GB2131636A - Gas flow control circuits - Google Patents

Abstract

A gas flow control circuit comprises a serial combination of SCR, an intermediate resistance (RF) which is preferably fusible and a gas control solenoid (SOL). A diode is connected in parallel with the solenoid (SOL). The anode of the SCR is connected to one end of the resistance (RF) and the anode of the diode is connected to the other end of the resistance (RF). <IMAGE>

Description

SPECIFICATION Gas flow control circuits The present invention relates to circuit arrangements by which the flow of gas is controlled by electrically driven solenoid valves. The intention is to provide a marked improvement in reliability in the solenoid output drive circuits while still maintaining the necessaryfail-safe characteristics required to prevent possible accumulations of unburnt gas and explosions which might result should such aflow control valve be unintentionally be held open and fail to close.

Solenoid valves are frequently driven by silicon controlled rectifiers, a three electrode semiconductor switch in which one electrode is commonly called the anode, one the cathode and the third electrode the gate. It has the propertythatwhen an AC source of power is applied in series with a load it can be switched into a conducting or low impedance state by applying a signal to the gate electrode with respectto the cathode and when the anode voltage is positive with respect to the cathode in what is known as the forward state. If however a signal is not applied to the gate the SCR remains in a non conducting state even while the anode is positive. While the anode is negative, or in the reverse state, the SCR remains non-conductive.and at a high impedance. However if the negative anode voltage exceeds the maximum rated negative value it may break down and destroy the SCR.The common failure mode being for itto becomeshortcircuit. Exceedingtheforward blocking voltage, or maximum rated forward voltage permitted in the non-conducting condition; that is with the anode positive but with no triggering signal present on the gate; the SCR may again break down in this case non destructively switching the SCR ON. Once switched ON at any ti me the SCR continues to conduct for as long as the anode positive and the currentflow exceeds a certain minimum value known as the holding current.

The breakdown voltages, both forward and reverse, of an SCR are found to be stable during its life and therefore it has been found that a circuit based on that shown in figure 1 is failsafe.

In this circuitthe SCR is switched on by the gate and conducts on the positive half cycle, actuating the solenoid. On the negative half cycle the SCR is in the non-conducting or blocking state. Ifthe solenoid is inductive the back emf generated bythe decaying currentflowing in the solenoid forward biases the flywheel diode. Thus current continues to flow in the solenoid coil during the negative half cycle holding the solenoid open and, in solenoids with suitable inductance, preventing audible noise. If however, the SCR should fail and become short circuited it will short circuittheAC supplyviatheflywheel diode during the negative half cycle and blowthe fuse. Thus in practice this circuit has proved flail-safe to a sufficient degree to permit its use in mass production to control the gas flow in domestic appliances.It is also usd in industrial circuits where a substantial gas flows are controlled.

Usually a requirement to avoid the possible failure mechanically ofthe solenoid is that two valves must be connected in seriesforthe gas flow (double block) and in parallel electrically. In place ofthe solenoid various alternative loads can be applied. For example a relay or series resistor and relaywith a hold in capacitor connected across the relay.

See Figure 2.

Various circuit variations have been tried but each has exhibited problems in application.

Overa number of years we have used variousfuses.

Initially a glass cartridge fuse. Laterthe electrical authorities required that a ceramic encased fuse be used instead. A wire wound resistorwas specifically designed so that a resistance of a few ohms would limit the fault current (5 ohms on 240 volts) and in blowing it would not arc and cause a fire hazard.

Recent developments in non-flammable enamels for coating resistance elements have resulted in the availability offusible or non flammable resistors which when overloaded by more than an order of magnitude above their normal ratings do not arc and fail open circuitwithout hazard. One such resistor is the Philips NFR25 resistor which is rated at one quarter of a watt continuous dissipation. Al ohms NFR25 resistor has proved effecient as such a fuse.

In the circuit of failure 1 it is evidentthat no terminal is common between the AC supply and the solenoid.

Although it is the obvious place to connectthe fuse so that in the event of a short circuitthefuse opens the active supply line a customer may connect the circuit in such a way that the solenoid can bypass the fuse by connecting itfrom the active supplytothe anode of the SCR. Ifthe3CR becomesshortcircuitthefuse is blown via theflywheel diode butthesolenoid isthen connected to the full AC supply via the short circuit SCR and it remains open.

Figure 3 shows a later circuit wherein the fuse is in the neutral supply line ensuring that the circuit is still failsafe and giving the advantage ofthe relay sharing the active terminal.

Other circuits in a gas controller also share the active supply. The active input is also required to drive the gas spark circuit, the flame detector, the logic control and timer power supply and finally provide any synchronizing signals required for SCR gate drives. All ofthese can be done with very low currents and will use resistors which are only known to fail to a high impedance or open circuit. None of these require protection bythefuse. In figure 3 R1 represents these various signals required from the active input.

However, this circuit has a potentially dangerous failure mode in that the operation of the circuit depends on the detection offlame presence by electronic means. This method ofdetectingtheflame is frequently by flame rectification such a method having been described in Australian Patent Applications Nos. 19555/76 and 19556/76. In these circuits an AC voltage is applied to an electrode positioned in the flame where the rectifying properties of the flame generate a negative DC voltage which is superimposed on the applied AC signal. In detecting the negative DC offset,the control circuitcan control the ignition and flow of the gas.

If the fuse Fwas to tend to drift to a high resistance value it is possible that the positive current flowing through the solenoid and its controlling SCR would give the control circuit a positive offset voltage with respectto the Neutral and Active supply. It is evident that if Active and Neutral were exchanged the control circuit would still adopt a positive potential with respectto the average of the AC supply. If the reference point is reversed it can be seen that in effect active and neutral have necome negative with respect to the control circuit and may falsely simulate a flame.

Further two the above considerations with respect to the placement ofthe fusible element in the circuit the reliability, or ratherthe inherent lack of reliability of this circuit has become a problem especially in country areas where it is not practicable to maintain close mains supplyvoltage limits over long transmission lines and where thunderstorms and other atmospherically generated transient effects exceed the SCR breakdown voltagethusfailure ofthecontroller results.

It is the object ofthis invention to not only overcome the problems which arise from the nature and position ofthe fuse in the circuit butto also substantially upgrade the circuit reliability. In doing thisthe purpose isto attempt to avoid having directly connected between active and neutral the two semiconductor junction devices oftheflywheel diode and the SCR. It is not a situation which one would accept happily having solid state junctions directly connected across such a low impedance power source as the AC supply.

After unsuccessfully attempting to include a series resistive impedance in the supply in the hope that such a resistor would effectively decouple anytransientvoltages, which applied across the SCR would result in its destruction, the circuit arrangement described by this invention was realized. It appears to overcome all the problems outlined above and also overcame the unsuccessful circuit trials where series impedances so limited the fult current that it was insufficientto blowthe fuse within a safely shorttime delay.

The circuit arrangement in accordance with this invention comprises the combination of the series impedance with the properties ofthefusible resist ance and to incorporatethis fuse and decoupling circuit in the anode of the SCR ratherthan in the active or neutral supply line. Thus all the requirements of such a circuit are metwithout any of those disadvantages outlined above.

Therefore in an embodiment of the present invention the circuit arrangement comprises a serial combination ofthe SCR, an intermediate resistance and a diode and gas control solenoid in parallel connected across alternating current supply mains terminals, with the a node terminal ofthe SCR connected to the intermediate resistor and the anode ofthe parallel solenoid diode being connected to the other end of the intermediate resistance.

Thus this circuit provides an electronic control of gas flow which is fail-safe in the eventoffailure ofthe SCR. The resistor is such that in the event of failure of one ofthe circuit components increased dissipation of power in the resistor results in itfailing open circuit without arcing or generating flame.

In anotherembodimentthesolenoid can replaced by a relay and further a charge storage capacitor can be connected in parallel across the relay to hold the relayclosed during the negative half cycle when the SCR is non-conducting.

In another embodiment a resistor can be connected in series with the parallel combination of relay and capacitor in orderto match the supply voltage to the voltage requirementofthe relay coil.

The circuit can further be modified by connecting a voltage dependant resistor or othervoltage dependant non-linear resistor or a resistance capacitance or series combination in parallel across the SCR. This includes the use of capacitor devices in which series resistance is inherent in the design ofthe capacitorto limit peak currentf!ow.

A resistor capable of continuous dissipation of 1 watt in series with a solenoid which typically consumes a current of 50mA can be shown to have a value of 1 watt = 12R = (0.05)2 R R =400 ohms.

An impedance of this value is sufficientto protect the SCR to some extent from overvoltage and transient effects. However if this is felt to be insufficient protection further measures can be adopted which rely on the isolating impedance provided by this resistor.

It is accepted practice to supply protective devices to the active and neutral input lines, and in particular an interference filter capacitor with a mains protection voltage dependant resistor is normally used connected across from active to neutral. Using the anode resistor in accordance with this invention fu rthertransient or over voltage filtering can be applied across the anode to cathode of the SCR. An over-voltage protection device such as a second voltage dependant resistor possibly with asmall transient filtering capacitor as shown in figure Swill provide a marked improvement in reliability.

Typical components used in the circuit shown in figure5wouldbeasfollows: Cm Capacitor 0.047MF250VAC Philipstype 222233040473 Rm VDR 250V Philips type 232259272512 Rf Fusible Resistor 100 Ohm 1 Watt Noble RSFB1 Flywheel BYV95C Philips diode Solenoid Goyen 240V AC Type 10AWL SCR BT149E Philips RSVDR 250V as above RL Resistor 100 ohm 1/2 watt Cs Capacitor 0.01 0M F 250VAC Philips type 222233040103 Typical Components Types and Values used in figure 5. RL is required as an inrush current limiting resistorto protectthe capacitor Cs and the SCR should the SCR be switched on when Cs is charged.

Claims (8)

1. A circuit arrangementto electronically control gas flow comprising a serial combination of a SCR, an intermediate resistance and a diode and gas control solenoid in parallel connected across alternating currentsupply mains terminals, said SCR having its anode terminal connected to the intermediate resis torandtheanodeoftheparalleldiode being connected to the other end of the intermediate resistance.

2. A circuit arrangement as claimed in claim 1 but with the solenoid replaced by a relay.

3. A circuit arrangement as claimed in claim 2 with a charge storage capacitor connected in parallel across the relayto hold the relay closed during the negative halfcyclewhen the SCR is non-conducting.

4. A circuit arrangement as claimed in claim 3 with a resistor in series with the parallel combination of relay and capacitor in order to match the supply voltage to the voltage requirement ofthe relay coil.

5. A circuit arrangement as claimed in any one of claims 1 to 4 wherein a voltage dependent resistor or othervoltage dependent non-linear resistor is connected in parallel across the SCR.

6. A circuit arrangement as claimed in any one of claims 1 to 5 wherein a resistance capacitance series combination is connected in parallel across the SCR.

7. A circuit arrangement as claimed in claim 6 wherein said resistsnce capacitance series combination comprises capacitor devices in which series resistance is inherent in the design ofthe capacitor to limit peakcurrentflow.

8. A circuit arrangement for controlling the flow of gas substantially as described herein with reference to figure 4 or 5 ofthe accompanying drawings.