GB2035725A - Ignition circuit for a discharge lamp - Google Patents

Abstract

The invention provides an ignition circuit for delivering an ignition pulse to an electric discharge lamp at the phase angle of the mains supply, specified by the lamp manufacturer. In the ignition circuit a main discharge capacitor C1 is connected in a phase shift network C1R2, so that the ignition pulse may be produced at a point on the charging cycle of the main discharge capacitor, at which the slope is relatively steep. In this way variations in the supply voltage will result in only small variations in the phase angle at which the ignition pulse is produced, thus enabling the ignition to stay within the manufacturers tolerances. Zener diode SD1 conducts at a capacitor voltage corresponding to the peak of the AC supply voltage, due to the phase lag produced by the circuit C1 R2 Figure 3 (not shown). Thyristor Q1 is thereby turned on to discharge capacitor C1 through the primary winding of step-up transformer T1 producing the ignition pulse. <IMAGE>

Description

SPECIFICATION Ignition circuit for discharge lamp This invention relates to an ignition circuit for an electric discharge lamp.

To effect ignition or striking of an electric discharge lamp, it is necessary that a relatively high voltage is established temporarily between the electrodes of the lamp. In order to do this an ignition pulse of the order to 4 KV is developed in an ignition circuit and delivered to the discharge lamp. This ignition pulse is normally established by discharging a capacitor through a suitable voltage step-up device, for example a transformer.

Manufacturers of discharge lamps normally specify that the ignition pulse should be applied to the discharge tube at a specific phase angle of the mains cycle, this normally being 90" i 20 . One method of doing this is by means of a voltage sensing device which when the circuit is at peak mains voltage, i.e.

90 , discharges a main discharge capacitor through the voltage step-up device. This method, however, suffers from the draw back that since the slope of the mains voltage supply at 90 , is zero, a small variation in the mains voltage will cause a large variation in the phase angle, with the result that over the variations experienced in the mains voltage, it is not possible to remain within the tolerances set by the manufacturers.

According to one aspect of the present invention an ignition circuit for delivering an ignition pulse to an electric discharge lamp at a phase angle 6 of an alternating voltage supply cycle, comprises a capacitor (herein called the main discharge capacitor) connected in a phase shift network, so that charging of the main discharge capacitor will lag the supply voltage; and electronic switch means associated with said main discharge capacitor, said switch means being maintained in an off condition until the voltage across the main discharge capacitor attains a value which coincides with the phase angle 6 of the supply voltage cycle, when the switch means turns on and causes the main discharge capacitor to discharge through a voltage step-up device to produce the ignition pulse.Preferably the main discharge capacitor is connected in series with a resistance to form the phase shift network.

By this means the voltage at which the switch means is turned on and thus the ignition pulse is delivered to the discharge lamp, occurs at a point on the main discharge capacitor voltage curve where the slope is relatively steep and consequently relatively large variations in voltage will only result in small variations in phase angle. The angle at which the ignition pulse is delivered to the discharge lamp can consequently be kept within the required tolerances for relatively large variations in supply voltage.

An embodiment of the invention is now described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram showing an ignition circuit in accordance with the invention, connected to a lamp, ballast and power factor correction capacitor; Figure 2 shows an ignition circuit according to the present invention, which may be employed in the circuit shown in Figure 1; and Figure 3 shows a plot of the supply voltage (curve Vs) and the voltage across the main discharge capacitor (curve Vc) for the circuit shown in Figure 2.

Assuming that initially all capacitors of the circuit are completely discharged, the ignition cycle is established by switching on the voltage supply across terminals L,N, Figure 1, which will cause the voltage to charge the power factor correction capacitor PFC and be applied between the terminals 1 and 3 of the ignition circuit.

During the first positive half cycle terminal 1 is assumed to be positive with respect to terminal 2 and the main discharge capacitor C1 begins to charge up through resistance R2. The main discharge capacitor C1 and resistance R2 form a phase shift network so that the voltage across the main discharge capacitor C1, lags the main supply voltage by approximately 40 , as illustrated in Figure 3.

The main discharge capacitor C1 is connected through a thyristor Q1 across the primary of output transformerTi. The thyristor 01 is controlled buy a Zener diode ZDl,the zener voltage of which is V1 (200 volts) which corresponds approximately to the voltage across the main discharge capacitor C1 at 50 of its charging cycle, that is when the supply voltage phase angle is 90". When the main discharge capacitor C1 reaches the zener voltage V1 the Zener diode ZD1 will start to conduct via the primary winding of tansformer T1, resistor R1, diode D1 and the gate of thyristor Ql,thereby turning on the thyristor Ql,which in turn discharges the main discharge capacitor C1 into the primary winding of transformerT1 (via resistance R1 and diode D1). The transformer T1 steps up the voltage V1 from main discharge capacitor C1 to a level suitable for ignition of the discharge lamp, for example from 200 v to 4 KV.

The capacitor C2 serves a dual purpose, firstly it ensures that there is a low impedence path from the ballast side of the transformer T1 to the neutral, thereby ensuring that the full secondary voltage is supplied across the discharge lamp, and secondly to suppress any high voltage impulses that may have been generated by lamps fitted with internal bimetallic switch type starters; the resistance R1 limits the current through thyristor Q1 to a safe level; resistance R3 prevents thyristor Q1 from being switched on by a leakage current; the diode D1 ensures that the main discharge capacitor C1 is not discharged on negative half cycles, via resistance R3, Zener diode ZDl,the primary of transformer T1 and resistance R1 and also ensures that the gate of thyristor Q1 is not allowed to go negative on negative half cycles.

As illustrated by Figure 3 the ignition circuit described above produces an ignition pulse at 90" of the mains voltage cycle. As the voltage sensed to trigger this pulse is taken at a point V1 on the capacitor voltage curve where the slope is relatively steep, in comparison with the slope of the curve at about 90" of the cycle i.e., at the peak voltage V2 variations in supply voltage, as illustrated by the curves drawn in broken line, would result in a far smaller deviation d61 from the required firing position, i.e. 90 , then the deviation d62 resulting from a similar variation in mains voltage, if the ignition circuit were triggered by sensing the voltage V2 at 90 of the mains cycle.

Claims (9)

1. An ignition circuit for delivery of an ignition pulse to an electric discharge lamp at a phase angled 0 of an alternating voltage supply cycle, comprises a capacitor (herein called the main dischargecapacitor) connected in a phase shift network, so that charging of the main discharge capacitor will lag the supply voltage; and electronic switch means associated with said main dischargecapacitor, said switch means being maintained in an off condition until the voltage accross the main discharge capacitor attains a value which coincides with the phase angle of the supply voltage cycle, when the switch means turns on and causes the main discharge capacitor to discharge through a voltage step up device to produce the ignition pulse.

2. An ignition circuit according to claim 1 in which the main discharge capacitor is connected in series with a resistance to form the phase shift network.

3. An ignition circuit according to claim 1 or 2 in which the electronic switch means comprises a bistable switching device controlled by a voltage sensing device.

4. An ignition circuit according to claim 3 in which the bistable switching device is a thyristor

5. An ignition circuit according to claim 3 or 4 in which the voltage sensing device is a zener diode.

6. An ignition circuit according to claim 5 in which the breakdown voltage of the zener diode is equal to the voltage across the main dischargecapa tor when the phase angle of the supply voltage is 8.

7. An ignition circuit according to any of the preceding claims in which the step up device is a transformer.

8. An ignition circuit according to claim 7 in which the main discharge capacitor is connected across the primary winding of the transformer via the electronic switching means, so that when the switching means is turned on the main discharge capacitor will discharge through the primary winding of the transformer and produce the ignition pulse in the secondary winding, which is connected to the discharge lamp.

9. An ignition circuit for delivering an ignition pulse to an electric discharge lamp substantially as described herein, with reference to and as shown in figure 2 of the accompanying drawing.