GB2034543A

GB2034543A - Fluorescent lamp starting circuit

Info

Publication number: GB2034543A
Application number: GB7844121A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-11-10
Filing date: 1978-11-10
Publication date: 1980-06-04
Also published as: GB2034543B

Abstract

A circuit for starting a fluorescent lamp comprises a capacitor C12 connected in parallel with a thyristor S1. When the lamp is connected to a power supply, a trigger circuit T1 fires the thyristor to conduct heating current through the lamp filaments. Due to the inductive nature of the circuit, the thyristor is switched off near each peak of the supply voltage so that a high voltage pulse is supplied to the lamp by the capacitor in each cycle of the supply voltage. When the lamp fires, the voltage across the lamp is reduced, preventing further operation of the starting circuit. The thyristor may be a triac triggered via a diac. <IMAGE>

Description

SPECIFICATION Fluorescent lamp circuit This invention relates to electrical switching circuits, and in particularto circuits for starting lamps of the fluorescent gas discharge type.

A conventional fluorescent discharge lamp comprises an elongated tube coated on its inner surface with a phosphor material and filled with a suitable gas mixture in which the discharge is struck. Ignition of the discharge is effected by ionising the gas via electrons emitted by heaters one of which is at each end of the tube. After the discharge has struck the heaters are then switched off.

Typically such a tube is arranged in series with a ballast choke and in parallel with an electromechanical starter device, the tube heaters being coupled in series with the starter. When electrical power is applied to the arrangement the initial voltage surge causes the starter to become a temporary short circuit thus allowing current to flow through the tube heaters so as to emit electrons into the gas in the tube thus preparing the tube for ignition. After a short period the starter goes open circuit so that the inductor has to produce a high voltage to keep its current flowing. This voltage is applied across the tube which then strikes. Once the tube has struck the voltage across it is reduced thus inhibiting further operations of the starter.If for any reason the tube fails to strike the starter again goes short circuit and the cycle of operations is repeated.

An electro-mechanical starter can damage the special electron emitting coating of the heaters by applying high voltages across the tube. Also the trigger voltage of the starter changes with use. This can lead to the starter operating when the lamp is already triggered, causing the lamp to flash continually.

The object of the invention is to minimise or to overcome these disadvantages.

According to the invention there is provided a circuit arrangement for igniting an AC fluorescent gas discharge lamp of the type having first and second heaters whereby the heaters are energised to effect ignition, the circuit including a capacitor in parallel wih a semiconductor switch device, the switch device being of the type which becomes conductive during alternate half cycles of an applied AC voltage.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a thyristor controlled switch circuit in part schematic form; Figure 2 shows a modificaton of the circuit of Figure 1; Figure 3 shows a further modification of the circuit of Figure 1; Figure 4 shows yet a further modification of the circuit of Figure 1; Figure 5 shows a further modification of the circuit of Figure 1; Referring to Figure 1, the switch circuit, which in use is coupled in parallel with a fluorescent discharge lamp may include a series connected resistor R11 and capacitor C12 arranged in parallel with a semiconductor switch device S1 which is advantageously a thyristor. The series resistor may be omitted if the internal resistance of the capacitor and surge resistance of the SCR is high enough.A power factor correction capacitor C1 is arranged across the main terminals D and E.

The initial high voltage across the strater circuit, when power is first applied to the tube, causes the thyristor to trigger either by over-voltaging the device or via a voltage responsive trigger circuit T1.

The thyristor stays on until that part of the alternating cycle at which the current falls to zero when the thyristor turns off. Due to the inductive nature of the thyristor load the supply voltage is near its peak value when the thyristor turns off. The capacitor Cl 2 then recharges through the inductor L1 and resistor R11.

The values of the various components should be chosen such that energy is stored in the inductor L1 when the capacitor is charged to the supply voltage.

This energy is then transferred to the capacitor increasing its voltage to nearly twice the supply peak voltage. This high voltage is applied across the tube and causes ignition when the tube heaters are relatively cool. To ensure that the tube does not ignite when the heaters H1 are cold (this could damage the surface coating on the heaters) a voltage surge attenuator (not shown) may be placed across the starter terminals A and B.

The starter circuit switches on and off once each supply voltage cucle. Hence the circuit applies a high voltage pulse to the tube at each successive cycle, the tube igniting when the heaters are hot enough to sufficiently lower the ignition voltage. Once the tube has ignited it draws current through the inductor L1 lowering the voltage across the circuit and thus inhibiting further operation of the thyristor.

The circuit arrangement of Figure 2 is adapted for use with a thyristor S2 of relatively low breakdown voltage. In this arrangement the thyristor is protected by a reverse blocking diode D1. To prevent the thyristor S2 from being broken down by diode leakage current a resistor R22 is provided in parallel with the anode and cathode of the thyristor. A thermally operated trip switch SW2 may also be provided to disable the starter in the event that a tube, to which the starter is coupled, does not ignite within a predetermined period.

Figure 3 shows the circuit arrangement of Figure 1 provided with a diac trigger. Resistors R32 and R33 form a potential divider to which one terminal of the diac is coupled, the other terminal of the diac being coupled to the gate of the thyristor. The values of the resistors R32 and R33 are selected with reference to the break-down voltage of the diac to achieve the desired switching characteristic.

Figure 4 shows the circuit arrangement of Figure 1 provided with a diac trigger and a triac S4 as the semiconductor switching device. Resistors R42 and R43 form a potential divider to which one terminal of the diax is coupled, the other terminal of the diac being coupled to the gate of the thyristor. The values of the resistors R42 and R43 are selected with reference to the break-down voltage of the diac to achieve the desired switching characteristic. If the tube is not ignited by process as described before, full mains voltage appears across terminals A and B causing the triac to retrigger in the opposite direction, after a time delay governed by the capacitor C43. This time delay is necessary as if it were not present the high voltage surger produced by C42 and L1 would be stopped by retriggering of the triac.

The cycle of operations continues until the heaters are hot enough for the tube to be triggered.

Figure 5 shows the circuit arrangement of Figure 1 provided with a thyristor S5 as the semiconductor switching device, and a diac trigger which has incorporated in its circuit a timer. When power is switched on, full mains voltage results across the sta rter terminais A and B. The thyristor S5 is triggered by the voltage responsive trigger circuit at a voltage governed by the ratio of a potential divider formed by resistors R54 and R55. The junction of the resistors R54 and R55 is connected to a diode D53 which charges a capacitor C53 to a voltage slightly greater than that of trigger diode D52. Resistor R53 is used to prevent the thyristor being triggered before the trigger diode D52 breaks over. The diode D53 prevents the resistors R54 and R55 from discharging the capacitor C53. When the trigger diode D52 breaks down the capacitor C53 discharges through a resistor R52 and this triggers the thyristor S5. The time the thyristor is held on for is governed by the values of the capacitor C53 and the resistor R52.

During this time interval the thyristor is always on immediately the supply voltage becomes positive, which allows maximum current through the heaters.

The lamp tries to strike after each time the current decreases to zero (one per cycle). Since the thyristor is on for a fixed time constant the lamp is allowed to start only at the end of the time constant when the current falls to zero. Due to the inductive nature of the thyristors' load, the mains voltage is at near maximum. The trigger circuit then operates as previously described with reference to Figure 1.

Claims

1. A circuit arrangement for igniting an AC fluorescent gas discharge lamp of the type having first and second heaters wherein the heaters are energised to effect ignition the circuit including a capacitor in parallel with a semiconductor switch device, the switch device being of the type which becomes conductive during alternate half cycles of an applied AC voltage.

2. A circuit arrangement for igniting an AC fluorescent gas discharge tube of the type having first and second heaters wherein the heaters are energised to effect ignition, the circuit including a triacwith its anode-cathode path arranged in parallel with the capacitor, and voltage level responsive means coupled to the gate of the triac.

3. A circuit arrangement for igniting an AC fluorescent gas discharge lamp of the type having first and second heaters whereby the heaters are energised to effect ignition, the circuit including a thyristor with its anode-cathode path arranged in parallel with the capacitor, and voltage level responsive means coupled to the gate of the thyristor and adapted to operate its thyristor during alternate harf cycles of an applied alternating waveform.

4. A circuit arrangement as claimed in claim 3 wherein a timing device is coupled to the gate of the thyristor, to hold the thyristor on for a predetermined time and at the end of this period to trigger the tube.

5. Starter circuit as claimed in claim 1,2, 3 or 4 used in conjunction with thermal trip, reverse blocking diode, surge attenuator and surge protection inductor.

6. A circuit arrangement substantially as described herein with reference to the accompanying drawings.

7. A fluorescent lamp fitting provided with an ignition circuit as claimed in any one of claims 1 to 6.

New claims or amendments to claims filed on 26 June1979 New or amended claims:

8. A circuit arrangement for igniting an AC fluorescent gas discharge lamp of the type having first and second heaters which are energised to effect ignition of the lamp, the circuit including a semiconductor switching device with its main conduction path arranged in parallel with a first capacitor, the switching device also being in parallel with the lamp, and voltage level responsive means coupled to the control electrode of the switching device and adapted to operate the switching device during alternate half cycles of an applied alternating waveform, the current thus caused to flow flowing in the lamp heaters to cause ignition of the lamp, and wherein the voltage responsive means coupled to said control electrode includes a timing device including a diac coupled to a voltage source via a blocking diode via a further capacitor connected to the junction between the diac and the blocking diode, the blocking diode preventing the further capacitor from being discharged via the voltage source.