GB2196500A - Time delay pulse circuit - Google Patents

Abstract

A control circuit providing a predetermined energising current to a coil of a gas supply valve solenoid for a predetermined period after a predetermined delay comprises a capacitor (43) connected in series with a resistor (42) across a power supply. A switch (49) is connected in series with the load (50), and the capacitor is connected in parallel with the series connected load and switch. A comparator (44) compares the voltage across the capacitor with a reference voltage, and causes the switch to close when the voltage across the capacitor exceeds a predetermined voltage. When the switch is closed, the capacitor is discharged through the load. The impedance of the resistor may be such that the current through the resistor is at all times substantially less than the said predetermined energising current, or the resistor may be positioned so that current through it is directed away from the load. <IMAGE>

Description

SPECIFICATION Control circuit The present invention relates to control circuits and in particular to control circuits suitable for controlling gas-consuming appliances.

Gas fired appliances are provided with an automatic and fail-safe gas ignition system. In the majority of cases the gas ignition system relies upon the generation of a spark to ignite a stream of gas which is supplied through a solenoid operated valve.

Spark-generating systems are unfortunately relatively noisy, the noise being particularly obtrusive when the gas burning appliances are positioned in living rooms.

To avoid the problem of noise associated with spark ignition systems it has been proposed to ignite the gas stream using an electrically heated coil.

However it takes a considerable period of time for the known coils to reach a temperature at which gas ignition can be relied upon, the time taken being typically from a few to several tens of seconds. It would not be safe to allow significant volumes of gas to escape in the time taken for the coil to heat up and accordingly it is necessary to defer the opening of an appropriate gas valve for a preset period after the electrically heated coil is energised. This preset period may also be used in conjunction with other hardware on appliances incorporating a fan to dispel waste flue products. The fan is operated to purge the combustion chamber before ignition.

It is also necessary to check whether or not a stream of gas which has been released has in fact been ignited. If ignition is not detected, the gas stream must be shut off relatively quickly and accordingly it is necessary to turn the supply of gas off a predetermined period after the supply was turned on if ignition is not detected.

It is not particularly difficult to provide a control circuit with two independent timing circuits, a first timing circuit holding a gas supply valve closed for a period of for example 40 seconds and then opening the valve and the second timing circuit reclosing the valve after for example 3 seconds if ignition is not detected. The supply of gas can be controlled by a solenoid operated valve to which energy is supplied by a switching circuit controlled by the two timing circuits. It is however necessary to ensure that the second timing circuit can only fail to a shorter period and also to ensure that the switching circuit cannot latch in a condition in which energy is supplied continuously to the valve solenoid.Because of these various requirements conventional control circuits of the general type described above are comparatively complex and as a re sult are relatively expensive and can be unreliable.

It is an object of the present invention to obviate or mitigate the problems outlined above.

According to a first aspect of the present invention there is provided a control circuit for providing a predetermined energising current to a load for a predetermined period after a predetermined delay, comprising a capacitor connected in series with a resistor across a power supply, a switch connected in series with the load, the capacitor being connected in parallel with the series connected load and switch, and a comparator connected to compare the voltage across the capacitor with a reference voltage, the comparator being operative to close the switch when the voltage across the capacitor exceeds a predetermined voltage so as to discharge the capacitor through the load.

The impedence of the resistor may be such that the current through the resistor is at all times substantially less than the said predetermined energising current. Alternatively, the switch may be connected in series with the resistor across the power supply. The capacitor when charged represents the only effective power source for the switch and load. Thus, even if the switch fails and as a result is effectively short circuited, the current delivered to the load via the resistor cannot be sufficient to energise the load, either because the current carrying capacity of the resistor is too low, or because the available current in diverted away from the load.The duration of the predetermined period is dependent upon the capacitance of the capacitor and the impedence of the load and switch, whereas the duration of the predetermined delay is dependent upon the capacitance of the capacitor and the impedence of the resistor. Thus using only a small number of components two different periods can be effectively timed.

In a control circuit for a gas fired appliance, the load may be the coil of a relay which when energised causes a low rate gas supply valve to open, and a flame sensing circuit may be provided which is operative to open-circuit the resistor when ignition of gas delivered by the low rate supply valve is detected. The flame sensing circuit may control a relay having contacts connected in a circuit which bypasses the resistor and timing circuit.

According to a second aspect of the present invention, there is provided a control circuit for a gas consuming appliance, the control circuit comprising a fail safe logic circuit incorporating components which when energised open one or more gas supply valves, a temperature controller circuit which provides at.least one output indicating a demand for the supply of gas, and a solid state switchcontrolled by the said at least one output, the switch being connected in series with a power supply of the fail safe logic circuit such that in the absence of the output indicating a demand for the supply of gas the fail safe logic circuit is de-energised, thereby preventing gas being released.

Thus faults within the fail safe logic circuit cannot compromise the safe operation of the appliance in the absence of a demand for the supply of gas. Furthermore, additional components such as relays are not required to control the energisation of a gas supply valve control solenoid.

According to a third aspect of the present invention, there is provided a control circuit for a gas consuming appliance, the control circuit comprising a fail safe logic circuit incorporating first and second solenoid coils which when energised open respective low and high flow rate gas supply valves, a temperature controller circuit providing first and second outputs to control the first and second solenoid coils, the first and second outputs indicating respectively a demand for low and high rates of gas supply, and a flame sensing circuit controlling contacts which switch when flame is sensed, wherein said contacts are arranged such that the second solenoid coil is disabled except when the contacts have been switched as a result of detection of a flame.

Thus, gas cannot be released at a high rate during ignition of the appliance until ignition of a low rate flow of gas has been detected.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a flow chart explaining the operation of a control circuit in accordance with the invention incorporated in a gas consuming appliance; Fig. 2 is a schematic illustration of two gas supply control valves incorporated in the appliance the operation of which is illustrated in Fig. 1; Fig. 3 is a circuit diagram of a control circuit for an appliance incorporating the valves of Fig. 2; Fig. 4 is a circuit diagram of a temperature control and fail-safe logic circuit which controls relay contacts illustrated in Fig. 3; and Fig. 5 illustrates a modified circuit which can be used as an alternative to part of the failsafe logic circuit illustrated in Fig. 4.

The flow chart of Fig. 1 is related to a gas consuming appliance comprising a burner, a low rate gas supply valve which when open delivers gas to the burner at a predetermined relatively low rate, a high rate gas control valve which when open delivers gas to the burner at a relatively high rate, an electrically heated coil positioned adjacent the burner so that when heated to a sufficient temperature it will ignite gas issuing from the burner, and a flame sensing device which detects the presence or absence of a flame in the vicinity of the burner. All of these components may be entirely conventional. The consumer is able to set a temperature to which he wishes the appliance to heat the water in a boiler incorporated in the appliance.Once turned on the appliance automatically opens both the low and high rate valves if the temperature is below a lower limit, opens the low rate valve but closes the high rate valve if the temperature is above the lower limit but below an upper limit determined by the temperature selected by the consumer, and turns both the valves off if the temperature is above the upper limit. Again this mode of operation is entirely conventional.

Referring to Fig. 1, when the appliance has been turned on by the consumer the water temperature issensed. If the water temperature is not below the preset upper limit a timing circuit which controls a gas ignition sequence is not enabled. Thus effectively nothing happens until the sensed temperature has dropped below the upper limit. If the sensed temperature is already below the upper limit or falls below that limit the output of the flame sensing device is monitored. If this indicates that a flame is already present at the burner the timing circuit is again not enabled.

Sensing of a flame at this stage in the ignition sequence will normally indicate a fault in the flame sensing circuit.

If no flame is sensed the timing circuit is enabled and the ignition coil is energised. The timing circuit then times out a 40 second delay to give the coil time to heat up and then opens the low rate gas supply valve. If everything is operating normally the gas will issue from the burner and be ignited by the coil.

The output of the flame sensor is monitored. If no flame is sensed within 3 seconds of opening of the low rate supply valve that valve is closed and a manual power supply interruption is needed before a further attempt can be made. Thus, the possibility of the ignition coil failing is effectively taken care of.

If flame is sensed within the 3 second period determined by the timing circuit the timing circuit is effectively turned off and by-passed so that the low rate gas valve is maintained open. If the sensed temperature is below the lower limit temperture the high rate gas valve is opened. If the temperature then rises above the upper limit temperature then as the temperature rises first the high rate valve is closed and then the low rate valve is closed so as to effectively switch off all supply of gas to the burner. The system then re-cycles so as to initiate a new ignition sequence after the sensed temperature has fallen below the preset upper limit temperature. When on low gas the heat generated will often not be sufficient to retain the water temperature at the desired level and under these circumstances the appliance will cycle between low and high rate gas supply without going through the ignition sequence. This helps to keep the noise level down.

Fig. 2 illustrates one gas valve configuration which can be adopted. A low rate valve 1 is connected in series with a high-rate valve 2 in a main gas supply conduit 3. The high rate valve 2 is by-passed by a conduit 4 of limited cross section. Thus when valve 1 is open but valve 2 is closed the supply of gas is limited by the capacity of the by-pass tube 4. When both valves 1 and 2 are open however a high rate of supply can be achieved as the restriction represented by the tube 4 is removed from the supply line. In practice this "tube" is in fact a hole through a plate or wall.

Referring now to Fig. 3, a power supply, flame sensing and valve solenoid control circuit is illustrated. An ignition coil (not shown) is connected to terminal 5, the solenoid of the low rate gas valve 1 of Fig. 2 is connected to terminals 6, the solenoid of the high rate gas valve 2 is connected to terminal 7, and a flame sensing probe is connected to terminal 8. A 240v mains supply is connected to the live, neutral and earth terminals indicated by L, N and E.

The power supply comprises the components outlined by broken line 9. The power supply comprises a transformer with an isolated secondary so that a boiler thermostat probe can be used which is in electrical contact with water inside the appliance boiler.

The flame sensing circuit is outlined by broken line 10 and comprises the coil 11 of a flame sense relay. The flame sensing circuit employs the conventional flame rectification principle. If there is no flame at the flame sensing probe connected to terminal 8 then a AC signal which is derived from the output of the power supply transformer is passed to ground through capacitors 12 and 13. An operational amplifier 14 receives a biasing signal from resistors 15, 16 and 17. Thus no significant current flows through the coil 11. When flame is present at the flame sensing probe however one half of each cycle of the AC supply is passed through the flame to earth and thus a DC signal remains which causes the operational amplifier 14 to energise the coil 11.

When the appliance is turned on live mains voltage is applied to terminal 1. If contacts 18 and 19 are closed a glow coil power control circuit 20 is energised and in turn energises the glow coil connected to terminal 5 for a maximum period of 60 seconds. The control circuit 20 can be any circuit providing a current and voltage appropriate to the particular glow coil used for a period of for example 60 seconds. The illustrated circuit is for a 500 watt 120 volts silicon carbide glow coil.

The live terminal L is also connected to contacts 21. When the contacts 21 are closed the solenoid connected to terminal 6 is energised so as to open the associated low rate gas supply valve. If the contacts 21 are closed the live terminal L is also connected to contacts 22. When contacts 21 and 22 are closed the solenoid connected to terminal 7 is energised, thereby opening the high rate gas supply valve.

Referring now to Fig. 4, a temperature controller and fail-safe logic circuit is illustrated which is adapted to control the various contacts illustrated in Fig. 3 and is in turn responsive to energisation of the flame sense coil 11 of Fig. 3. Thus, Figs. 3 and 4 should be considered together.

The temperature controller of Fig. 4 comprises the components outlined by broken line 23. The 24 volt power supply derived by the power supply unit of Fig. 3 is applied to line 24 and pulled down to 15 volts by a Zener diode 25. This gives a reasonably controlled constant voltage supply. Two comparators 26 and 27 each give a high output when the respective positive input thereto is higher than the negative input. A temperature sensor 28 which may be for example a Mullard-type KTY81-110 provides a resistance which increases with increasing temperature. The temperature sensor is in thermal contact with the water inside the boiler The sensor 28 is connected in series with a resistor 29 and the temperature sensor output is taken from the common connection between the sensor and the resistor and supplied via a further resistor 30 to the positive inputs of the comparators 26 and 27.The negative inputs of the comparators are supplied from a chain of resistors including a potentiometer 31 which may be fitted remote from the remainder of the circuitry and operated by the consumer so as to adjust the demanded boiler temperature. The negative inputs of the two comparators are taken from different parts of the resistor chain so that they switch at different temperatures. Thus as the temperature rises the "high gas" output 32 of the comparator 27 goes high when the sensed temperature reaches a predetermined lower limit and the output 33 of the comparator 26 goes high when the sensed temperature reaches a predetermined upper limit. Thus the outputs 32 and 33 indicate the intended condition of the high and low rate gas supply valves The outputs 32 and 33 are applied to the fail-safe logic circuit which comprises the components outlined by broken line 34.The output 32 is applied to transistor 35 and the output 33 is applied to transistor 36. The outputs 32 and 33 switch the transistors 35 and 36 and are also used to feed back a bias voltage to the positive inputs of the comparators so as to adjust their switching points.

The resistors 37 to 40 control the amount of this bias so as to provide a "hysteresis" effect to the temperature differential of the boiler thermostat. This means that for example if one of the comparators switches when the temperature falls to 70"C the same comparator would not switch back until the temperature reached 70 plus a differential of for example 5"C, that is 75"C. It should be noted that the comparator 26 is used to switch the fail-safe logic circuit and achieve the necessary control without there being a requirement for an additional relay to switch one of the gas valve solenoids.The output from comparator 27 is used in conjunction with the fail-safe logic circuit to switch a relay comprising coil 41 which controls the high rate gas supply valve and also contributes to the control of the glow coil connected to terminal 5 by means of the contacts 19.

When transistor 36 is turned on by the output 33, that is when the output 33 is low, power is supplied to a timing circuit comprising resistor 42, capacitor 43 and comparator 44. Power is supplied through normally closed contacts 45 controlled by the flame sense coil 11 (Fig. 3) to the capacitor 43 through resistor 42. The capacitor and resistor are selected so that it takes approximately 40 seconds to build up the charge on the capacitor 43 to a level which is sufficient to cause the output of switch comparator 44 to go high. The comparator 44 is connected to a potential divider formed by resistors 46 and 47, and is also provided with a feedback resistor 48. When the output of comparator 44 goes high it turns on transistor 49. The transistor 49 is in series with a coil 50 which controls contact 51 (Fig. 4) and contact 21 (Fig. 3).When the transistor 49 turns on the transistor 49 and coil 50 are connected in parallel with the capacitor 43 and therefore the capacitor 43 discharges through the coil 50 closing contacts 21 and 51. Resistor 42 has a value sufficiently high to limit the current through the resistor 42 to a level substantially below the current required to cause the coil 50 to switch the contacts 21 and 51. Thus the capacitor 43 is effectively the only power supply to the relay coil 50. The capacitor 43 has a capacitance such that it discharges rapidly through the coil 50 so as to maintain effective energisation on the coil 50 for only approximately three seconds. Once the capacitor 43 has been discharged the resistor 42 cannot supply a current sufficiently high to maintain energisation of the coil 50.Thus the above described portion of the fail-safe logic circuit limits the time for which gas is released to the glow coil to 3 seconds. This 3 second period cannot be increased regardless of any component faults which may occur. The remainder of the fail-safe logic circuit relies upon the way in which the relay contacts respond to energisation of the various coils.

When the appliance is turned on and the sensed temperature is sufficiently low to indicate a heating demand, the comparator outputs are low and thus transistor 36 conducts and supplies power to the fail-safe logic circuit. Capacitor 43 starts charging. As the capacitor 43 charges up, the contact 45 controlled by the flame sensing relay coil 11 prevents power from reaching transistor 35 and thus the base of a transistor 53 is not clamped. Thus the transistor 53 conducts and energises coil 41 of the high rate gas flow relay. The contact 19 (Fig. 3) and the normally closed contact 18 which are controlled by the flame sense coil 11 deliver power to the glow coil via the power control circuit 20. The glow coil needs approximately 30 seconds to reach a temperature at which it can reliably ignite gas.It should be noted that the contact 22 has been opened by the coil 41 and power can therefore not reach the high flow rate valve solenoid connected to terminal 7. Assuming that the released gas is ignited during the 3 second inigition stage the relay 11 is energised. The contact 45 then moves to its alternative position (to the right in Fig. 4) and supplies power via contacts 51 to maintain energisation of the relay coil 50. This switching has to occur within the 3 second time limit provided by the discharge time of the capacitor 43. Thus the low rate solenoid connected to terminal 6 continues to be energised and gas continues to flow. The contact 18 controlled by the flame sense coil 11 opens and therefore cuts off power to the glow coil. Contact 45 of the flame sense relay is also closed and supplies power to transistor 35.The circuit is now in its normal operating condition and will remain so until the temperature of the boiler water rises to a level sufficient for the output of comparator 27 to go high. When the comparator 27 output is high transistor 35 is made conductive and then turns off transistor 53 and closes the high rate gas control contacts 22 (Fig. 3). The circuit will then cycle between low and high rate gas supply under the. control of the thermostat sensor 28. If the temperature continues to rise whilst gas is supplied at the low rate then eventually the comparator 26 will cut off the 24 volt supply to the fail safe logic circuit to thereby return the circuit to its normal start-up condition. When the temperature drops to the relevant pre-set temperature the start-up procedure is recommenced.

If during operation of the appliance the flame is extinguished then coil 11 is de-energised and the contact 45 returns to the position shown in Fig. 4 thereby cutting off the supply of power via contact 51 to the relay coil 50. Contacts 21 and 51 then open. Transistor 53 then conducts and relay coil 41 is energised so that the glow coil is energised for the 60 seconds allowed by the power control circuit 20. The start up procedure will be followed but gas will not be ignited and accordingly the energisation of the relay coil 50 cannot be maintained and therefore the low rate gas supply valve will be shut off.

The system cannot carry on cycling however because transistor 49 is conducting and there fore prevents capacitor 43 re-charging to the required operating voltage. As mentioned above the resistance of the resistor 42 is too great to enable direct energisation of the coil 50 through the resistor. The appliance is therefore shut down and needs a manual interruption of the electrical power supply before another attempt can be made to start up.

If any part of the flame sensing circuit fails so that a condition arises indicating the presence of a flame when in fact no such flame is present, for example if coil 11 of the flame sensing circuit is energised when it should not be, then the flame sense contact 18 prevents the glow coil from being energised and contact 45 cuts off the supply of power to the resistor 42 and the capacitor 43, thereby preventing the supply of gas through the low rate gas supply valve. The appliance is therefore shut down.

Referring again to Fig. 3, a broken line 52 is shown bypassing relay contacts 18 and 19. If the line 52 is not provided, the contacts 18 and 19 must carry the current necessary to energise the glow coil connected to terminal 5. This current can be considerable and there is therefore a risk of the relay contacts being damaged, e.g. welded together. To avoid this problem the line 52 can be provided, the voltage delivered through contacts 18 and 19, merely serving to control a solid state switch (not shown), for example a Triac, connected between the line 52 and the terminal 5.

Referring now to Fig. 5, an alternative circuit to part of the fail-safe logic circuit of Fig. 4 is illustrated. The same reference numerals are used for equivalent components in Figs. 4 and 5.

In Fig. 5, resistor 54 and Zener diode 55 provide a stable reference voltage to the comparator 44 from line 56 which carries the 24v output of transistor 36 (Fig. 4) when that transistor is conducting. The capacitor 43. is charged via resistor 42 and a diode 57, and the voltage on capacitor 43 is monitored by the potential divider formed by resistors 46 and 47. When the voltage at the junction of resistors 46 and 47 exceeds the reference voltage, the transistor 49 is turned on by comparator and the capacitor 43 is discharged into the relay coil 50. The low gas contacts 51 close. The capacitor is chosen so that the relay is energised for about 3 seconds before the capacitor becomes discharged and the relay drops out. If during this 3 second period the flame is sensed then the flame sense relay is energised and the normally open contact 45 closes. Thus the supply to the low gas relay is sustained via the relay contacts 45 and 51 and transistor 49. The only source of continuous current for the relay is via the flame sense normally open contact, therefore if flame is not sensed, the relay will only be energised for the 3 second ignition period.

If the flame sensing circuit is falsely sensing flame at the commencement of the safety logic timing period, then the capacitor cannot charge at all since resistor 42 has no supply due to the normally open contact 45 of the flame sense relay now being open.

Since resistor 42 is connected to the bottom of the relay coil 50, its resistive value can be made as low as desired since it cannot supply relay current directly. It is however necessary to provide diode 57 to bypass the capacitor charging current around the relay coil.

Claims (12)

1. A control circuit for providing a predetermined energising current to a load for a predetermined period after a predetermined delay, comprising a capacitor connected in series with a resistor across a power supply, a switch connected in series with the load, the capacitor being connected in parallel with the series connected load and switch, and a comparator connected to compare the voltage across the capacitor with a reference voltage, the comparator being operative to close the switch when the voltage across the capacitor exceeds a predetermined voltage so as to discharge the capacitor through the load.

2. A control circuit according to claim 1, wherein the impedence of the resistor is such that the current through the resistor is at all times substantially less than the said predetermined energising current.

3. A control circuit according to claim 1, wherein the resistor, load and capacitor are connected in series, and the switch is connected in series with the resistor across the power supply.

4. A control circuit according to claim 3, comprising a diode connected in parallel with the load, the diode being connected to pass current to the capacitor from the resistor but to prevent current from the capacitor by passing the load.

5. A control circuit according to any preceding claim, wherein the load is the coil of a relay which when energised causes a gas supply valve of a gas fired appliance to open.

6. A control circuit according to claim 5, wherein a flame sensing circuit is provided which is operative to open-circuit the resistor when ignition of gas delivered by the supply valve is detected.

7. A control circuit according to claim 6, wherein the flame sensing circuit controls a relay having contacts connected to a circuit which by-passes the resistor when flame is sensed.

8. A control circuit according to claim 5, 6 or 7, comprising a temperature controller circuit which provides at least one output indicating a demand for the supply of gas, and a solid state switch controlled by the said at least one output, the switch being connected in series with the said power supply such that in the absence of the output indicating a demand for the supply of gas the control circuit is de-energised, thereby preventing gas being released.

9. A control circuit according to claim 5, 6, 7 or 8 comprising first and second solenoid coils which when energised open respective low and high flow rate gas supply valves, wherein the flame sensing circuit controls contacts which switch when flame is sensed, the said flame sensing circuit contacts being arranged such that the second solenoid coil is disabled except when the contacts have been switched as a result of detection of a flame.

10. A control circuit for a gas consuming appliance, the control circuit comprising a fail safe logic circuit incorporating components which when energised open one or more gas supply valves, a temperature controller circuit which provides at least one output indicating a demand for the supply of gas, and a solid state switch controlled by the said at least one output, the switch being connected in series with a power supply of the fail safe logic circuit such that in the absence of the output indicating a demand for the supply of gas the fail safe logic circuit is de-energised, thereby preventing gas being released.

11. A control circuit for a gas consuming appliance, the control circuit comprising a fail safe logic circuit incorporating first and second solenoid coils which when energised open respective low and high flow rate gas supply valves, a temperature controller circuit providing first and second outputs to control the first and second solenoid coils, the first and second outputs indicating respectively a demand for low and high rates of gas supply, and a flame sensing circuit controlling contacts which switch when flame is sensed, wherein said contacts are arranged such that the second solenoid coil is disabled except when the contacts have been switched as a result of detection of a flame.

12. A control circuit, substantially as hereinbefore described with reference to the accompanying drawings.