GB2034134A - Improvements relating to circuit protection arrangement - Google Patents

Abstract

A circuit arrangement includes a power consuming load 1 having connected in series with it across a power supply 2; a switch 4 which under normal operating conditions of the circuit arrangement is operated by regularly recurring pulses Vc produced in a pulse generating circuit 5-11, whereby load 1 is operated by current pulses of such amplitude and frequency as to afford an average current value corresponding to the rated current value of the load and current sensing means 3 operable to disconnect load 1 when the average current therethrough exceeds a predetermined value; the pulse generating circuit including circuit protection means effective in the event of mal-operation or breakdown of components to provide an abnormal control input to switch 4 so that the average current level in the load circuit either falls thereby causing the load to be rendered inoperative or the current rises so that the current sensing means operates to disconnect the load from the supply. <IMAGE>

Description

SPECIFICATION Improvements relating to circuit protection arrangements This invention relates to protection arrangements for providing fail-safe conditions in the event of circuit breakdown or failure.

According to the present invention as broadly conceived there is provided a circuit arranged including power consuming load means having connected in series with it across a power supply switching means which under normal operating conditions of the circuit arrangement is operated by regularly recurring pulses produced in a pulse generating circuit whereby the load means is operated by current pulses of such amplitude and frequency as to afford an average current value corresponding to the rated current value of the load means and current sensing trip means operable to disconnect the load means from said supply when the average current through the load means exceeds a predetermined value, the pulse generating circuit including circuit protection means effective in the event of mal-operation or breakdown of components in said circuit arrangement or in control means for the aforesaid power consuming load means to provide an abnormal control input to the switching means so that the average current level in the load circuit either falls thereby causing the load means to be rendered inoperative or the current rises to that the current sensing trip means operates to disconnect the load from the supply.

In operation of the circuit arrangement according to the present invention it is expected that the failure or malfunction of components in the pulse generating circuit or control circuit will either cause a continuous signal input to be supplied to the switching means which will accordingly be held operated continuously so that the current sensing trip means will be operated to disconnect the load means or there will be zero input to the switching means whereby the switching means disconnects the load means from the supply. There may also be a significant lengthening of pulses applied to the switching means which will cause an increase in the average current of the load circuit which again will cause the current sensing trip means to respond in order to disconnect the load from the supply.

In carrying out the present invention the switching means may comprise a silicon controlled rectifier or triac but other forms of switching devices such as transistors, thyristors, reed and other relays could possibly be used according to the nature of the load means and the response characteristics (e.g. switching speed) of the switching means.

The pulse generating circuit may comprise oscillator means which for the purpose of ensuring a relatively constant average current level in the load circuit under normal operating conditions will need to be related to the mains supply frequency when the load is supplied from an alternating current supply. If a direct current supply is utilised, however, no such relationship is necessary.

For the purpose of achieving fail-safe conditions it is preferred to provide in the pulse generating circuit two oscillators having phasedisplaced pulse outputs these oscillators feeding their pulse outputs into respect "AND" gates. The other inputs to these "AND" gates may be derived from control circuit means for the load means whilst the respective outputs from the "AND" gates are applied to an "EXCLUSIVE-OR" gate.

This gating circuit arrangement ensures that when the two oscillators are functioning and continuous signals are derived from the load control circuit means then the "AND" gates provide output pulse trains the pulses of one train being phasedisplaced by a small amount from the corresponding pulses of the other pulse train.

These pulse trains are fed to the "EXCLUSIVE-OR" gate which itself produces a pulse train output the pulses of which have a duration corresponding to the phase difference between the two input pulse trains. It is this pulse train that operatively controls the switching means. Any failure or malfunctioning of one or more of the oscillators, gates or control circuit means will cause the "EXCLUSIVE-OR" gate to provide a continuous or zero signal output which in turn will cause the load means to be disconnected from the supply. A similar outcome will result from the lengthening of the pulses supplied to the input of the switching means.

Use of the present invention to provide fail-safe in control circuits for gas cookers and gas-fired boilers is especially contemplated but it could clearly be used in other applications such as motor, heater and solenoid control circuits.

By way of example the present invention will now be described with reference to the accompanying drawings in which: Fig. 1 shows a schematic circuit diagram of a fail-safe control circuit arrangement for supplying power to a power consuming load; Fig. 2 is a pulse diagram showing the waveforms of various pulses produced in the circuit diagram of Fig. 1 when the load is connected to an a.c. supply; Fig. 3 is a diagram similar to that of Fig. 2 when the load is connected to a d.c. supply; and, Fig. 4 shows one specific application of the Figure 1 circuit arrangement to the fail-safe operation of a gas or other fuel valve such as in a gas cooker control circuit.

Referring to Fig. 1 a power consuming load 1 is connected to an a.c. mains supply 2 through a current sensing trip device 3 which may simply comprise a suitably rated fuse, and a solid state switch 4 such as a silicon controlled rectifier or triac.

The load' 1 has for the purpose of fail-safe operation duplicated control circuits 5 and 6 and these'control circuits may be designed to control the on/off operation of the load according to requirements. These load control circuits are arranged to provide continuous corresponding output signals Ca and Cb when operation of the load 1 is required. These signals Ca and Cb are fed to respective "AND" gates 7 and 8. Further input signals to these "AND" gates are derived from respective oscillators 9 and 10 which produce the square waveform pulse trains Va and Vb, respectively, shown in Fig. 2.As can be seen the pulse trains have the same frequency and periodicity but the pulses are phase displaced by 0. The "AND" gates 7 and 8 provide pulse outputs Vai and Vbl corresponding to the pulse trains Va and Vb as long there is correspondency between the outputs from the control circuit 5 and 6 and the oscillators 9 and 10 are functioning. The pulse outputs Vai and Vbl are fed to an "EXCLUSIVE OR" gate 11 which accordingly provides a pulse output of a duration corresponding to the phase displacement between the pulses Va and Vb. The resultant pulse output Vc provides pulse operation of the switch 4 and since the oscillator outputs signals are related to the a.c. mains supply the pulses Vd will be applied through switch V4 to the load 1.It will be seen that the pulses Vd are of the same amplitude to provide a constant average current due to the time relationship between the oscillators and the mains supply. The average current flowing in the load circuit will usually be the rated load current and the current sensing trip device will be arranged to operate when the average load current exceeds the rated current of the load. According to the nature of the load it may be necessary to take into account such factors as the mechanical and thermal time constants.

Referring to Fig. 3 of the drawings this shows a pulse diagram similar to Fig. 2 but in which the mains supply is a d.c. supply which obviates the need for relating the oscillator outputs to the main supply in the interest of achieving a steady average load current as hereinbefore explained.

In Fig. 4 a specific application of the Fig. 1 arrangement is shown in which the power load is a solenoid fuel valve 12 (e.g. gas valve) associated with a burner of a gas cooker for example. The control circuits 13 and 14 for the valve 12 include flame sensing means and spark ignition control means.

The operation of the circuit arrangement is such that when initial ignition of fuel at the burner is required the control circuits are powered in response to the actuation of a manually-operable means such as one of the gas taps in the case of a cooker. This powering of the control circuits causes the latter to provide corresponding "valve opens" signals for a short period on the lines LA and LB and the operation of the circuit is then similar to that already described in connection with Figs. 1 and 2 in order to effect the opening of solenoid valve 12. The current pulses operating the solenoid valve 1 2 must be of such frequency and duration as to avoid any tendency of the valve to oscillate or chatter or to overheat.The valve 1 2 in operating releases fuel to the burner and since the flame sensing means contemporaneously senses the absence of a flame at the burner a fuel ignition circuit (not shown) will be triggered by the control~circuits to produce repetitive sparking of the burner. If the gas ignites then the flame sensing means detects the presence of the flame and the ignition circuit means will be disabled and the "valve-open" signals from the control circuit will persist in order to maintain the valve 12 in the open condition.

Should the fuel fail to ignite initially then the "valve-open" signals will be removed automatically by the control circuits within a safe time period (e.g. eight seconds). If during normal operation of the system the flame should be accidentally extinguished then the "valve-open" signals will be removed to cause the valve to become closed due to the removal of the pulse train from the input to the switch 4.

The fail-safe provision of the circuit arrangement resulting from duplication of control circuits, oscillators, gates etc. as previously described ensures that the valve will be effectively de-energised to interrupt the supply of fuel tQ the burner in the event of circuit or component failure.

Should the switch 4 itself fail the current sensing trip means will operate to disconnect the valve from the supply if the switch short circuits or if the switch 4 goes open circuit the valve will be deenergised. A partial or soft failure of the silicon controlled rectifier switch will result in the generation of heat causing it rapidly to fail completely to the short circuit or open circuit condition. In either case the valve 12 will be disabled for safety reasons.

Claims (9)

1. A circuit arrangement including power consuming load means having connected in series with it across a power supply switching means which under normal operating conditions of the circuit arrangement is operated by regularly recurring pulses produced in a pulse generating circuit whereby the load means is operated by current pulses of such amplitude and frequency as to afford an RMS current value corresponding to the rated current value of the load means and current sensing trip means operable to disconnect the load means from said supply when the RMS current through the load means exceeds a predetermined value, the pulse generating circuit including circuit protection means effective in the event of mal-operation or breakdown of components in said circuit arrangement or in control means for the aforesaid power consuming load means to provide an abnormal control input to the switching means so that the RMS current level in the load circuit either falls thereby causing the load means to be rendered inoperative or the current rises so that the current sensing trip means operates to disconnect the load from the supply.

2. A circuit arrangement as claimed in claim 1, in which the switching means comprises a silicon controlled rectifier or triac.

3. A circuit arrangement as claimed in claim 1, in which the switching means comprises a transistor, thyristor or reed relay.

4. A circuit arrangement as claimed in any preceding claim, in which the pulse generating circuit comprises oscillator means which for the purpose of ensuring a relatively constant RMS current level in the load circuit under normal operating conditions will need to be related to the mains supply frequency when the load is supplied from an alternating current supply.

5. A circuit arrangement as claimed in any preceding claim, in which the pulse generating circuit comprises two oscillators having phasedisplaced pulse outputs, these oscillators feeding their pulse outputs into respective "AND" gates.

6. A circuit arrangement as claimed in claim 5, jn which other inputs to the "AND" gates are derived from control circuit means for the load means whilst the respective outputs from the "AND" gates are applied to an "EXCLUSIVE-OR" gate.

7. A gas cooker embodying a fail-safe circuit arrangement as claimed in any preceding claim, in which the load means comprises a solenoid gas valve.

8. A gas-fired boiler embodying-a fail-safe circuit arrangement as claimed in any of claims 1 to 6, in which the load means comprises a solenoid gas valve..

9. A fail-safe circuit arrangement substantially as hereinbefore described with reference to the accompanying drawings.