is connected to line 35 via seriesconnected resistors 87 and 88.

The receiver part 89 ofthe optical couples is connected between line 36 and the junction of resistors 87 and 88.

The low output of gate 77 drives the output of gate 79 high which clamps the input of gate 84 high, so causing line B to be high. This signal is fed via a resistor 90 to the base ofan NPN transistor91 whose emitter is connected to line 36 and whose collector is connected to line 35 via a resistor 92, to the base of a PNP transistor 93 via a resistor 94 and to the base of a PNPtransistor 95 via a capacitor 96.

The emitters oftransistors 93 and 95 are connected to line 35. Transistor 93 has a load resistor 97 connected to line 36, and its collector is connected to diodes 53,60 and 65 via respective diodes 98,99 and 100, and to a forward-biased diode 101 which in turn is connected to line 36 via series-connected resistor 102 and LED 103.

Thus transistors 91 and 93 are non-conducting and diodes 98,99 and 100 are reverse-biased via resistor 97 and do not disable the TRIAC drives.

The collector of transistor 95 is connected to line FB feedbackwhich is connected via diode 104and resistor 1 OS to the input of NAND gate 106, and the junction of diode 104 and resistor 105 is connected via parallel-connected resistor 107 and capacitor 108 to line 36. The output of gate 106 is connected to line A which is connected to the base of a PNPtransistor109 via a resistor 11 0, the collector is connected to line 36 and the emitter is connected to line 35 via a resistor 111 and to the base of a PNPtransistor 11 2via a resistor 113. Transistor 112 has a load resistor 114 connected to line 36 and its collector is connected to diodes 53,60 and 65 via respective diodes 115,116 and 117 and to resistor 102 via diode 118.

Initially, transistor 95 will be non-conducting because transistor91 is non-conducting, and the input of gate 106 will be held high via resistors 105 and 107.

Thus line Awill be low and transistors 109 and 112 will be non-conducting and diodes 115,116 and 117 are reverse-biased via resistor 114 and do notdisablethe TRIAC drives.

At the end ofthe first delay period of 15 seconds the input of gate 43 goes low and hence the signal on line V goes low resulting in control transistor 49 becoming non-conducting andTRIAC 51 switching onto ener gise the solenoid valve forthe flow of oil to the nozzle.

Also, the output of gate 45 goes low and removes the clamping action on the input of gate 77 and allows the second delay period of 1 2 seconds to begin, with capacitor 73 charging via resistors 74 and 75.

Atthe end ofthis second delay period the input of gate 77 goes low and the signal on line goes high which results in control transistor 67 turning on.

TRIAC 62 is thus switched off and the ignition device is de-energised. Also, the output of gate 79 goes low and removes the clamping action of the input of gate 84 and allows a further delay period of about 1/4 second to begin, with capacitor 82 charging via resistors 87 and 88 intheeventthattheflame has notestablished itself and receiver 89 is thus non-conducting.

Atthe end of this further delay period (i.e. for no established flame) the input of gate 84 goes low and the signal on line B goes low which turns on transistor 91. This results in transistor 93 turning on and switching off all the TRIACs via diodes 98,99 and 100, and energising the alarm LED 103. Also capacitor96 starts to charge via the base-emitterjunction of transistor 95 causing itto conduct while capacitor 96 charges, and pull the input of gate 106 lowvia diode 104. The signal on line Athus goes high which causes transistors 109 and 112 to conduct whereby diodes 115,116and 117 acttoswitch offtheTRlACs,and LED 103 is also energised via diode 118. The collector load for transistor 95 is resistor 107 and capacitor 108 which charges two nearly 12volts.

When capacitor 96 becomes fully charged, there is no base currentfortransistor 95 which then ceases conducting and allows capacitor 108 to start discharging via resistor 107 backto its normal level. The delay set forthis is approximately 75 seconds and during this period the signal on line A remains high thus inhibiting any action of the TRIAC circuits in response to signals on lines T and V, and energising the alarm LED 103.

A manual reset push-button 119 is connected across capacitor 38 and, upon actuation, shortcircuits capacitor 38 to provide a high signal to the input of gate 43. Thus gates 45 and 79 provide high outputs which discharge capacitors 73 and 82 via diodes 72 and 81, respectively, and the ignition timing sequence is readyto recommence.

The signal on line B goes high when push-button 119 is actuated, but as explained,the signal on line A will inhibitthe TRIAC circuits for approximately 75 seconds. If an operator actuates the push-button 119 during this period, it will be immediately apparent because the alarm LED 103 will not extinguish.The operatorwillremainpressingthepush-button 119 until at the end ofthis period the signal on line A goes low and the LED 103 extinguishes.

Ifa flame establishes normally during the ignition sequence, then receiver 89 ofthe optical couplerwill be conducting and will provide a high signal via resistor 87 tothe input of gate 84. Thus atthe end of the furtherdelay period the system is in a steady state with the blower on, the solenoid valve open, the ignition device offand an established flame.

Should for any reason the flame sensor circuit determine that the flame is not present (this will normally be dueto toss of radiation falling on resistor 32, but includes a component failure in the flame sensor circuit), then the receiver 89 of the optical couplerwill be non-conducting and capacitor 82 will charge rapidly via resistors 87 and 88. Thus a low signal will be present on line B and the system will be "locked-out" as described for the period determined by capacitor 108 and resistor 107.

After the end ofthis lock-out period the signal on lineAwill be low but the signal on lineBwill below, so the system remains off with LED 103 energised.

The ignition sequence will now startthe instant the operator presses the reset push-button 119.

In the above-described system capacitor 38 will start charging via resistors 40 and 41 as soon as the operator releases the reset push-button 119. Thus if he attempts a restart during the lock-out period he will have to keep the reset push-button actuated until the LED 103 extinguishes in order to start the timing sequence at the correct time. If the push-button 1 is released sometime before the end ofthe lock-out period then the 15 second purge period could finish without any purge action having taken place, and the system could go straight into the ignition phase as the signal on line A goes low.If it desired to ensure that capacitor38can be discharged by the reset pushbutton 119 only after the end of the lock-out period, then a suitable logic circuit could be included which would inhibit the reset action while the signal on line Awas high.

The system is switched on when the thermostat switch 10 opens, and the capacitors then discharge.

Diode 56 provides protection forthe input circuitry of gate 43 when the voltage on line 36 collapses and the positive plate of capacitor 38 tries to go more positive than line 35. Gates 77 and 84 have their input circuitry similarly protected by diodes 120 and 121, respectively.

In the above described system the signals on lines Aand Bfor inhibiting theTRIACs drive separate transistors 93 and 112. If desired, a single transistor could be used with appropriate gating circuitry receiving the signals on lines A and B.

Although TRIACs have been used in the above circuits to switch the solenoid valve etc., other solid-state switching devices-for example silicon controlled rectifiers-could be used with appropriate circuitry.

The described system has, apartfromthethermostat switch and the manual reset, no mechanical contacts or moving parts. The switches and their drive circuits, and the logic timing circuits, are all based on solid-state devices.

Claims (9)

1. An oil burner control system comprising a thermostat which energises the system upon a call for heat; first, second and third solid-state switching means for coupling an air blower motor, an ignition device and a solenoid valve, respectively, to a power source; first solid-state drive means arranged to actuate the first switching means upon energising of the system; second and third solid-state drive means for actuating the second and third switching means, respectively, in response to respective drive signals; a firsttiming means responsiveto energising ofthe system ancI a rrangecita provide the drivesignalfor the third drive means after a first period and to trigger a second timing means arranged to provide the drive signal forthe second drive means from energising of the system until the end of a second period; means responsive to the drive signal provided by the second timing means and to the signal from a flame sensor and arranged to provide a first inhibiting signal for inhibiting all three drive means when the ignition sequence has failed to establish a flame orwhen an established flame goes out; third timing means responsiveto the first inhibiting signal for providing a second inhibiting signal for inhibiting all three drive means for a third period (known as a lock-out period); and a manually-operable reset means arranged to resetthefirsttiming means to start a fresh ignition sequence.

2. A system according to Claim 1 and comprising an interlock logic circuit to prevent the first timing means from being reset until after the end ofthe lock-out period.

3. AsystemaccordingtoClaim 1 orClaim2 wherein the means arranged to provide the first inhibiting signal is in the form of another timing means which provides the first inhibiting signal atthe end of another period unless disabled bya "flame absent" signal from the flame sensor.

4. Asystem accordingto anyofthe preceding Claims wherein there is provided an indicator arranged to be energised whilst the three drive means are inhibited.

5. A system according to any of the preceding Claims wherein each of the drive means comprises a resistor arranged to supply to a control electrode of its associated switching means from a voltage supply line, the resistor being coupled bya respective clamping diode to the output of a transistor switch responsive to an inhibiting signal.

6. A system according to Claim 5 and comprising first and second such transistor switches responsive to the first and second inhibiting signals respectively.

7. A system according to Claim Sand comprising a singletransistorswitch with logicgating circuitry for receiving the first and second inhibiting signals.

8. A system according to any of the preceding Claims wherein each of the drive means comprises a TRIAC.

9. An oil burner control system substantially as described herein with reference to and as illustrated in the accompanying drawings.