GB2193758A - Spark generator for gas fired appliances - Google Patents

Abstract

The spark generator includes power supply 8 for charging a capacitor 4 arranged to discharge at a pre- determined voltage through a transformer primary 2 to produce an induced voltage in the transformer secondary 3 for generating a spark at a spark gap 7. The charging and discharging of the capacitor is controlled by a controlled rectifier (SCR) 11 which is non-conducting up to the pre-determined voltage allowing the capacitor 4 to charge and a voltage dependent resistor (VDR) 12 connected to the SCR 11 and operable at the pre-determined voltage to trigger the SCR 11 which becomes conducting allowing the capacitor 4 to discharge. After discharging, the SCR 11 becomes non-conducting allowing the capacitor 4 to charge and discharge automatically and periodically for generating a series of separate sparks as long as the power supply 8 is energised. <IMAGE>

Description

SPECIFICATION Spark generator This invention relates to spark generators and in particular, though not exclusively, to spark generators capable of continuous operation to provide automatically and periodically a series of separate sparks.

Spark generators are commonly used to ignite a burner in a wide range of gas or similar fuel-fired appliances including fires, cookers, water heaters, central heating boilers and the like for both domestic and industrial use. Both in-built spark generators incorporated in the appliance and hand-held portable spark generators separate from the appliance are known.

Generally, in both types of known spark generator, energy is stored by charging a capacitor via a resistor from a d.c. supply up to a pre-determined level at which a switch automatically closes to discharge the capacitor through the primary of a high voltage pulse transformer to produce an induced voltage in the secondary of the transformer which generates a spark at a spark gap. When the capacitor has discharged, the switch automatically re-opens allowing the capacitor to re-charge and the cycle repeats providing a series of separate sparks at the spark gap for as long as the d.c. voltage is applied. Typical times are 1/4 second to charge and 10 microseconds to discharge.

One type of switch which has previously been used for automatically controlling the charging and discharging of the capacitor comprises a gas tube surge arrestor in which a sealed tube contains two metal electrodes and a gas which is normally non-conducting but, at the pre-determined capacitor voltage becomes conducting allowing the capacitor to discharge until the voltage falls to a low level at which the gas becomes non-conducting again allowing the capacitor to re-charge. The disadvantage of this type of switch is that it is prone to gas leaks, has a limited life and is expensive.

Another known switch comprises a thyristor which is normally non-conducting allowing the capacitor to charge up to the pre-determined capacitor voltage at which the thyristor is triggered by a short duration pulse applied to the gate electrode of the thyristor by a resistancecapacitance discharge circuit rendering the thyristor conducting allowing the capacitor to discharge until the voltage falls to a low level at which the thyristor becomes non-conducting again allowing the capacitor to re-charge. The disadvantage of this type of switch is that the known discharge circuit for applying the pulse to the thryistor gate electrode is complex and expensive including separate components for sensing the trigger voltage and generating the trigger pulse.

Yet another known switch comprises a SI DAC having an internal gate electrode connection so as to be self-triggering when the voltage across its anode and cathode reaches a pre-determined voltage. The disadvantage of this switch is that the triggering voltage of known SIDACs is low so that known SIDACs can only be used with very large storage capacitors which adds considerably to the cost.

It is an object of the present invention to provide a spark generator of the general type above-described having a switch for controlling the capacitor which overcomes or reduces the disadvantages of the known switches above-described.

According to the present invention a spark generator includes a transformer having a primary and a secondary, a capacitor connected to the primary, power supply means for charging the capacitor, and switch means for controlling charging and discharging of the capacitor, the switch means comprising a silicon controlled rectifier (SCR) which is nonconducting up to a pre-determined capacitor voltage and a voltage dependent resistor (VDR) connected to the SCR and operable at the pre-determined capacitor voltage to trigger the SCR which becomes conducting allowing the capacitor to discharge through the primary and produce an induced voltage in the secondary for generating a spark.

In the invented spark generator, the nonlinear resistance characteristics of the VDR are utilised to enable the VDR both to sense the charging voltage of the capacitor and to trigger the SCR at the pre-determined capacitor voltage.

By using one component, the VDR, to provide both these functions, the construction and operation of the spark generating circuit is considerably simplified as compared with the known circuits incorporating the switch means above-described.

After discharge of the capacitor through the primary, the voltage drops and the SCR becomes non-conducting again enabling the capacitor to charge and discharge automatically and periodically for generating a series of separate sparks whilst the power supply means is energised.

The SCR conveniently comprises a thyristor having a gate electrode to which the VDR is connected.

Preferably the VDR comprises a metal oxide varistor (MOV), for example a zinc oxide varistor.

The power supply means comprises a DC power source, for example battery means or rectified AC voltage energised through any suitable switch device.

The spark is generated at a spark gap provided by a pair of spaced electrodes. Both electrodes may be connected to the secondary or alternatively one electrode may be connected to the secondary and positioned in use adjacent to the other electrode provided separately from the spark generator.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing in which the single Figure shows a circuit diagram for spark generation in a spark generator according to the present invention.

Referring to the spark generating circuit shown in the accompanying drawing, conventional symbols are used to indicate the various components of the circuit which comprises a transformer 1 having a primary 2 and a secondary 3, a capacitor 4 connected to the primary 2, a pair of electrodes 5,6 defining a spark gap 7 connected to the secondary 3, power supply means 8 such as one or more batteries or rectified AC mains voltage providing a DC power source for charging the capacitor 4 through a resistor 9 and switch means 10 comprising a thyristor 11 and a zinc oxide varistor 12 connected to the gate electrode 13 of the thyristor 11 for controlling charging and discharging of the capacitor 4.

In use, when the power supply means 8 is energised by any appropriate switch device (not shown), the thyristor 11 is non-conducting enabling the capacitor 4 to charge. As the capacitor 4 charges the varistor 12 passes a very low current insufficient to trigger the thyristor 11 until the capacitor voltage reaches a pre-determined level at which the current passed by the varistor 12 increases sharply providing sufficient current to trigger the thyristor 11 which becomes conducting allowing the capacitor 4 to discharge through the transformer primary 2 thereby producing an induced voltage in the transformer secondary 3 and generating a spark at the spark gap 7.

On discharge of the capacitor voltage, the thyristor 11 becomes non-conducting again and, provided that the power supply means 8 remains energised, the charging and discharging cycle of the capacitor 4 repeats continuously to produce automatically and periodically, a series of separate sparks at the spark gap 7.

In this way the non-linear resistance characteristics of the varistor 12 are used to sense the capacitor voltage and to trigger the thyristor 11 by way of current flow sufficient to render the thyristor 11 conducting when the capacitor voltage reaches the pre-determined level required for spark generation.

As will be appreciated, the capacitor discharge voltage is controlled by appropriate selection of the characteristics of the thyristor 11 and varistor 12 and may be varied as desired. For example, for a capacitor discharge voltage of approximately 200 volts, the thyristor 11 is typically selected to become conducting at currents between 20 and 50 microamps and the varistor 12 is designed to pass a current between these limits to trigger the thyristor 11 at voltages between 188 and 231 volts and to pass a current of less than 1 micro-amp at lower voltages to preclude triggering of the thyristor 11 until the required capacitor discharge voltage is obtained.

Similarly the induced voltage is controlled by the ratio of windings on the primary 2 and secondary 3 and this ratio is selected to step up or step down the discharge voltage as necessary to provide the required discharge voltage. For example in the circuit illustrated the ratio is 1:100 thereby stepping up the discharge voltage to produce a higher induced voltage.

The spark generator circuit above-described may be incorporated in any suitable housing, casing or body to provide a spark generator for igniting a fuel-fired appliance for domestic or industrial use, in particular gas-fired appliances. The spark generator may be adapted for incorporation in the appliance as an integral part thereof or the spark generator may comprise a self-contained hand-held portable unit separate from the appliance.

Claims (13)

1. A spark generator including a transformer having a primary and a secondary, a capacitor connected to the primary, power supply means for charging the capacitor, and switch means for controlling charging and discharging of the capacitor, the switch means comprising a silicon controlled rectified (SCR) which is non-conducting up to a pre-determined capacitor voltage and a voltage dependent resistor (VDR) connected to the SCR and operable at the pre-determined capacitor voltage to trigger the SCR which becomes conducting allowing the capacitor to discharge through the primary and produce an induced voltage in the secondary for generating a spark.

2. A spark generator according to claim 1 wherein the SCR comprises a thyristor having a gate electrode to which the VDR is connected.

3. A spark generator according to claim 1 or claim 2 wherein the VDR comprises a metal oxide varistor (MOV).

4. A spark generator according to claim 3 wherein the MOV comprises a zinc oxide varistor.

5. A sparl < generator according to any one of the preceding claims wherein the power supply means comprises a DC power source.

6. A spark generator according to claim 5 wherein the DC power source comprises battery means or rectified AC voltage.

7. A spark generator according to any one of the preceding claims wherein the spark is generated at a spark gap provided by a pair of electrodes.

8. A spark generator according to claim 7 wherein both electrodes are connected to the secondary.

9. A spark generator according to claim 7 wherein one electrode is connected to the secondary and is positioned, in use, adjacent to the other electrode.

10. A spark generator according to any one of the preceding claims including a switch device for energising the power supply means.

11. A spark generator according to any one of the preceding claims wherein the capacitor is operable to charge and discharge automatically and periodically to produce a series of separate sparks as long as the power supply means is energised.

12. A spark generator according to any one of the preceding claims incorporated in a selfcontained hand-held portable unit.

13. A spark generator substantially as hereinbefore described with reference to the accompanying drawing.