Electricity: electrical systems and devices – Igniting systems – For electric spark ignition
Reexamination Certificate
1999-07-30
2001-09-18
Fleming, Fritz (Department: 2836)
Electricity: electrical systems and devices
Igniting systems
For electric spark ignition
C361S256000, C361S257000
Reexamination Certificate
active
06292347
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electric arc generation circuits. More specifically, the present invention relates to circuits in which such arcs are generated from an A.C. voltage.
2. Discussion of the Related Art
FIG. 1
illustrates a conventional example of a circuit
1
generating electric arcs adapted to igniting a fuel gas, for example, in an industrial enclosure, or in a domestic application to a gas oven, to the burners of a gas cooker, or to an independent gas lighter. Circuit
1
includes two input terminals I
1
and I
2
for receiving an A.C. supply voltage Vac, for example, the mains voltage, typically 220 volts at 50 Hz (or 110 volts at 60 Hz). Circuit
1
includes, in series between its terminals I
1
and I
2
, a resistor R
1
, a diode DR, a capacitor C, and a primary winding L
1
of an isolation transformer including one or several secondary windings L
2
. A pair of electrodes (not shown) is associated with each of secondary windings L
2
of the transformer. Each pair is at a location where an electric arc is required, and its electrodes are at a small distance from each other. Circuit
1
also includes, in parallel with capacitor C and winding L
1
, a means FLC for organizing the charge and discharge of capacitor C. Means FLC includes, in antiparallel, a cathode-gate thyristor Th
1
and a diode D
1
. The gate of thyristor Th
1
is connected to the anode of a zener diode DZ
1
, the cathode of which is connected to the anode of thyristor Th
1
.
The operation of circuit
1
is described hereafter in relation with
FIGS. 2 and 3
.
FIG. 2
illustrates the shape of current Ic in capacitor C (FIG.
1
).
FIG. 3
schematically illustrates the shape of voltage Vc across the capacitor.
During positive halfwaves of voltage Vac, capacitor C charges via diode DR and resistor R
1
. More precisely, diode DR is on when voltage Vac is greater than charge level Vc and the capacitor charges. In all other cases and, in particular, during negative halfwaves of voltage Vac, diode DR is blocked (nonconducting). All along the charge of capacitor C, thyristor Th
1
is off.
It should be noted that several positive halfwaves of voltage Vac are necessary to reach the required charge level, controlled as described hereafter.
Indeed, as illustrated in
FIG. 3
, as long as voltage Vac has not reached a level VZ
1
, the capacitor pursues its charge at each halfwave. This results, for the shape of voltage Vc, in an increase by steps corresponding to several halfwaves and, for current Ic, to an exponential decrease of the maximum amplitudes of the charge current peaks along the halfwaves.
When voltage Vc reaches threshold VZ
1
, for example, 250 volts, determined by zener diode DZ
1
, the latter starts an avalanche (time t1) and a gate current triggers transistor Th
1
. Diode DR blocks and the capacitor abruptly discharges into primary winding L
1
via the thyristor which is then used as a free wheel component. This discharge creates in the primary winding an increase (negative peak P with the sign conventions of
FIG. 2
) of the current which is reproduced at the secondary (by conservation of the magnetic energy). On the secondary side, this current variation results in an overvoltage across each winding L
2
and thus across the corresponding electrodes, which creates an electric arc across these electrodes.
Thyristor Th
1
turns off as the current flowing therethrough disappears, that is, when the capacitor is completely discharged. Diode D
1
is then used as a free wheel diode to discharge the current associated with the reactive energy of the transformer as long as diode DR has not turned back on.
Then, at the beginning of the positive halfwave following this discharge (this arc), diode DR turns back on and capacitor C starts charging again in the way previously described until a time t1′ when voltage Vc reaches level VZ
1
again.
The time interval (t1′−t1) between two discharges (arcs) corresponds to the capacitor charge time before reaching threshold VZ
1
. This time thus conventionally depends on the level of supply voltage Vac.
This is a disadvantage of this type of electric arc generation circuits, since supply voltage Vac, for example, the mains voltage, can have random level variations. Such variations cause variations of the electric arc emission frequency.
If the level of voltage Vac increases, the capacitor charges faster, the arc frequency increases and can become too high, typically almost 10 Hz, and problems of electromagnetic compatibility arise.
This frequency problem also occurs in circuits where for other reasons, the arcs are synchronized on the frequency of voltage Vac. A drawback is then that the arc frequency is equal to the frequency (50 Hz) of voltage Vac, which is too high. This is the case of the gas lighter disclosed in UK application No. 2 130 646.
If the level of voltage Vac decreases, the capacitor charges slower, the arc frequency decreases and can become too low, typically under 2 Hz, which causes an accumulation of the gas to be ignited. Such an accumulation can raise security problems.
Now, electric distribution companies do not guarantee a constant voltage. The voltage can vary, for example, by ±20% with respect to the 220-volt mains voltage at 50 Hz. A value of 170 volts then corresponds to a 1.6-Hz arc frequency, and a value of 250 volts corresponds to a 6-Hz arc frequency.
Another disadvantage of a conventional circuit such as shown in
FIG. 1
is that threshold VZ1 of zener diode DZ
1
is likely to have variations due to technological dispersion and to operating drifts (temperature, etc.). Now, this threshold determines the arc periodicity.
SUMMARY OF THE INVENTION
The present invention aims at providing a novel electric arc generation circuit in which the capacitor charge rate is always sufficient to enable generation of electric arcs at a sufficiently high frequency to avoid gas accumulation.
The present invention also aims at providing an electric arc generation circuit at a sufficiently low frequency to avoid disturbing its environment by electromagnetic disturbances.
More generally, the present invention aims at providing an electric arc generation circuit of approximately regular frequency.
To achieve these and other objects, the present invention provides a circuit of electric arc generation from an A.C. voltage, including means for making the electric arc frequency substantially independent from possible amplitude variations of the A.C. voltage, while being lower than the frequency of the A.C. voltage.
According to an embodiment of the present invention, the arcs are generated by means of an isolation transformer, the primary winding of which is connected in series with a capacitor, and the circuit includes means for organizing the capacitor charge and discharge, the charge being performed at a substantially constant current.
According to an embodiment of the present invention, the means for organizing the capacitor charge and discharge include a constant current source and a switching block.
According to an embodiment of the present invention, the current source and the switching block are made in the form of a single integrated circuit.
According to an embodiment of the present invention, the current source includes, on the charge path of the capacitor, a first resistive element associated with a means setting the voltage thereacross.
According to an embodiment of the present invention, the means setting the voltage across the first resistive element is formed of a zener diode having a low threshold with respect to the A.C. voltage, and the current source includes a means for limiting the avalanche current in the zener diode.
According to an embodiment of the present invention, the current limiting means further is a means of control of a switch in series with the first resistive element.
According to an embodiment of the present invention, the switching block includes a controllable switch on the discharge path of the capacitor, a means of detection of the charg
Bremond Andre
Merceron Philippe
Fleming Fritz
Galanthay Theodore E.
Morris James H.
STMicroelectronics S.A.
Wolf Greenfield & Sacks P.C.
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