GB2225498A - Fluorescent lamp ignition circuit - Google Patents

Abstract

The circuit comprises a triac SCR1 connected across the lamp 1 and triggered by a control circuit R2, C2, SCR2 so that the triac SCR1 turns on to pass heating current through the lamp filaments 2, 3, and the triac SCR1 then turns off to generate a lamp ignition pulse with the aid of a ballast inductor L1. A diode D1 and resistor R3 may be provided to limit the ignition pulse amplitude. A capacitor C1 in series with inductor L1 also acts to reduce the ignition pulse amplitude, as well as increasing the power factor and reducing fluctuation in light output due to mains voltage fluctuations. A further inductor L2 suppresser harmonics that might be produced by the lamp 1, capacitor C1 and inductor L1 acting as a resonant circuit. <IMAGE>

Description

FLUORESCENT LAMP IGNITION CIRCUIT DESCRIPTION This invention relates to an ac fluorescent lamp ignition circuit.

A conventional fluorescent lamp consists of a low pressure gas mixture which, in use, is struck by an ignition voltages which is substantially higher than the normal arc voltage. Conventionally, the ignition voltage is reduced substantially by pre-heating the lamp electrodes. then ignition has been achieved, the lamp exhibits a negative voltage/current characteristic such that an increase in current produces a reduction in voltage and consequently, the tube is inherently unstable. To produce stability, a ballast inductor is usually connected in series with the lamp.

A conventional starter circuit for the lamp permits, prior to ignition, current fro the main supply voltage to flow through the lamp electrodes to produce heating.

The circuit includes a switch so arranged that when the electrodes are sufficiently heated, the switch opens and the interruption of current produces a voltage surge from the ballast inductor across the lamp which makes it start.

Conventionally, the switch comprises a glow switch comprising a glass envelope filled with a rare gas and containing two electrodes of which at least one consists of a bi-metallic strip. This strip, composed of two thin layers of metal, having different coefficients of expansion, has the property of bending when heated. When a sufficiently large voltage, here the mains voltage, is applied to the switch, a glow discharge takes place in the rare gas and the resulting heat generation causes the bi-metallic strip to deflect thereby closing the contacts so as to permit current to flow through the lamp electrodes, which are consequently heated. The glow discharge then ceases since the current path through the gas is short circuited by closure of the bi-metallic strip. The bi-metallic strip then cools off and opens the contacts.The resulting voltage surge from the ballast inductor then ignites the lamp. The closing voltage of the glow switch is selected to lie between an upper limit which is the lowest voltage where the switch is expected to operate and a lower limit which corresponds to the highest value which the arc voltage of the lamp can reach. Thus, the normal lamp voltage, which is lower than the starting voltage, will not cause the starter switch to attempt to start the lamp during normal operation.

It is well known that a fluorescent lamp is provided with a coating of electron emissive material on its tube body in order to facilitate ignition and that as soon as this material is exhausted, the lamp will fail to operate. For normal tube running, a small amount of the emissive material is continuously consumed, but this erosion is relatively insignificant when compared with the large amount of material that is dislodged during the starting cycle of a conventional starter circuit, particularly due to the relatively high over voltage produced to strike ignition in the lamp.

The present invention seeks to provide an improved starter circuit for a fluorescent lamp which operates at â lower striking voltage and permits easy turn-on of the lamp.

According to the present invention there is provided an ac fluorescent lamp ignition circuit, including means to receive a fluorescent lamp, a circuit for producing a flow of a heating current through electrodes of the lamp, and a circuit breaking device for breaking flow of the heating current and producing an overvoltage for striking the lamp, wherein the circuit breaking device comprises semiconductor switching means arranged to be conductive for a first portion of a cycle of an applied ac mains voltage in order to permit the heating current to flow, and to be rendered non-conductive for a second portion of the ac cycle so as to break the flow of heating current and produce said overvoltage.

Preferably, the switching means comprises a controlled semiconductor rectifier having a control electrode connected to receive a control voltage of a polarity dependent upon the polarity of the applied ac voltage.

In this way, it is possible to arrange for the semiconductor switching means to be repeatedly switched on for a first half cycle of the applied ac waveform and subsequently switched off for the next half cycle.

Thus, the heater current is applied for the first half cycle, and the overvoltaae to ignite the lamp is applied for the next half cycle. This repetitive action produces ignition of the lamp readilv without the need for a substantial overvoltage, thus reducing the erosion of the electron emissive material on the inside of the lamp.

In order that the invention may be more fully understood and embodiment thereof will now be described by way of example with reference to the accompanying drawings, wherein: Figure 1 is a schematic circuit diagram of a fluorescent lamp ignition circuit according to the invention.

The circuit is adapted to receive a fluorescent lamp tube 1 having electrodes 2, 3 at opposite ends. Live and neutral terminals 4, 5 of a domestic ac supply are connected to the electrodes 2, 3 through a ballast circuit 6 shown in dotted outline. Also, a starter circuit 7 shown in dotted outline is provided.

The ballast circuit 6 will now be described in more detail. An inductor coil L1 is connected in series with a stabilisation capacitor C1 between the neutral input terminal 5 and the lamp electrode 3. The capacitor C1 is bridged in parallel by a resistor R1.

An additional stabilising inductor L2 is connected in parallel across the electrodes 2, 3, between the live and neutral input terminals 4, 5.

The starter circuit 7 comprises a triac SCR1 having its anode and cathode connected to the electrodes 2, and 3 respectively, and its control or gate electrode is connected to a potential divider circuit comprising a resistor R2 in series with capacitor C2 and an diac SCR2.

In use, a domestic ac supply is connected to the live terminals 4, 5. During a positive going half cycle of the applied waveform, the potential divider R2, C2, SCR2 applies to the gate of SCR1 a control potential which switches on the anode cathode path thereof to permit a heater current to flow through the electrode 2, SCR1 and electrode 3. During the next negative going half cycle of the applied ac wave form, the potential divider R2, C2, SCR2 applies 2 control voltage of opposite polarity to the gate of SCR2 and consequently it is switched off. As a result, the flow of the heating current ceases which causes energy stored in inductor L1 to produce an overvoltage between the electrodes 2, 3 of the lamp tube 1, causing it to strike. As a result, an arc is established through the fluorescent lamp tube which presents a substantially lower impedance to current flow than the starter circuit 7. Thusr the SCR1 remains switched off.

In the event that the switch off of SCR1 does not produce ignition of the lamp tube 1, the triac is again switched on for the next positive going half cycle of the applied ac wave form and the process just described is repeated repetitively until ignition of the lamp tube 1 occurs.

In a modification to the circuit, shown as an optional feature in Figure 1 a diode D1 and resistor R3 are connected in parallel to the thyristor SCR1 so as to limit in use the overvoltage produced upon switch-off of the SCR1. This can improve operation of the circuit in some circumstances.

Upon ignition of the lamp tube 1 occurs, the resulting arc is stabilised by the ballast inductor L1. As previously stated, an increase in current flow through the lamp tube 1 is accompanied by a decrease in arc voltage and hence the arc established in the fluorescent tube 1 is inherently unstable. Stability however is provided by the ballast inductor L1. The capacitor C1 and associated resistor R1 also assist in stabilisation of the arc.The capacitor Cl reduces the level of overvoltage produced upon ignition of the arc (i.e. at switch off of SCR1). Also, the capacitor C1 increases the power factor of the ballast circuit and also reduces fluctuations in lighting levels produced by the lamp in response to fluctuations of applied mains voltage and frequency.

The lamp 1, when struck, in combination with the capacitor C1 and ballast inductor L1 can provide a course resonant circuit which produces harmonics that modulate stroboscopically the output of the lamp. The inductor L2 is provided to suppress such harmonics. From the foregoing, it will be appreciated that the starter circuit according to the invention can be used for fluorescent lamps situated in factories, offices, warehouses, shops and other situations. Furthermore, the power factor of the ballast circuit is increased in comparison with circuits provided hitherto with consequential economies in current consumed. No conventional glow discharge starter is required and the embodiment of the invention described hereinbefore produces no flicker, no harsh striking, no sparking and results in low heat dissipation.

Furthermore although the invention has been described in relation to a single fluorescent lamp tube it will be appreciated that plural lamp tube circuits can be provided.

Claims (8)

1. An ac fluorescent lamp ignition circuit, including means to receive a fluorescent lamp, a circuit for producing a flow of a heating current through electrodes of the lamp, and a circuit breaking device for breaking irlow of the heating current and producing an overvoltage for striking the lamp, wherein the circuit breaking device comprises semiconductor switching means arranged to be conductive for a first portion of a cycle of an applied ac mains voltage in order to permit the heating current to flow, and to be rendered non-conductive for a second portion of the ac cycle so as to break the flow of heating current and produce said overvoltage.

2. A circuit according to claim 1 wherein said semiconductor switching means comprises a controlled semiconductor rectifier having a control electrode connected to receive a control voltage of a polarity dependent upon the polarity of the applied ac voltage.

3. A circuit according to claim 2 wherein the control voltage is developed by a voltage divider connected across said applied ac voltage.

4. A circuit according to claim 2 wherein said voltage divider includes a resistor and a capacitor connected in series across said applied voltage, and a diac connected between the control electrode of the controlled semiconductor rectifier and the series connection of the resistor and the capacitor.

5. A circuit according to any preceding claim including a ballast inductor winding connected to be in series with the lamp, and stabilising capacitor connected in series with the ballast inductor.

6. A circuit according to claim 5 including a secondary winding connected across said applied ac voltage.

7. A circuit according to claim 5 or 6 including a stabilising resistor connected in parallel with the stablising capacitor.

8. An ac fluorescent lamp circuit substantially as hereinbefore described with reference to the accompanying drawing.