GB2194400A

GB2194400A - Starter & discharge lamp including it

Info

Publication number: GB2194400A
Application number: GB08716920A
Authority: GB
Inventors: Stephen Kenneth Ashworth
Original assignee: TRANSTAR Ltd
Current assignee: TRANSTAR Ltd
Priority date: 1986-08-04
Filing date: 1987-07-17
Publication date: 1988-03-02
Anticipated expiration: 2007-07-17
Also published as: GB8716920D0; GB2194400B

Abstract

A starter for a gas discharge tube (1), (Fig. 1), includes a fluoractor 11 which conducts a current to heat the electrodes (2, 3) of the tube (1) and switches off to cause a high voltage to be induced across the tube by a ballast inductor (5) to ignite it. Multiple triggering means (DC1, C2, R6) is operable to trigger fluoractor 11 into its conducting and non-conducting state at least twice during a half cycle of applied alternating voltage. When a main switch (10) is initially closed, the triggering means DC1 turns the fluoractor 11 on to pass electrode heating current until a capacitor C1 has charged to a level at which the impedance of a thyristor SCR1 has reduced sufficiently to cause fluoractor 11 to turn off and generate an ignition pulse on the trailing portion of a main half cycle. This action is repeated to generate one ignition pulse per half cycle for a few half cycles. The decreased impedance of thyristor SCR1 due to the gradual build up of charge on capacitor C1 then causes fluoractor 11 to turn off on the leading portion of each half cycle whereupon the multiple triggering means DC1, C2, R6 becomes effective to cause the fluoractor to generate multiple ignition pulses (e.g. three or four) in each half cycle. Multiple triggering during the first half of each half cycle allows the maximum current to flow through the tube before the supply polarity reverses. If the tube does not ignite, build up of charge on capacitor C1 causes the gate of fluoractor 11 to be clamped to ground line 18 to prevent it being turned on. A resistor R5 provides a potential source for capacitor C1 when fluoractor 11 is off to ensure that the impedance of thyristor SCR1 is held low inhibiting further operation of the starter. A zener diode 202 in series with resistor R5 reduces problems of residual voltages which-may prevent capacitor C1 discharging when the main switch (10) is off. Residual charge on capacitor C1 would reduce the preheat time. <IMAGE>

Description

SPECIFICATION Starter and discharge lamp including it This invention relates to starters for discharge lamps and also embraces gas discharge lamps including such starters.

The most common gas discharge lamp is a fluorescent tube used for domestic or industrial lighting but non-fluorescent gas discharge lamps are also used to achieve particular effects including their use in illuminated signs.

Gas discharge lamps include a starter which is connected in parallel with the gas discharge tube and which is conventionally includes a bimetallic strip supporting one of a pair of contacts. When the tube is first switched on the bimetallic strip is heated which opens the contacts. The resulting interruption of the current generates a high voltage in a ballast forming part of the gas discharge lamp circuit which produces a voltage surge high enough to ignite the gas discharge tube.

Recently a solid state device has been designed for use in replacements of this conventional starter and this device is called a fluoractor. The fluoractor comprises a high power switch with a regenerative triggering circuit and a high voltage, high power clamp. The device includes an anode and cathode and a gate electrode and, in response to changes in potential applied to the gate electrode establishes or interrupts connection between the anode and cathode. So far it has been proposed to use such a fluoractor by connecting it in a starter including a bridge rectifier circuit to provide full wave rectification. The fluoractor is connected in series with a pair of diodes across the output of the rectifier. A potential divider formed by a resistor and a thyristor is also connected in parallel with the output of the rectifier.A resistor and capacitor timing network is connected in parallel with the diodes and the trigger of the thyristor is connected to the junction point between the capacitor and resistors. The junction between the resistor and thyristor is connected to the gate electrode of the fluoractor.

When current is switched on the fluoractor is switched ON when a voltage is applied to its gate electrode as a result of the thyristor having a high impedance, With the fluoractor switched ON a current passes between its anode and cathode and this provides the heating current for the electrodes of the gas discharge tube. As the potential builds up on the capacitor the impedance of the thyristor decreases until the potential of the fluoractor gate is reduced to a level to trigger interruption of the connection between the fluoractor anode and cathode. This generates an ignition pulse as the flow of current through the ballast is interrupted.

The fluoractor is triggered on the trailing edge part of each half cycle for each of the first few cycles and then, starts to trigger on the leading edge of each half cycle. If the gas discharge tube is not ignited after say ten ignition pulses build-up of the current on the capacitor reduces the impedance of the thyristor to such an extent that it is always ON and further switching ON of the fluoractor is inhibited.

According to one aspect of this invention a starter for a gas discharge tube including a fluoractor which conducts a current to heat the electrodes of a gas discharge tube and which switches to a non-conductive state, in use, to cause a high voltage to be induced across the tube by a ballast to ignite the gas discharge tube also includes multiple triggering means for triggering the fluoractor into its conducting and non-conducting state at least twice during a half cycle of applied alternating voltage.

Preferably the multiple triggering means triggers the fluoractor into multiple switching between its conducting and non-conducting states during half cycles only when the fluoractor is triggered during the first half of the half cycle. Multiple triggering the fluoractor during the first half of the half cycles allows the maximum current to flow through the gas discharge tube before the polarity of the applied alternating current reverses. This facilitates the starting process in the gas discharge tube.

According to another aspect of this invention a gas discharge lamp includes a starter in accordance with the one aspect of this invention connected in series between the electrodes at opposite ends of a gas discharge tube, and a ballast connected in series inbetween a terminal of the lamp and one of the electrodes. The gas discharge lamp may also include a power factor correction capacitor connected across the input terminals of the lamp.

Preferably the starter includes a rectifier with the anode and cathode of the fluoractor coupled across its output, a silicon controlled rectifier connected between the gate electrode of the fluoractor and one of the outputs of the rectifier, the control terminal of the silicon controlled rectifier connected to a first timing circuit, a driver circuit including a second timing circuit connected across the outputs of the rectifier, and a triggering device connected between the driver circuit and the gate electrode of the fluoractor, the time constant of the first timing circuit being many times greater than that of the second timing circuit.

With such a device the application of an alternating voltage to the rectifier results in a build-up of voltage across the output of the rectifier which results in the driver circuit causing the triggering device to switch ON the fluoractor. This allows a current to pass through the starter and, in use, heats up the electrodes of a gas discharge tube connected across the starter. This continues until the time constant of the first timing circuit is exceeded whereupon a potential is applied to the control electrode of the silicon controlled rectifier which lowers its impedance and hence that coupled to the gate electrode of the fluoractor sufficiently to turn the fluoractor OFF and cause the ballast forming part of the gas discharge lamp to generate and apply an ignition pulse to the gas discharge tube.One such ignition pulse is generated during the trailing edge portion of each half cycle of applied voltage. However, when the fluoractor begins switching OFF during the leading portion of each half cycle multiple ignition pulses are produced. These occur because turning OFF the fluoractor at such a time results in a high voltage being output by the rectifier and being applied to the driver circuit. Such a high voltage rapidly exceeds the time constant of the second timing circuit and causes triggering of the triggering device so that a pulse is applied to the gate electrode of the fluoractor.

This turns the fluoractor ON once again and, in use, conducts a current through the ballast.

Once the pulse from the triggering device is over the fluoractor again switches OFF resulting in another ignition pulse. The OFF condition of the fluoractor again causes a high vol tage- to be developed across the rectifier and being applied to the driven circuit so leading to the generation of a further ignition pulse.

This is repeated until the potential difference developed across the rectifier is insufficient to trigger the triggering device.

Preferably the triggering device is formed by a semiconductor switching device which is self triggering and so changes from a nonconducting to a conducting state when a pre determined threshold potential difference is applied across it. Examples of such devices are unilateral semiconductor switches, bilateral semiconductor switches, a unipolar or bipolar switching device such as a SIDAC or prefera bly a DIAC.

Preferably the driver circuit includes a potential divider with a capacitor being connected to the mid-point of the potential divider and the semiconductor switching device is con nected to the mid-point of the potential divider so that it conducts and discharges the capacitor each time that a particular potential difference is established across the capacitor to apply a trigger pulse to the gate electrode of a fluoractor to turn it ON. Preferably a zener diode is connected in series between the switching device and the gate of the fluoractor to increase the predetermined threshold voltage at which the switching device turns ON to prevent the fluoractor being turned ON once the gas discharge tube has ignited and a current is being conducted by it.

Typically a pair of series connected diodes are connected in series with the fluoractor across the rectifier and the first timing circuit is connected in parallel with the pair of diodes to establish a non-linear current to voltage characteristic across the first timing circuit.

Preferably discharge means are connected in parallel with the first timing circuit to discharge it after the power supply to the gas discharge lamp is interrupted to allow the starter to be reset.

Preferably a resistor is coupled in parallel with the anode and cathode of the fluoractor to provide a source of potential for the first timing circuit when the fluoractor is in the OFF condition to ensure that the impedance of the silicon controlled rectifier is maintained low to inhibit further operation of the starter. Preferably another zener diode is connected in series with the resistor coupled in parallel with the fluoractor to prevent any residual voltage being applied to the starter when the discharge lamp is OFF. Such residual voltage may result from the power correction factor capacitor of the gas discharge lamp or from electrical noise or pick-up induced into cables located between a switch and the gas discharge lamp.

The residual voltage would otherwise interfere with the operation of the first timing circuit by partly charging its capacitor. This ensures that the preheat time for which conduction takes place between the anode and cathode of the fluoractor before the first ignition pulse is generated is not shortened.

A particular example of a fluorescent lamp and a starter in accordance with this invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of the fluorescent lamp; Figure 2 is a circuit diagram of the starter circuit; Figure 3 is a block diagram of the fluoractor; and, Figure 4 is a simple equivalent circuit diagram of fluoractor.

Figure 1 shows a fluorescent tube 1 having electrodes 2 and 3, a starter 4 connected in series between the electrodes 2 and 3 and an inductive ballast 5 connected in series between a terminal 6 of the fluorescent lamp and the electrode 2. A power factor correction capacitor 7 is connected between terminals 6 and 8 of the fluorescent lamp. Figure 1 shows the fluorescent lamp instalied in a building and connected to a mains supply 9 via a switch 10.

The starter circuit 4 is shown in Figure 2 and includes a fluoractor 11 such as a fluoractor type Y1 112 manufactured by Texas Instruments, the block diagram and equivalent circuit diagram of which is shown in Figures 3 and 4 respectively. The fluoractor 11 includes an anode terminal A and a cathode terminal K with a high power regenerative switch in parallel with a high power high voltage clamp capable of clamping up to 300 Watts at 1,500 volts connected between them. The high power regenerative switch is actuated by a high sensitivity regenerative trigger which is fired by the application of a potential to a gate electrode input terminal G.

The starter 4 includes a full wave bridge rectifier circuit 12 having its input terminals 13 and 14 connected between the electrodes 2 and 3 of the fluorescent tube and its output terminals 15 and 16 connected to power supply and ground lines 17 and 18 respectively.

The fiuoractor 11 is connected in series with diodes D1 and D2 between the lines 17 and 18. A resistor R4 is connected in parallel with the diodes D1 and D2 and a diode D3 and capacitor C1 is connected in parallel with resistor R4 and diodes D1 and D2. Capacitor C1 is charged from the output of the fluoractor 11 via a resistor chain R2 and R3, direct charge being blocked by the diode D3.

A thyristor SCRl is connected between the gate electrode G of the fluoractor 11 and the ground line 18 and the trigger electrode of the thyristor SCRl is connected to the junction between resistors R2 and R3. Potential divider formed by resistors R6 and R7 is also connected between the lines 17 and 18 and has a capacitor C2 connected in parallel to resistor R7 and has the junction between resistors R6 and R7 connected via a diac DC1 and a zener diode ZD1 to the gate electrode G of fluoractor 11. A further resistor R5 and a further zener diode ZD2 are connected in series, in parallel with the anode and cathode of the fluoractor 11.

When a current is first applied to the fluorescent lamp by closure of the wall switch 10, mains alternating voltage is applied to terminals 13 and 14 of the starter circuit 4. It is rectified by the rectifier 12 to provide a full wave varying DC potential on line 17. Initially, the fluoractor 1 1 is non-conducting so that the rectified mains voltage is developed across the terminals 15 and 16. This establishes a potential difference across the diac DC1 and zener diode ZD1 sufficient to cause breakdown of both to apply a voltage to the gate electrode G of the fluoractor 11 to turn it ON. As soon as the fluoractor 11 turns ON it passes a current, the voltage developed across the terminals 15 and 16 falls and a heating current is applied through the electrodes 2 and 3 of the fluorescent tube 1.The current through the fluoractor 11 is developed across the diodes D1 and D2 and the resulting developed voltage charges up capacitor C1 via resistor chain R2 and R3. The time constant of capacitor C1 and resistors R2 and R3 allows sufficient time to heat the electrodes 2 and 3 to their operating temperature.As the charge on capacitor C1 increases, the potential of the anode between resistors R2 and R3 increases to increase the potential applied to the trigger of the thyristor SCR1. When the potential of the trigger has been raised to a sufficient extent the impedance of the thyristor SCR1 is reduced sufficiently to overcome the self sustaining action of the fluoractor 11 so that it interrupts the connection between the anode A and cathode K. This causes the ballast to generate a high back EMF which is clamped at about 1,500 volts by the fluoractor 11 and provides the first ignition pulse.

This first ignition pulse is generated on the trailing portion of a rectified voltage half cycle.

Once the fluoractor 11 has been switched OFF insufficient voltage is developed across the terminals and hence at the node between resistors R6 and R7 to overcome the breakdown voltage of the diac DC1 and zener diode ZD 1. During the following rectified voltage half cycle the fluoractor 11 is initially OFF but as the voltage builds up across the output terminals 15, 16 of the rectifier and hence the node between resistors R6 and R7 the diac DC1 breaks down to turn the fluoractor 11 ON. It is sustained ON by the current it conducts until it again switches OFF on the trailing portion of the rectified voltage half cycle.

This is repeated over the following few half cycles until the fluoractor switches OFF on the leading portion of a rectified voltage half cycle. The charge on capacitor C1 gradually builds up and so doing reduces still further the impedance of the thyristor SCR1. This change over from switching OFF on the trailing portion of each half cycle to switching OFF on the leading portion is an intrinsic property of the fluoractor 11 as the impedance of the thyristor SCR1 falls so connecting the ground line 18 directly to the gate electrode G. When the fluoractor 11 turns OFF an ignition pulse is generated by the ballast 5 which is again clamped by the fluoractor 11 to 1500 volts.

With the fluoractor 11 OFF a high voltage is developed across the terminals 1 5 and 16 and the capacitor C2 is charged up via resistor R6.

When the capacitor C2 has been charged sufficiently and the voltage at the node between resistors R6 and R7 exceeds the breakdown potential of both the diac DC1 and the zener diode ZD1, typically 180v, the capacitor C2 is discharged into gate G of the fluoractor 11 which switches it ON for a time comparable to that of the current pulse. The fluoractor 11 then switches OFF so breaking the connection between its anode A and cathode K causing a further ignition pulse as the ballast generates a high back EMF which is again clamped at about 1,500 volts by the fluoractor 11. Since the voltage developed across the terminals 1 5 and 16 is high the capacitor C2 is again charged via resistor R6 again causing the diac DC1 and zener diode ZD1 to break over and again switch the fluoractor 11 ON and OFF so generating a further ignition pulse.This process is repeated until the half wave voltage generated across the terminals 15 and 16 is insufficient to raise the voltage on capacitor C2 to the diac DC1 and zener diode ZD1 break over point. The circuit shown in Figure 2 typically produces three or four pulses in each half wave.

Eventually if the tube 1 does not ignite the build-up of charge on capacitor C1 clamps the gate electrode G of the fluoractor 11 to the ground potential line 18 and so prevents the fluoractor 11 being switched ON after a predetermined number of attempts have been made to ignite the tube 1.

When the tube 1 has ignited the potential difference appearing across the ends of the tube 1 and hence across the terminals 13 and 14 is insufficient to establish break over of the diac DC1 and the zener diode ZD1 and hence the fluoractor 11 is prevented from being switched ON.

With a fluorescent tube 1 in peak condition it may be ignited by the starter circuit on the production of the first ignition pulse or at least on the production of one of the one ignition pulses per half cycle that are produced on the trailing portions of the half cycles. However with aged tubes which are not in such good condition they are only ignited when the multiple pulses per half cycle are produced. The first pulse of each set of multiple pulses in each half cycle occurs approximately 20 before the peak voltage in each half cycle which, since the current typically lags behind the voltage allows more than half a cycle of mains current to flow through the fluorescent tube 1 before the polarity reverses.Also the provision of multiple ignition pulses in each half cycle enables them to have a cumulative effect so creating a gradual build-up of ionised gas in the fluorescent tube 1. The starter in accordance with this invention is suitable for starting gas discharge tubes rated at from 20 watts to 125 watts and, because a greater number of ignition pulses are produced this greatly enhances the ignition capabilities of the starter which results in extended discharge tube life and wider operating tolerances of temperature and supply voltage.

Any residual voltage applied to the starter circuit 4 when the switch 10 is open would prevent capacitor CI from discharging by sourcing further charging current via resistor R5. Typically this can occur in two ways, firstly, the power factor correction capacitor 7 may remain partially charged after the switch 10 is opened or, secondly, electrical noise or pick-up in the cables between the switch 10 and the fluorescent lamp can induce voltages into the circuit. The zener diode ZD2 having a nominal break-down voltage of 120 volts reduces this problem since electrical noise and picked-up voltages less than this value are not applied to the capacitor C1 via the resistor chain R5, R2 and R3. Further as the power factor correction capacitor discharges it only has an effect whilst its potential is above 120v which is very short compared to its total discharge time. Thus, the starter circuit is still reset satisfactorily even if voltages up to 120 volts are applied to it. This ensures that the preheat time determined by the time constant of capacitor C1, R3 and R2 is not shortened as a result of partial charging of capacitor C1. Shortening of the preheat time is likely to prevent the tube 1 from igniting.

Claims

1. A starter for a gas discharge tube including a fluoractor which conducts a current to heat the electrodes of a gas discharge tube and which switches to a non-conductive state, in use, to cause a high voltage to be induced across the tube by a ballast to ignite the gas discharge tube also including multiple triggering means for triggering the fluoractor into its conducting and non-conducting state at least twice during a half cycle of applied alternating voltage.

2. A starter according to Claim 1, in which the multiple triggering means triggers the fluoractor into multiple switching between its conducting and non-conducting states during half cycles only when the fluoractor is triggered during the first half of the half cycle.

3. A starter according to Claims 1 or 2, which includes a rectifier with the anode and cathode of the fluoractor coupled across its output, a silicon controlled rectifier connected between the gate electrode of the fluoractor and one of the outputs of the rectifier, the control terminal of the silicon controlled rectifier connected to a first timing circuit, a driver circuit including a second timing circuit connected across the outputs of the rectifier, and a triggering device connected between the driver circuit and the gate electrode of the fluoractor, the time constant of the first timing circuit being many times greater than that of the second timing circuit.

4. A starter according to Claim 3, in which the triggering device is formed by a semiconductor switching device which is self triggering and so changes from a non-conducting to a conducting state when a predetermined threshold potential difference is applied across it.

5. A starter according to Claims 3 or 4, in which the triggering device is a unipolar or bipolar switching device such as a SIDAC or a DIAC.

6. A starter according to Claims 3, 4 or 5, in which the driver circuit includes a potential divider with a capacitor being connected to the mid-point of the potential divider and the triggering device is connected to the mid-point of the potential divider so that it conducts and discharges the capacitor each time that a particular potential difference is established across the capacitor to apply a trigger pulse to the gate electrode of a fluoractor to turn it ON.

7. A starter according to any one of Claims 3 to 6, in which a zener diode is connected in series between the triggering device and the gate of the fluoractor to increase the predetermined threshold voltage at which the triggering device turns ON to prevent the fluoractor being turned ON once the gas discharge tube has ignited and a current is being conducted by it.

8. A starter according to any one of Claims 3 to 7, in which a pair of series connected diodes are connected in series with the fluoractor across the rectifier and the first timing circuit is connected in parallel with the pair of diodes to establish a non-linear current to voltage characteristic across the first timing circuit.

9. A starter according to any one of Claims 3 to 8, in which a resistor is coupled in parallel with the anode and cathode of the fluoractor to provide a source of potential for the first timing circuit when the fluoractor is in the OFF condition to ensure that the impedance of the silicon controlled rectifier is maintained low to inhibit further operation of the starter.

10. A starter according to any one of Claims 3 to 9, in which another zener diode is connected in series with the resistor coupled in parallel with the fluoractor to prevent any residual voltage being applied to the starter when the discharge lamp is OFF.

11. A starter substantially as described with reference to the accompanying drawings.

12. A gas discharge lamp including a starter in accordance with any one of the preceding Claims connected in series between electrodes at opposite ends of a gas discharge tube, and a ballast connected in series in between a terminal of the lamp and one of the electrodes.

13. A gas discharge lamp according to Claim 12, which also include a power factor correction capacitor connected across the input terminals of the lamp.

14. A gas discharge lamp according to Claims 12 or 13, when dependent upon Claim 3 or any Claim dependent upon Claim 3, in which discharge means are connected in parallel with the first timing circuit to discharge it after the power supply to the gas discharge lamp is interrupted to allow the starter to be reset.

15. A gas discharge lamp substantially as described with reference to the accompanying drawings.